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标 题: Re: Incompatibilities Between ISO C and ISO C++
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Incompatibilities Between
ISO C and ISO C++
------------------------------------------------------------------------------
--
By David R. Tribble
david@tribble.com
Revision 1.0, 2001-08-05
Contents
Introduction
C++ versus C
Changes to C99 versus C++98
Aggregate Initializers
Comments
Conditional expression declarations
Digraph punctuation tokens
Implicit function declarations
Implicit variable declarations
Intermixed declarations and statements
C99 versus C++98
Alternate punctuation token spellings
Array parameter qualifiers
Boolean type
Character literals
clog identifier
Comma operator results
Complex floating-point type
Compound literals
const linkage
Designated initializers
Duplicate typedefs
Dynamic sizeof evaluation
Empty parameter lists
Empty preprocessor function macro arguments
Enumeration constants
Enumeration declarations with trailing comma
Enumeration types
Flexible array members
Function name mangling
Function pointers
Hexadecimal floating-point literals
IEC 60559 arithmetic support
Inline functions
Integer types headers
Library function prototypes
Library header files
long long integer type
Nested structure tags
Non-prototype function declarations
Old-style casts
One definition rule
_Pragma keyword
Predefined identifiers
Reserved keywords in C99
Reserved keywords in C++
restrict keyword
Returning void
static linkage
String initializers
String literals are const
Structures declared in function prototypes
Type-generic math functions
Typedefs versus type tags
Variable-argument function declarators
Variable-argument preprocessor function macros
Variable-length arrays
Void pointer assignments
Wide character type
References
Acknowledgments
Revision History
Bottom
------------------------------------------------------------------------------
--
Introduction
The C programming language began to be standardized some time around 1985 by t
he ANSI X3J9 committee. Several years of effort went by, and in 1989 ANSI appr
oved the new standard. An ISO committee ratified it a year later in 1990 after
adding an amendment dealing with internationalization issues. The 1989 C stan
dard is known officially as ANSI/ISO 9899-1989, Programming Languages - C, and
this document refers to the 1989 C standard as C89. The 1990 ISO revision of
the standard is known officially as ISO/IEC 9899-1990, Programming Languages -
C, which is referred to in this document as "C90".
The next version of the C standard was ratified by ISO in 1999. Officially kno
w as ISO/IEC 9899-1999, Programming Languages - C, it is referred to in this d
ocument as "C99".
The C++ programming language was based on the C programming language as it exi
sted shortly after the ANSI C standardization effort had begun. Around 1995 an
ISO committee was formed to standardize C++, and the new standard was ratifie
d in 1998, which is officially known as ISO/IEC 14882-1998, Programming Langua
ges - C++. It is referred to in this document as "C++98" or simply as "C++".
Though the two languages share a common heritage, and though the designers inv
olved in the standardization processes for each language tried to keep them as
compatible as possible, some incompatibilities unavoidably arose. Once the pr
ogrammer is aware of these potential problem spots, they are easy, for the mos
t part, to avoid when writing C code.
When we say that C is incompatible with C++ with respect to a specific languag
e feature, we mean that a C program that employs that feature either is not va
lid C++ code and thus will not compile as a C++ program, or that it will compi
le as a C++ program but will exhibit different behavior than the same program
compiled as a C program. In other words, an incompatible C feature is valid as
C code but not as C++ code. All incompatibilities of this kind are addressed
in this document. Avoiding these kinds of incompatibilities allows the program
mer to write correct C code that is intended to interact with, or be compiled
as, C++ code.
Another form of incompatible feature is one that is valid when used in a C++ p
rogram but is invalid in a C program. We call this an incompatible C++ feature
. Huge portions of the C++ language fall into this category (e.g., classes, te
mplates, exceptions, references, member functions, anonymous unions, etc.), so
very few of these kinds of incompatibilities are addressed in this document.
Yet another form of incompatible feature occurs when a C++ program uses a feat
ure that has the same name as a C90 feature but which has a different usage or
meaning in C. This document covers these kinds of incompatibilities.
This document lists only the incompatibilities between C99 and C++98. (Incompa
tibilities between C90 and C++ have been documented elsewhere; see Appendix B
of Stroustrup [STR], for example.)
New additions to the C99 standard library are also not addressed in this docum
ent unless they specifically introduce C++ incompatibilities.
------------------------------------------------------------------------------
--
C++ versus C
As discussed in the Introduction, no attempt is made in this document to cover
incompatible C++ features, i.e., features of the C++ language or library that
are not supported in C. Huge portions of C++ and its library fall into this c
ategory. A partial list of these features includes:
anonymous unions
classes
constructors and destructors
exceptions and try/catch blocks
external function linkages (e.g., extern "C")
function overloading
member functions
namespaces
new and delete operators and functions
operator overloading
reference types
standard template library (STL)
template classes
template functions
------------------------------------------------------------------------------
--
Changes to C99 versus C++98
The following items are incompatibilities between C90 and C++98, but have sinc
e been changed in C99 so that they no longer cause problems between the two la
nguages.
Aggregate Initializers
C90 requires automatic and register variables of aggregate type (struct, array
, or union) to have initializers containing only constant expressions. (Many c
ompilers do not adhere to this restriction, however.)
C99 removes that restriction, allowing non-constant expressions to be used in
such initializers.
C++ allows non-constant expressions to be used in initializers for automatic a
nd register variables. (It also allows arbitrary non-constant expressions to b
e used to initialize static and external variables.)
For example:
// C and C++ code
void foo(int i)
{
float x = (float)i; // Valid C90, C99, and C++
int m[3] = { 1, 2, 3 }; // Valid C90, C99, and C++
int g[2] = { 0, i }; // Invalid C90
}
[C99: §6.7.8]
[C++98: §3.7.2, 8.5, 8.5.1]
Comments
C++ recognizes //... comments as well as /*...*/ comments.
C90 only recognizes the /*...*/ form of comments. The //... form usually produ
ces a syntax error in C90, but there are rare cases that may compile erroneous
ly without warning:
i = (x//*y*/z++
, w);
C99 recognizes both forms of comments.
[C99: §5.1.1.2, 6.4.9]
[C++98: §2.1, 2.7]
Conditional expression declarations
C++ allows local variable declarations within conditional expressions (which a
ppear within for, if, while, and switch statements). The scope of the variable
s declared in this context extends to the end of the statement containing the
conditional expression. For example:
for (int i = 0; i < SIZE; i++)
a[i] = i + 1;
C90 does not allow this feature.
C99 allows this feature, but only within for statements.
[C99: §6.8.5]
[C++98: §3.3.2, 6.4, 6.5]
Digraph punctuation tokens
C++ recognizes two-character punctuation tokens, called digraphs, which are no
t recognized by C90. The digraphs and their equivalent tokens are:
<: [
:> ]
<% {
%> }
%: #
%:%: ##
C99 recognizes the same set of digraphs.
The following program is valid in both C99 and C++:
%:include <stdio.h>
%:ifndef BUFSIZE
%:define BUFSIZE 512
%:endif
void copy(char d<::>, const char s<::>, int len)
<%
while (len-- >= 0)
<%
d<:len:> = s<:len:>;
%>
%>
[C99: §6.4.6]
[C++98: §2.5, 2.12]
Implicit function declarations
C90 allows a function to be implicitly declared at the point of its first use
(call), assigning it a return type of int by default. For example:
/* No previous declaration of bar() is in scope */
void foo(void)
{
bar(); /* Implicit declaration: extern int bar() */
}
C++ does not allow implicit function declarations. It is invalid to call a fun
ction that does not have a previous declaration in scope.
C99 no longer allows functions to be implicitly declared. The code above is in
valid in both C99 and C++.
