C++ Migration Guide

Chapter 6

Moving From C to C++

This chapter describes how to move programs from C to C++.

C programs generally require little modification to compile as C++ programs. C and C++ are link compatible; you