[C99: §6.5.2.2]
[C++98: §5.2.2]
Implicit variable declarations
C90 allows the declaration of a variable, function argument, or structure memb
er to omit the type specifier, implicitly defaulting its type to int.
C99 does not allow this omission, and neither does C++.
The following code is valid in C90, but invalid in C99 and C++:
static sizes = 0; /* Implicit int, error */
struct info
{
const char * name;
const sz; /* Implicit int, error */
};
static foo(register i) /* Implicit ints, error */
{
auto j = 3; /* Implicit int, error */
return (i + j);
}
[C99: §6.7, 6.7.2]
[C++98: §7, 7.1.5]
Intermixed declarations and statements
C90 syntax specifies that all the declarations within a block must appear befo
re the first statement in the block.
C++ does not have this restriction, allowing statements and declarations to ap
pear in any order within a block.
C99 also removes this restriction, allowing intermixed statements and declarat
ions.
void prefind(void)
{
int i;
for (i = 0; i < SZ; i++)
if (find(arr[i]))
break;
const char * s; /* Invalid C90, valid C99 and C++ */
s = arr[i];
prepend(s);
}
[C99: §6.8.2]
[C++98: §6, 6.3, 6.7]
------------------------------------------------------------------------------
--
C99 versus C++98
The following items comprise the differences between C99 and C++98. Some of th
ese incompatibilities existed between C89 and C++98 and remain unchanged betwe
en C99 and C++98, while others are new features that were introduced into C99
that are incompatible with C++98.
Note that features that are specific to C++ and which are not legal C (e.g., c
lass member function declarations) are not included in this section; only lang
uage features that are common to both C and C++ are discussed. Most of the fea
tures are valid as C but invalid as C++.
Some of these features are likely to be implemented as extensions by many C++
compilers in order to be more compatible with C compilers.
Alternate punctuation token spellings
C++ provides the following keywords as synonyms for punctuation tokens:
and &&
and_eq &=
bitand &
bitor |
compl ~
not !
not_eq !=
or ||
or_eq |=
xor ^
xor_eq ^=
These keywords are also recognized by the C++ preprocessor.
C90 does not have these built-in keywords, but it does provide a standard <iso
646.h> header file that contains definitions for the same words as macros, beh
aving almost like built-in keywords.
C++ requires implementations to provide an empty <iso646.h> header. Including
it in a C++ program has no effect on the program. However, C code that does no
t include the <iso646.h> header is free to use these words as identifiers and
macro names, which may cause incompatibilities when such code is compiled as C
++.
enum oper { nop, and, or, eq, ne };
extern int instr(enum oper op, struct compl *c);
The recommended practice for code intended to be compiled as both C and C++ is
to use these identifiers only for these special meanings, and only after incl
uding <iso646.h>.
// Proper header inclusion allows for the use of 'and' et al
#ifndef __cplusplus
#include <iso646.h>
#endif
int foo(float a, float b, float c)
{
return (a > b and b <= c);
}
[C99: §7.9]
[C++98: §2.5, 2.11]
Array parameter qualifiers
C99 provides new declaration syntax for function parameters of array types, al
lowing type qualifiers (the cv-qualifiers const and volatile, and restrict) to
be included within the first set of brackets of an array declarator. The qual
ifier modifies the type of the array parameter itself. For example, the follow
ing declarations are semantically identical:
extern void foo(int str[const]);
extern void foo(int *const str);
In both declarations, parameter str is a const pointer to an int object.
C99 also allows the static specifier to be placed within the brackets of an ar
ray declaration immediately preceding the expression specifying the size of th
e array. The presence of such a specifer indicates that the array is composed
of at least the number of contiguous elements indicated by the size expression
. (Presumably this is a hint to the compiler for optimizing access to elements
of the array.) For example:
void baz(char s[static 10])
{
// s[0] thru s[9] exist and are contiguous
...
}
None of these new syntactic features are recognized by C++.
(These features might be provided as an extension by some C++ compilers.)
[C99: §6.7.5, 6.7.5.2, 6.7.5.3]
[C++98: §7.1.1, 7.1.5.1, 8.3.4, 8.3.5, 8.4]
Boolean type
C99 supports the _Bool keyword, which declares a two-valued integer type (capa
ble of representing the values true and false). It also provides a standard <s
tdbool.h> header that contains definitions for the following macros:
bool Same as _Bool
false Equal to (_Bool)0
true Equal to (_Bool)1
C++ provides bool, false, and true as reserved keywords and implements bool as
a true built-in boolean type.
C programs that do not include the <stdbool.h> header are free to use these ke
ywords as identifiers and macro names, which may cause compatibility problems
when such code is compiled as C++. For example:
typedef short bool; // Different
#define false ('\0') // Different
#define true (!false) // Different
bool flag = false;
The recommended practice is therefore to use these identifiers in C only for t
hese special meanings, and only after including <stdbool.h>.
(It is likely that an empty <stdbool.h> header will be provided by most C++ im
plementations as an extension.)
[C99: §6.2.5, 6.3.1.1, 6.3.1.2, 7.16, 7.26.7]
[C++98: §2.11, 2.13.5, 3.9.1]
Character literals
In C, character literals such as 'a' have type int, and thus sizeof('a') is eq
ual to sizeof(int).
In C++, character literals have type char, and thus sizeof('a') is equal to si
zeof(char).
This difference can lead to inconsistent behavior in some code that is compile
d as both C and C++.
memset(&i, 'a', sizeof('a')); // Questionable code
In practice, this is probably not much of a problem, since character constants
are implicitly converted to type int when they appear within expressions in b
oth C and C++.
[C99: §6.4.4.4]
[C++98: §2.13.2]
clog identifier
C99 declares clog() in <math.h> as the complex natural logarithm function.
C++ declares std::clog in <iostream> as the name of the standard error logging
output stream (analogous to the stderr stream). This name is placed into the
global namespace if the <math.h> header is included, and refers to the logarit
hm function. If <math.h> defines clog as a preprocessor macro name, it can cau
se problems with other C++ code.
// C++ code
#include <iostream>
using std::clog;
#include <math.h> // Possible conflict
void foo(void)
{
clog << clog(2.718281828) << endl;
// Possible conflict
}
Including both the <iostream> and the <cmath> headers in C++ code places both
clog names into the std:: namespace, one being a variable and the other being
a function, which should not cause any conflicts.
// C++ code
#include <iostream>
#include <cmath>
void foo(void)
{
std::clog << std::clog(2.718281828) << endl;
// No conflict; different types
}
void bar(void)
{
complex double (* fp)(complex double);
fp = &std::clog; // Unambiguous
}
It would appear that the safest approach to this potential conflict would be t
o avoid using both forms of clog within the same source file.
[C99: §7.3.7.2]
[C++98: §27.3.1]
Comma operator results
The comma operator in C always results in an r-value even if its right operand
is an l-value, while in C++ the comma operator will result in an l-value if i
ts right operand is an l-value. This means that certain expressions are valid
in C++ but not in C:
int i;
int j;
(i, j) = 1; // Valid C++, invalid C
[C99: §6.5.3.4, 6.5.17]
[C++98: §5.3.3, 5.18]
Complex floating-point type
C99 provides built-in complex and imaginary floating point types, which are de
clared using the _Complex and _Imaginary keywords.
There are exactly three complex types and three imaginary types in C99:
_Complex float
_Complex double
_Complex long double
_Imaginary long double
_Imaginary double
_Imaginary long double
C99 also provides a standard <complex.h> header that contains definitions of c
omplex floating point types, macros, and constants. In particular, this header
defines the following macros:
complex Same as _Complex
imaginary Same as _Imaginary
I i (the complex identity)
C code that does not include this header is free to use these words as identif
iers and macro names. This was an intentional part of the design of the _Compl
ex and _Imaginary keywords, since this allows existing code that employs the n
ew words to continue working as it did before under C89.
Implicit widening conversions between the complex and imaginary types are prov
ided, which parallel the implicit widening conversions between the non-complex
floating point types.
// C99 code
#include <complex.h>
complex double square_d(complex double a)
{
return (a * a);
}
complex float square_f(complex float a)
{
complex double d = a; // Implicit conversion
return square_d(a); // Implicit conversion
}
C++ provides a template class named complex, declared in the <complex> standar
d header file. This type is incompatible with the C99 complex types.
C++ supports more complex types than C99, in theory, since complex is a templa
te class.
// C++ code
#include <complex>
complex<float> square(complex<float> a)
{
return (a * a);
}
complex<int> square(complex<int> a)
{
return (a * a);
}
It is possible to define typedefs that will work in both C99 and C++, albeit w
ith some limitations:
#ifdef __cplusplus
#include <complex>
typedef complex<float> complex_float;
typedef complex<double> complex_double;
typedef complex<long double> complex_long_double;
#else
#include <complex.h>
typedef complex float complex_float;
typedef complex double complex_double;
typedef complex long double complex_long_double;
typedef imaginary float imaginary_float;
typedef imaginary double imaginary_double;
typedef imaginary long double imaginary_long_double;
#endif
Including these definitions allows for portable code that will compile as both
C and C++ code, such as:
complex_double square_cd(complex_double a)
{
return (a * a);
}
[C99: §6.2.5, 6.3.1.6, 6.3.1.7, 6.3.1.8]
[C++98: §26.2]
Compound literals
C99 allows literals having types other than primitive types (e.g., user-define
d structure or array types) to be specified in constant expressions; these are
called compound literals. For example:
struct info
{
char name[8+1];
int type;
};
extern void add(struct info s);
extern void move(float coord[2]);
void predef(void)
{
add((struct info){ "e", 0 }); // A struct literal
move((float[2]){ +0.5, -2.7 }); // An array literal
}
C++ does not support this feature.
C++ does provides a similar capability through the use of non-default class co
nstructors, but which is not quite as flexible as the C feature:
void predef2()
{
add(info("e", 0)); // Call constructor info::info()
}
(This C feature might be provided as an extension by some C++ compilers, but w
ould probably be valid only for POD structure types and arrays of POD types.)
[C99: §6.5.2, 6.5.2.5]
[C++98: §5.2.3, 8.5, 12.1, 12.2]
const linkage
C specifies that a variable declared with a const qualifier is not a modifiabl
e object. In all other regards, though, it is treated the same as any other va
riable. Specifically, if a const object with file scope is not explicitly decl
ared static, its name has external linkage and is visible to other source modu
les.
const int i = 1; // External linkage
extern const int j = 2; // 'extern' optional
static const int k = 3; // 'static' required
C++ specifies that a const object with file scope has internal linkage by defa
ult, meaning that the object's name is not visible outside the source file in
which it is declared. A const object must be declared with an explicit extern
specifier in order to be visible to other source modules.
const int i = 1; // Internal linkage
extern const int j = 2; // 'extern' required
static const int k = 3; // 'static' optional
The recommended practice is therefore to define constants with an explicit sta
tic or extern specifier.
[C99: §6.2.2, 6.7.3]
[C++98: §7.1.5.1]
Designated initializers
C99 introduces the feature of designated initializers, which allows specific m
embers of structures, unions, or arrays to be initialized explicitly by name o
r subscript. For example:
struct info
{
char name[8+1];
int sz;
int typ;
};
struct info arr[] =
{
[0] = { .sz = 20, .name = "abc" },
[9] = { .sz = -1, .name = "" }
};
Unspecified members are default-initialized.
C++ does not support this feature.
(This feature might be provided as an extension by some C++ compilers, but wou
ld probably be valid only for POD structure types and arrays of POD types. How
ever, C++ already provides a similar capability through the use of non-default
class constructors.)
[C99: §6.7.8]
[C++98: §8.5.1, 12.1]
Duplicate typedefs
C does not allow a given typedef to appear more than once in the same scope.
C++ handles typedefs and type names differently than C, and allows redundant o
ccurrences of a given typedef within the same scope.
Thus the following code is valid in C++ but invalid in C:
typedef int MyInt;
typedef int MyInt; // Valid C++, invalid C
This means that typedefs that might be included more than once in a program (e
.g., common typedefs that occur in multiple header files) should be guarded by
preprocessing directives if such source code is meant to be compiled as both
C and C++. For example:
//========================================
// one.h
#ifndef MYINT_T
#define MYINT_T
typedef int MyInt;
#endif
...
//========================================
// two.h
#ifndef MYINT_T
#define MYINT_T
typedef int MyInt;
#endif
...
Thus code can include multiple header files without causing an error in C:
// Include multiple headers that define typedef MyInt
#include "one.h"
#include "two.h"
MyInt my_counter = 0;
[C99: §6.7, 6.7.7]
[C++98: §7.1.3]
Dynamic sizeof evaluation
Because C99 supports variable-length arrays (VLAs), the sizeof operator does n
ot necessarily evaluate to a constant (compile-time) value. Any expression tha
t involves applying the sizeof operator to a VLA operand must be evaluated at
runtime (any other use of sizeof can be evaluated at compile time). For exampl
e:
size_t dsize(int sz)
{
float arr[sz]; // VLA, dynamically allocated
if (sz <= 0)
return sizeof(sz); // Evaluated at compile time
else
return sizeof(arr); // Evaluated at runtime
}
C++ does not support VLAs, so C code that applies the sizeof operator to VLA o
perands will cause problems when compiled as C++.
[C99: §6.5.3.4, 6.7.5, 6.7.5.2]
[C++98: §5.3, 5.3.3]
Empty parameter lists
C distinguishes between a function declared with an empty parameter list and a
function declared with a parameter list consisting of only void. The former i
s an unprototyped function taking an unspecified number of arguments, while th
e latter is a prototyped function taking no arguments.
// C code
extern int foo(); // Unspecified parameters
extern int bar(void); // No parameters
void baz()
{
foo(0); // Valid C, invalid C++
foo(1, 2); // Valid C, invalid C++
bar(); // Okay in both C and C++
bar(1); // Error in both C and C++
}
C++, on the other hand, makes no distinction between the two declarations and
considers them both to mean a function taking no arguments.
// C++ code
extern int xyz();
extern int xyz(void); // Same as 'xyz()' in C++,
// Different and invalid in C
For code that is intended to be compiled as either C or C++, the best solution
to this problem is to always declare functions taking no parameters with an e
xplicit void prototype. For example:
// Compiles as both C and C++
int bosho(void)
{
...
}
Empty function prototypes are a deprecated feature in C99 (as they were in C89
).
[C99: §6.7.5.3]
[C++98: §8.3.5, C.1.6.8.3.5]
Empty preprocessor function macro arguments
C99 allows preprocessor function macros to be specified with empty (missing) a
rguments.
#define ADD3(a,b,c) (+ a + b + c + 0)
ADD3(1, 2, 3) => (+ 1 + 2 + 3 + 0)
ADD3(1, 2, ) => (+ 1 + 2 + + 0)
ADD3(1, , 3) => (+ 1 + + 3 + 0)
ADD3(1,,) => (+ 1 + + + 0)
ADD3(,,) => (+ + + + 0)
C++ does not support empty preprocessor function macros arguments.
(This feature is likely to be provided as an extension by many C++ compilers.)
[C99: §6.10.3, 6.10.3.1]
[C++98: §16.3., 16.3.1]
Enumeration constants
Enumeration constants in C are essentially just named constants of type signed
int. As such, they are constrained to having an initialization value that fal
ls within the range [INT_MIN,INT_MAX]. This also means that for any given enum
eration constant RED, the values of sizeof(RED) and sizeof(int) are always the
same.
C++ enumeration constants have the same type as their enumeration type, which
means that they have the same size and alignment as their underlying integer t
ype. This means that the values of sizeof(RED) and sizeof(int) are not necessa
rily the same for any given enumeration constant RED. Enumeration constants al
so have a wider range of possible underlying types in C++ than in C: signed in
t, unsigned int, signed long, and unsigned long. As such, they also have a wid
er range of valid initialization values.
This may cause incompatibilities for C code compiled as C++, if the C++ compil
er chooses to implement an enumeration type as a different size than it would
be in C, or if the program relies on the results of expressions such as sizeof
(RED).
enum ControlBits
{
CB_LOAD = 0x0001,
CB_STORE = 0x0002,
...
CB_TRACE = LONG_MAX+1, // (Undefined behavior)
CB_ALL = ULONG_MAX
};
[C99: §6.4.4.3, 6.7.2.2]
[C++98: §4.5, 7.2]
Enumeration declarations with trailing comma
C99 allows a trailing comma to follow the last enumeration constant initialize
r within an enumeration type declaration, similar to structure member initiali
zation lists. For example:
enum Color { RED = 0, GREEN, BLUE, };
C++ does not allow this.
(This feature is likely to be provided as an extension by many C++ compilers.)
[C99: §6.7.2.2]
[C++98: §7.2]
Enumeration types
C specifies that each enumerated type is a unique type, distinct from all othe
r enumerated types within the same program. The implementation is free to use
a different underlying primitive integer type for each enumerated type. This m
eans that sizeof(enum A) and sizeof(enum B) are not necessarily the same. This
also means, given that RED is an enumeration constant of type enum Color, tha
t sizeof(RED) and sizeof(enum Color) are not necessarily the same (since all e
numeration constants are of type signed int).
All enumeration constants, though, convert to values of type signed int when t
hey appear in expressions. Since enumeration constants cannot portably be wide
r than int, it might appear that int is the widest enumeration type; however,
implementations are free to support wider enumeration integer types. Such exte
nded types may be different than the types used by a C++ compiler, however.
In C, objects of enumeration types may be assigned integer values without the
need for a explicit cast. For example:
// C code
enum Color { RED, BLUE, GREEN };
int c = RED; // Cast not needed
enum Color col = 1; // Cast not needed
C++ also specifies that all enumerated types are unique and distinct types, bu
t it goes further than C to enforce this. In particular, a function name can b
e overloaded to take an argument of different enumerated types. While objects
of enumerated types implicitly convert to integer values, integer values requi
re an explicit cast to be converted into enumerated types. Implicitly converte
d enumeration values are converted to their underlying integer type, which is
not necessarily signed int. For example:
// C++ code
enum Color { ... };
enum Color setColor(int h)
{
enum Color c;
c = h; // Error, no implicit conversion
return c;
}
int hue(enum Color c)
{
return (c + 128); // Implicit conversion,
// but might not be signed int
}
Since a C++ enumeration constant has the same type and size as its enumeration
type, this means, given that RED is an enumeration constant of type enum Colo
r, that the values of sizeof(RED) and sizeof(enum Color) are exactly the same,
which differs from the rules in C.
There is no guarantee that a given enumeration type is implemented as the same
underlying type in both C and C++, or even in different C implementations. Th
is affects the calling interface between C and C++ functions. This may also ca
use incompatibilities for C code compiled as C++, if the C++ compiler chooses
to implement an enumeration type as a different size that it would be in C, or
if the program relies on the results of expressions such as sizeof(RED).
// C++ code
enum Color { ... };
extern "C" void foo(Color c);
// Parameter types might not match
void bar(Color c)
{
foo(c); // Enum types might be different sizes
}
[C99: §6.4.4.3, 6.7.2.2]
[C++98: §4.5, 7.2]
Flexible array members (FAMs)
This is also known as the struct hack. This specifies a conforming way to decl
are a structure containing a set of fixed-sized members followed by a flexible
array member that can hold an unspecified number of elements. Such a structur
e is typically allocated by calling malloc(), passing it the number of bytes b
eyond the fixed portion of the structure to add to the allocation size. For ex
ample:
struct Hack
{
int count; // Fixed member(s)
int fam[]; // Flexible array member
};
struct Hack * vmake(int sz)
{
struct Hack * p;
p = malloc(sizeof(struct Hack) + sz*sizeof(int));
// Allocate a variable-sized structure
p->count = sz;
for (int i = 0; i < sz; i++)
p->fam[i] = i;
return p;
}
C++ does not support flexible array members.
(This feature might be provided as an extension by some C++ compilers, but wou
ld probably be valid only for POD structure types.)
[C99: §6.7.2.1]
[C++98: §8.3.4]
Function name mangling
In order to implement overloaded functions and member functions, C++ compilers
must have a means of mapping the source names of functions into unique symbol
s in the object code resulting from the compile. For example, the functions ::
foo(int), ::foo(float), and Mine::foo() all have identical names (foo) but dif
ferent calling signatures. In order for the linker to distinguish between the
functions during program link time, they must be mangled into different symbol
ic names.
This differs from the way functions names are mapped into symbolic object name
s in C, which allows for certain cases of type punning (between signed and uns
igned integer types) and non-prototyped extern functions. Therefore C programs
compiled as C++ will produce different symbolic names, unless the functions a
re explicitly declared as having extern "C" linkage. For example:
int foo(int i); // Different symbolic names in C and C++
#ifdef __cplusplus
extern "C"
#endif
int bar(int i); // Same symbolic name in both C and C++
C++ functions are implicitly declared with extern "C++" linkage.
Another consequence of C++ function name mangling is that identifiers in C++ a
re not allowed to contain two or more consecutive underscores (e.g., the name
foo__bar is invalid). Such names are reserved for the implementation, ostensib
ly so that it may have a guaranteed means of mangling source function names in
to unique object symbolic names. (For example, an implementation might choose
to mangle the member function Mine::foo(int) into something like foo__4Mine_Fi
, which is a symbolic name containing consecutive underscores.)
C does not reserve such names, so a C program is free to use such names in any
manner. For example:
void foo__bar(int i) // Improper C++ name
{ ... }
[C99: §5.2.4.1, 6.2.2, 6.4.2.1]
[C++98: §2.10, 3.5, 17.4.2.2, 17.4.3.1.2, 17.4.3.1.3]
Function pointers
C++ functions have extern "C++" linkage by default. In order to call C functio
ns from C++, the functions must be declared with extern "C" linkage. This is t
ypically accomplished by placing C function declarations within an extern "C"
block:
extern "C"
{
extern int api_read(int f, char *b);
extern int api_write(int f, const char *b);
}
extern "C"
{
#include "api.h"
}
But simply declaring functions with extern "C" linkage is not enough to ensure
that C++ functions can call C functions properly. Specifically, pointers to e
xtern "C" functions and pointers to extern "C++" functions are not compatible.
When compiled as C++ code, function pointer declarations are implicitly defin
ed as having extern "C++" linkage, so they cannot be assigned addresses of ext
ern "C" functions. (Function pointers can thus be a source of problems when de
aling with C API libraries and C callback functions.)
extern int mish(int i); // extern "C++" function
extern "C" int mash(int i);
void foo(int a)
{
int (*pf)(int i); // C++ function pointer
pf = &mish; // Okay, C++ function address
(*pf)(a);
pf = &mash; // Error, C function address
(*pf)(a);
}
To make the combination of function pointers and extern "C" functions work cor
rectly in C++, function pointers that are assigned addresses of C functions mu
st be changed to have extern "C" linkage.
One solution is to use a typedef with the proper linkage:
extern "C"
{
typedef int (*Pcf)(int); // C function pointer
}
void bar(int a)
{
int (*pf)(int i); // C++ function pointer
pf = &mish; // Okay, C++ function address
(*pf)(a);
Pcf pc; // C function pointer
pc = &mash; // Okay, C function address
(*pc)(a);
}
[C99: §6.2.5, 6.3.2.3, 6.5.2.2]
[C++98: §5.2.2, 17.4.2.2, 17.4.3.1.3]
Hexadecimal floating-point literals
C99 recognizes hexadecimal floating-point literals, having a "0x" prefix and a
"p" exponent specifier. For example:
float pi = 0x3.243F6A88p+03;
C99 also provides additional format specifiers for the printf() and scanf() fa
mily of standard library functions:
printf("%9.3a", f);
printf("%12.6lA", d);
(These features are likely to be provided as extensions by many C++ compilers.
)
[C99: §6.4.4.2, 6.4.8]
[C++98: §2.9, 2.13.3]
IEC 60559 arithmetic support
C99 allows an implementation to pre-define the __STD_IEC_559 preprocessor macr
o, indicating that it conforms to certain required behavior of the IEC 60559 (
a.k.a. IEEE 599) specification regarding floating-point arithmetic and library
functions. Implementations that do not pre-define this macro are not require
to provide conforming floating-point behavior.
C++ does not make any special provisions for implementations that explicitly s
upport the IEC 60559 floating-point specification.
Conformance to IEC 60559 floating-point arithmetic, and the pre-definition of
the __STD_IEC_559 macro, is likely to be provided as an extension by many C++
compilers.
C99 also allows an implementation to pre-define the __STD_IEC_559_COMPLEX prep
rocessor macro to indicate that it conforms to the behavior specified by IEC 6
0559 for complex floating-point arithmetic and library functions. This affects
the way the _Complex and _Imaginary types are implemented.
C++ provides library functions for complex floating-point arithmetic by provid
ing the complex<> template class, declared in the standard <complex> header fi
le. This type is incompatible with the C99 complex types.
Conformance to the complex arithmetic specification, and the pre-definition of
the __STD_IEC_559 macro, might also be provided by many C++ compilers, and th
is would indicate how the complex<> template class is implemented.
[C99: §6.10.8, F, G]
[C++98: §16.8]
Inline functions
Both C99 and C++ allow functions to be defined as inline, which is a hint to t
he compiler that invocations of such functions can be replaced with inline cod
e expansions rather than actual function calls. Inline functions are not suppo
sed to be a source of incompatibilities between C99 and C++ in practice, but t
here is a small difference in the semantics of the two languages.
C++ requires all of the definitions for a given inline function to be composed
of exactly the same token sequence.
C99, however, allows multiple definitions of a given inline function to be dif
ferent, and does not require the compiler to detect such differences or issue
a diagnostic.
Thus the following two example source files, which define two slightly differe
nt versions of the same inline function, constitute acceptable C99 code but in
valid C++ code:
//========================================
// one.c
inline int twice(int i) // One definition
{
return i * i;
}
int foo(int j)
{
return twice(j);
}
//========================================
// two.c
typedef int integer;
inline integer twice(integer a) // Another definition
{
return (a * a);
}
int bar(int b)
{
return twice(b);
}
This should not be a problem in practice, provided that multiple inline functi
on definitions occur only in shared header files (which ensures that the multi
ple function definitions are composed of the same token sequences).
[C99: §6.7.4]
[C++98: §7.1.2]
Integer types headers
C99 provides the header file <stdint.h>, which contains declarations and macro
definitions for standard integer types. For example:
int height(int_least32_t x);
int width(uint16_t x);
C++ does not provide these types or header files.
(This feature is likely to be provided as an extension by many C++ compilers.
Some C++ compilers might also provide a <cstdint> header file as an extension.
)
[C99: §7.1.2, 7.18]
[C++98: §17.4.1.2, D.5]
Library function prototypes
The C++ standard library header files amend some of the standard C library fun
ction declarations so as to be more type-safe when used in C++. For example, t
he standard C library function declaration:
// <string.h>
extern char * strchr(const char *s, int c);
is replaced with these near-equivalent declarations in the C++ library:
// <cstring>
extern const char * strchr(const char *s, int c);
extern char * strchr(char *s, int c);
These slightly different declarations can cause problems when C code is compil
ed as C++ code, such as:
// C code
const char * s = ...;
char * p;
p = strchr(s, 'a'); // Valid C, invalid C++
This kind of code results in an attempt to assign a const pointer returned fro
m a function to a non-const variable. A simple cast corrects the code, making
it valid as both C++ and C code, as in:
// Corrected for C++
p = (char *) strchr(s, 'a'); // Valid C and C++
[C99: §7.21.5, 7.24.4.5]
[C++98: §17.4.1.2, 21.4]
Library header files
C++ provides the standard C89 library as part of its library.
C99 adds a few header files that are not included as part of the standard C++
library, though:
<complex.h>
<fenv.h>
<inttypes.h>
<stdbool.h>
<stdint.h>
<tgmath.h>
Even though C++ provides the C89 standard C library headers as part of its lib
rary, it deems their use as deprecated. Instead, it encourages programmers to
prefer the equivalent set of C++ header files which provide the same functiona
lity as the C header files:
<math.h> replaced by <cmath>
<stddef.h> replaced by <cstddef>
<stdio.h> replaced by <cstdio>
<stdlib.h> replaced by <cstdlib>
etc. etc.
Deprecating the use of the C header files thus makes the following valid C++98
program possibly invalid under a future revision of standard C++:
#include <stdio.h> // Deprecated in C++
int main(void)
{
printf("Hello, world\n");
return 0;
}
The program can be modified by removing the use of deprecated features in orde
r to make it portable to future implementations of standard C++:
#ifdef __cplusplus
#include <cstdio> // C++ only
using std::printf;
#else
#include <stdio.h> // C only
#endif
int main(void)
{
printf("Hello, world\n");
return 0;
}
[C99: §7.1.2]
[C++98: §17.4.1.2, D.5]
long long integer type
C99 provides signed long long and unsigned long long integer types to its repe
rtoire of primitive types, which are binary integer types at least 64 bits wid
e.
C99 also has enhanced lexical rules to allow for integer constants of these ty
pes. For example:
long long int i = -9000000000000000000LL;
unsigned long long int u = 18000000000000000000LLU;
C99 also provides several new macros in <limits.h>, new format specifiers for
the printf() and scanf() family of standard library functions, and additional
standard library functions that support these types. For example:
void pr(long long i)
{
printf("%lld", i);
}
C++ does not recognize these integer types.
(These features are likely to be provided as extensions by many C++ compilers,
especially those that provide the same runtime library for both C and C++ env
ironments.)
[C99: §5.2.4.2.1, 6.2.5, 6.3.1.1, 6.4.4.1, 6.7.2, 7.12.9, 7.18.4, 7.19.6.1, 7
.19.6.2, 7.20.1, 7.20.6, 7.24.2.1, 7.24.2.2, 7.24.4, A.1.5, B.11, B.19, B.23,
F.3, H.2]
[C++98: §2.13.1, 3.9.1, 21.4, 22.2.2.2.2, 27.8.2, C.2]
Nested structure tags
Nested structure types may be declared within other structures. The scope of t
he inner structure tag extends outside the scope of the outer structure in C,
but does not do so in C++. Structure declarations possess their own scope in C
++, but do not in C. This applies to any struct, union, and enumerated types d
eclared within a structure declaration. For example:
struct Outer
{
struct Inner // Nested structure declaration
{
int a;
float f;
} in;
enum E // Nested enum type declaration
{
UKNOWN, OFF, ON
} state;
};
struct Inner si; // Nested type is visible in C,
// Not visible in C++
enum E et; // Nested type is visible in C,
// Not visible in C++
In order to be visible in C++, the inner declarations must be explicitly named
using its outer class prefix, or they must be declared outside the outer stru
cture so that they have file scope. The former case, for example:
// C++ code
Outer::Inner si; // Explicit type name
Outer::E et; // Explicit type name
And the latter case:
// C and C++ code
struct Inner // Declaration is no longer nested
{
int a;
float f;
};
enum E // Declaration is no longer nested
{
UKNOWN, OFF, ON
};
struct Outer
{
struct Inner in;
enum E state;
};
[C99: §6.2.1, 6.2.3, 6.7.2.1, 6.7.2.3]
[C++98: §9.9, C.1.2.3.3]
Non-prototype function declarations
C supports non-prototype (a.k.a. K&R-style) function definitions. (Like C90, C
99 deems this as deprecated practice.) For example:
int foo(a, b) // Deprecated syntax
int a;
int b;
{
return (a + b);
}
C++ allows only prototyped function definitions. So in order to compile the ex
ample above as C++ code, it must be rewritten in function prototype form:
int foo(int a, int b)
{
return (a + b);
}
[C99: §6.2.7, 6.5.2.2, 6.7.5.3, 6.9.1, 6.11.6, 6.11.7]
[C++98: §5.2.2, 8.3.5, 8.4, C.1.6]
Old-style casts
C++ provides four typecast operators:
const_cast
dynamic_cast
reinterpret_cast
static_cast
While the following C code is also valid C++98 code, it may not be considered
valid code in a future revision of the C++ standard:
char * p;
const char * s = (const char *) p;
One possible work-around is to use macros in C that simulate the C++ typecast
operators:
#ifdef __cplusplus
#define const_cast(t,e) const_cast<t>(e)
#define dynamic_cast(t,e) dynamic_cast<t>(e)
#define reinterpret_cast(t,e) reinterpret_cast<t>(e)
#define static_cast(t,e) static_cast<t>(e)
#else
#define const_cast(t,e) ((t)(e))
#define dynamic_cast(t,e) ((t)(e))
#define reinterpret_cast(t,e) ((t)(e))
#define static_cast(t,e) ((t)(e))
#endif
const char * s = const_cast(const char *, p);
All four casts are included above even though dynamic_cast is not really usefu
l in C code. Perhaps a better definition for dynamic_cast in C would be:
#define dynamic_cast(t,e) _Do_not_use_dynamic_cast
// Produces a compile-time error
C++ also provides functional typecasts, which are not recognized in C:
f = float(i); // C++ cast to float; invalid C
These kinds of typecasts cannot be used in code that is compiled as both C and
C++.
[C99: §6.3, 6.54]
[C++98: §5.2, 5.2.3, 5.2.7, 5.2.9, 5.2.10, 5.2.11, 5.4, 14.6.2.2, 14.6.2.3]
One definition rule
C allows tentative definitions for variables, e.g.:
int i; // Tentative definition
int i = 1; // Explicit definition
C++ does not allow this. Only one definition of any given variable is allowed
within a program.
C also allows, or at least does not require a diagnostic for, different source
files containing conflicting definitions. For example:
//========================================
// one.c
struct Capri // A declaration
{
int a;
int b;
};
//========================================
// two.c
struct Capri // Conflicting declaration
{
float x;
};
C++ deems this invalid, requiring both definitions to consist of the same sequ
ence of tokens.
C allows definitions of the same function or object in different source files
to be composed of different token sequences, provided that they are semantical
ly identical.
The C++ rules are more strict, requiring the multiple definitions to be compos
ed of identical token sequences. Thus the following code, which contains multi
ple definitions that are semantically equivalent but syntactically (token-wise
) different, is valid in C but invalid in C++:
//========================================
// file1.c
struct Waffle // A declaration
{
int a;
};
int syrup(int amt) // A definition
{
return amt*2;
}
//========================================
// file2.c - Valid C, but invalid C++
typedef int IType;
struct Waffle // Equivalent declaration,
{ // but a different token sequence
IType a;
};
IType syrup(IType quant) // Equivalent definition,
{ // but a different token sequence
return (quant*2);
}
[C99: §6.9.2, J.2]
[C++98: §3.2, C.1.2.3.1]
_Pragma keyword
C99 provides the _Pragma keyword, which operates in a similar fashion to the #
pragma preprocessor directive. For example, these two constructs are equivalen
t:
#pragma FLT_ROUND_INF // Preprocessor pragma
_Pragma(FLT_ROUND_INF) // Pragma statement
C++ does not support the _Pragma keyword.
(This feature is likely to be provided as an extension by many C++ compilers.)
[C99: §5.1.1.2, 6.10.6, 6.10.9]
[C++98: §16.6]
Predefined identifiers
C99 provides a predefined identifier, __func__, which acts like a string liter
al containing the name of the enclosing function. For example:
int incr(int a)
{
fprintf(dbgf, "%s(%d)\n", __func__, a);
return ++a;
}
(While this feature is likely to be provided as an extension by many C++ compi
lers, it is unclear what its value would be, especially for member functions w
ithin nested template classes declared within nested namespaces.)
[C99: §6.4.2.2, 7.2.1.1, J.2]
Reserved keywords in C99
C99 has a few reserved keywords that are not recognized by C++:
restrict
_Bool
_Complex
_Imaginary
_Pragma
This will cause problems when C code containing these tokens is compiled as C+
+. For example:
extern int set_name(char *restrict n);
[C99: §6.4.1, 6.7.2, 6.7.3, 6.7.3.1, 6.10.9, 7.3.1, 7.16, A.1.2]
[C++98: §2.11]
Reserved keywords in C++
C++ has a few keywords that are not recognized by C99:
bool mutable this
catch namespace throw
class new true
const_cast operator try
delete private typeid
dynamic_cast protected typename
explicit public using
export reinterpret_cast virtual
false static_cast wchar_t
friend template
C++ also specifically reserves the asm keyword, which may or may not be reserv
ed in C implementations.
C code is free to use these keywords as identifiers and macro names. This will
cause problems when C code containing these tokens is compiled as C++. For ex
ample:
extern int try(int attempt);
extern void frob(struct template *t, bool delete);
[C99: §6.4.1]
[C++98: §2.11]
restrict keyword
C99 supports the restrict keyword, which allows for certain optimizations invo
lving pointers. For example:
void copy(int *restrict d, const int *restrict s, int n)
{
while (n-- > 0)
*d++ = *s++;
}
C++ does not recognize this keyword.
A simple work-around for code that is meant to be compiled as either C or C++
is to use a macro for the restrict keyword:
#ifdef __cplusplus
#define restrict /* nothing */
#endif
(This feature is likely to be provided as an extension by many C++ compilers.
If it is, it is also likely to be allowed as a reference modifier as well as a
pointer modifier.)
[C99: §6.2.5, 6.4.1, 6.7.3, 6.7.3.1, 7, A.1.2, J.2]
[C++98: §2.11]
Returning void
C++ allows functions of return type void to explicitly return expressions of t
ype void. C does not allow void functions to return any kind of expression.
For example:
void foo(someType expr)
{
...
return (void)expr; // Valid C++, invalid C
}
This is allowed in C++ primarily to allow template functions to accept any fun
ction return type, including void, as a template parameter. For example:
// C++ code
template <typename T>
T bar(someType expr)
{
...
return (T)expr; // Valid even if T is void
}
[C99: §6.8.6.4]
[C++98: §3.9.1, 6.6.3]
static linkage
Both C and C++ allow objects and functions to have static file linkage, also k
nown as internal linkage. C++, however, deems this as deprecated practice, pre
ferring the use of unnamed namespaces instead. (C++ objects and functions decl
ared within unnamed namespaces have external linkage unless they are explicitl
y declared static. C++ deems the use of static specifiers on objects or functi
on declarations within namespace scope as deprecated.)
While it is not a problem for C code compiled under C++98 rules, it may become
a problem in a future revision of the C++ language. For example, the followin
g fragment uses the deprecated static feature:
// C and C++ code
static int bufsize = 1024;
static int counter = 0;
static long square(long x)
{
return (x * x);
}
The preferred way of doing this in C++ is:
// C++ code
namespace /*unnamed*/
{
static int bufsize = 1024;
static int counter = 0;
static long square(long x)
{
return (x * x);
}
}
(Note that the use of the static specifiers is unnecessary.)
A possible work-around is to use preprocessor macros and wrappers:
// C and C++ code
#ifdef __cplusplus
#define STATIC static
#endif
#ifdef __cplusplus
namespace /*unnamed*/
{
#endif
STATIC int bufsize = 1024;
STATIC int counter = 0;
STATIC long square(long x)
{
return (x * x);
}
#ifdef __cplusplus
}
#endif
[C99: §6.2.2, 6.2.4, 6.7.1, 6.9, 6.9.1, 6.9.2, 6.11.2]
[C++98: §3.3.5, 3.5, 7.3.1, 7.3.1.1, D.2]
String initializers
C allows character arrays to be initialized with string constants. It also all
ows a string constant initializer to contain exactly one more character than t
he array it initializes, i.e., the implicit terminating null character of the
string may be ignored. For example:
char name1[] = "Harry"; // Array of 6 char
char name2[6] = "Harry"; // Array of 6 char
char name3[] = { 'H', 'a', 'r', 'r', 'y', '\0' };
// Same as 'name1' initialization
char name4[5] = "Harry"; // Array of 5 char, no null char
C++ also allows character arrays to be initialized with string constants, but
always includes the terminating null character in the initialization. Thus the
last initializer (name4) in the example above is invalid in C++.
[C99: §6.7.8]
[C++98: §8.5, 8.5.2]
String literals are const
In C, string literals have type char[n], but are not modifiable (i.e., attempt
ing to modify the contents of a string literal is undefined behavior).
In C++, string literals have type const char[n] and are also not modifiable.
When a string literal is used in an expression (or passed to a function), both
C and C++ implicit convert it into a pointer of type char *. (The C++ convers
ion is considered to be two conversions, the first being an array-to-pointer c
onversion from type const char[n] to type const char *, and the second being a
qualification conversion to type char *.)
The following code is valid in both C and C++.
extern void frob(char *s);
// Argument is not const char *
void foo(void)
{
frob("abc"); // Valid in both C and C++,
// since literal converts to char *
}
This language feature does not present an incompatibility between C99 and C++9
8. However, the implicit conversion has been deprecated in C++ (presumably to
be replaced by a single implicit conversion to type const char *), which means
that a future revision of C++ may no longer accept the code above as valid co
de.
[C99: §6.3.2.1, 6.4.5, 6.5.1, 6.7.8]
[C++98: §2.13.4, 4.2, D.4]
Structures declared in function prototypes
C allows struct, union, and enum types to be declared within function prototyp
e scope, e.g.:
extern void foo(const struct info { int typ; int sz; } *s);
int bar(struct point { int x, y; } pt)
{ ... }
C also allows structure types to be declared as function return types, as in:
extern struct pt { int x; } pos(void);
C++ does not allow either of these, since the scope of the structure declared
in this fashion does not extend outside the function declaration or definition
, making it impossible to define objects of that structure type which could be
passed as arguments to the function or to assign function return values into
objects of that type.
Both C and C++ allow declarations of incomplete structure types within functio
n prototypes and as function return types, though:
void frob(struct sym *s); // Okay, pointer to incomplete type
struct typ * fraz(void); // Ditto
[C99: §6.2.1, 6.7.2.3, 6.7.5.3, I]
[C++98: §3.3.1, 3.3.3, 8.3.5, C.1.6.8.3.5]
Type-generic math functions
C99 supports type-generic mathematical functions. These are functions that are
essentially overloaded on the three floating-point types (float, double, and
long double) and the three complex floating-point types (complex float, comple
x double, and complex long double). To use them, the header file <tgmath.h> mu
st be included; the functions are defined as macros, presumably replaced by im
plementation-defined names.
For example, the following is one possible implementation of the type-generic
functions:
/* Equivalent <tgmath.h> contents:
* extern float sin(float x);
* extern double sin(double x);
* extern long double sin(long double x);
* extern float complex sin(float complex x);
* extern double complex sin(double complex x);
* extern long double complex sin(long double complex x);
* etc...
*/
// Macro definitions
#define sin __tg_sin // Built-in compiler symbol
#define cos __tg_cos // Built-in compiler symbol
#define tan __tg_tan // Built-in compiler symbol
etc...
C++ can also provide type-generic functions, since it is quite capable of prov
iding multiple overloaded function definitions.
(Support for type-generic mathematical functions might be provided by many C++
implementations as an extension, although the exact nature of such generic/ov
erloaded functions would most likely differ substantially from the correspondi
ng C99 implementation. In particular, pointers to type-generic functions would
probably behave differently.)
[C99: §7.22]
[C++98: §13, 13.1, 13.3.1, 13.3.2, 13.3.3]
Typedefs versus type tags
C requires type tags to be preceded by the struct, union, or enum keyword.
C++ treats type tags as implicit typedef names.
Thus the following code is valid C but invalid C++:
// Valid C, invalid C++
typedef int type;
struct type
{
type memb; // int
struct type * next; // struct pointer
};
void foo(type t, int i)
{
int type;
struct type s;
type = i + t + sizeof(type);
s.memb = type;
}
This difference in the treatment of typedefs can also lead to code that is val
id as both C and C++, but which has different semantic behavior. For example:
int sz = 80;
int size(void)
{
struct sz
{ ... };
return sizeof(sz); // sizeof(int) in C,
// sizeof(struct sz) in C++
}
[C99: §6.2.1, 6.2.3, 6.7, 6.7.2.1, 6.7.2.2, 6.7.2.3]
[C++98: §3.1, 3.3.1, 3.3.7, 3.4, 3.4.4, 3.9, 7.1.3, 7.1.5, 7.1.5.2, 9.1]
Variable-argument function declarators
C90 syntax allows a trailing ellipsis in the parameter list of a function decl
arator, which specifies that the function can take zero or more additional arg
uments after the last named parameter.
C++ also allows variable function argument lists, but provides two syntactical
forms for this feature.
/* Variable-argument function declarations */
int foo(int a, int b, ...); // Valid C++ and C
int bar(int a, int b ...); // Valid C++, invalid C
[C99: §6.7.5]
[C++98: §8.3.5]
Variable-argument preprocessor function macros
C99 supports preprocessor function macros that may take a variable number of a
rguments. Such macros are defined with a trailing '...' token in their paramet
er lists, and may use the __VA_ARGS__ reserved identifier in their replacement
text.
For example:
#define DEBUGF(f,...) \
(fprintf(dbgf, "%s(): ", f), fprintf(dbgf, __VA_ARGS__))
#define DEBUGL(...) \
fprintf(dbgf, __VA_ARGS__)
int incr(int *a)
{
DEBUGF("incr", "before: a=%d\n", *a);
(*a)++;
DEBUGL("after: a=%d\n", *a);
return (*a);
}
C++ does not provide this feature.
(This feature is likely to be provided as an extension by many C++ compilers.)
[C99: §6.10.3, 6.10.3.1, 6.10.3.4, 6.10.3.5]
[C++98: §16.3, 16.3.1]
Variable-length arrays (VLAs)
C99 supports variable-length arrays, which are arrays of automatic storage who
se size is determined dynamically at program execution time. For example:
size_t sum(int sz)
{
float arr[sz]; // VLA, dynamically allocated
while (sz-- > 0)
arr[sz] = sz;
return sizeof(arr); // Evaluated at runtime
}
C99 also provides new declaration syntax for function parameters of VLA types,
allowing a variable identifier or a '*' to occur within the brackets of an ar
ray function parameter declaration in place of a constant integer size express
ion. The following example illustrates the syntax involved in passing VLAs to
a function:
extern float sum_square(int n, float a[*]);
float sum_cube(int n, float a[m])
{
...
}
void add_seq(int n)
{
float x[n]; // VLA
float s;
...
s = sum_square(n, x) + sum_cube(n, x);
...
}
VLA function parameter declarations using a '*' can only appear in function de
clarations (with prototypes) and not in function definitions. Note that this c
apability also affects the way sizeof expressions are evaluated.
C++ does not support VLAs.
[C99: §6.7.5, 6.7.5.2, 6.7.5.3, 6.7.6]
[C++98: §8.3.4, 8.3.5, 8.4]
Void pointer assignments
C allows a pointer to void (void *) value to be assigned to an object of any o
ther pointer type without requiring a cast. This allows such things as assigni
ng the return value of malloc() to a pointer variable without the need for an
explicit cast.
C++ does not allow assigning a pointer to void directly to an object of any ot
her pointer type without an explicit cast. This is considered a breach of type
safety, so an explicit cast is required. Thus the following code is valid C b
ut invalid C++:
extern void * malloc(size_t n);
struct object * allocate(void)
{
struct object * p;
p = malloc(sizeof(struct object));
// Direct assignment without a cast,
// valid C, invalid C++
return p;
}
(Both languages allow values of any pointer type to be assigned to objects of
type pointer to void without requiring an explicit cast.
void * vp;
Type * tp;
vp = tp; // No explicit cast needed,
// valid C and C++
Such usage is considered type safe.)
(Note that there are situations in C++ where pointers are implicitly converted
to type pointer to void, such as when comparing a pointer of type pointer to
void to another pointer of a different type, but such situations are considere
d type safe since no pointer objects are modified in the process.)
[C99: §6.2.5, 6.3.2.3, 6.5.15, 6.5.16, 6.5.16.1]
[C++98: §3.8, 3.9.2, 4.10, 5.4, 5.9, 5.10, 5.16, 5.17, 13.3.3.2]
Wide character type
C provides a wide character type, wchar_t, that is capable of holding a single
wide character from an extended character set. This type is defined in the st
andard header files <stddef.h>, <stdlib.h>, and <wchar.h>.
C++ also provides a wchar_t type, but it is a reserved keyword just like int.
No header file is required to enable its definition.
This means that C code that does not include any of the standard header files
listed above is free to use wchar_t as an identifier or macro name; such code
will not compile as C++ code.
// Does not #include <stddef.h>, <stddef.h>, or <wchar.h>
typedef unsigned short wchar_t;
wchar_t readwc(void)
{
...
}
The recommended practice is therefore to use the wchar_t type only for its spe
cial meaning, and only after including <stddef.h>, <stdlib.h>, or <wchar.h>.
(It is likely that a <wchar.h> header will be provided by most C++ implementat
ions as an extension. Some C++ compilers might also provide an empty <cwchar>
header as an extension.)
[C99: §3.7.3, 6.4.4.4, 6.4.5, 7.1.2, 7.17, 7.19.1, 7.20, 7.24]
[C++98: §2.11, 2.13.2, 2.13.4, 3.9.1, 4.5, 7.1.5.2]
------------------------------------------------------------------------------
--
References
[C89]
Programming Languages - C
ANSI/ISO 9899:1989, 1989,
Available at www.ansi.org.
[C90]
Programming Languages - C
(with ISO amendments)
ISO/IEC 9899:1990, 1990, ISO/IEC JTC1/SC22/WG14.
Available at www.ansi.org.
[C99]
Programming Languages - C
ISO/IEC 9899:1999, 1999, ISO/IEC JTC1/SC22/WG14.
Available at www.ansi.org.
[C++98]
Programming Languages - C++
ISO/IEC 14882:1998(E), 1998-09-01, 1st ed., ISO/IEC JTC1/SC22.
Available at www.ansi.org.
[STR]
The C++ Programming Language, Appendix B - Compatibility
Bjarne Stroustrup.
Third ed., 1997, AT&T.
Available in PDF format at www.research.att.com/~bs/3rd_compat.pdf.
------------------------------------------------------------------------------
--
Acknowledgments
My thanks to the the people who gave helpful comments on early drafts of this
document, especially to the following individuals who emailed me suggestions a
nd corrections or posted comments on the comp.std.c and comp.std.c++ newsgroup
s:
Nelson H. F. Beebe, beebe@math.utah.edu
Greg Brewer, greg@brewer.net.
David Capshaw, capshaw@metrowerks.com.
Steve Clamage, clamage@eng.sun.com.
Yaakov Eisenberg, Yaakov@Digisoft.com.
Clive D. W. Feather, clive@demon.net.
Francis Glassborow, francisG@robinton.demon.co.uk.
Doug Gwyn, gwyn@arl.mil or dagwyn@home.com.
James Kanze, James.Kanze@dresdner-bank.com.
Matt Seitz, mseitz@snapserver.com.
Vesa A J Karvonen, vkarvone@cc.helsinki.fi.
Nick Maclaren, nmm1@cam.ac.uk.
Joe Maun, reply_to@yahoo.com.
Gabriel Netterdag, gabriel.netterdag@quidsoft.se.
Cesar Quiroz, Cesar.Quiroz@CoWare.com.
Bjarne Stroustrup, bs@research.att.com and www.research.att.com/~bs.
Keith Thompson, kst@cts.com.
Martin van Loewis, loewis@informatik.hu-berlin.de.
Daniel Villeneuve, danielv@crt.umontreal.ca.
Bill Wade, bill.wade@stoner.com.
------------------------------------------------------------------------------
--
Revision History
1.0, 2001-08-05
Completed revision.
0.12, 2000-11-13
Minor corrections made.
Better HTML anchor names.
0.11, 2000-09-20
Sixth public review revision.
Added ISO section reference numbers to most of the sections.
0.10, 2000-07-30
Sixth public review revision.
0.09, 2000-02-17
Fifth public review revision, still incomplete.
0.08, 1999-10-31
Fourth public review revision, still incomplete.
Minor corrections made.
Changed "C9X" to "C99" after the ratification of ISO C-1999.
0.07, 1999-10-13
Third public review revision, still incomplete.
0.06, 1999-10-05
Second public review revision, still incomplete.
0.05, 1999-09-14
First public review revision, still incomplete.
0.00, 1999-08-24
First attempt, incomplete.
------------------------------------------------------------------------------
--
Copyright ©1999-2001 by David R. Tribble, all rights reserved.
References and links to this document may be created without permission from t
he author. Portions of this document may be used or quoted without permission
from the author provided that appropriate credit is given to the author. This
document may be printed and distributed without permission from the author on
the condition that the authorship and copyright notices remain present and una
ltered.
The author can be reached by email at david@tribble.com.
The author's home web page is at david.tribble.com.
------------------------------------------------------------------------------
--
--
段正游低声向范骅、华赫艮、巴天石诸人道：“这位萧大侠于我有救命之恩，待会危急
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为，以死相报。”大理众士齐声道：“原当如此！”
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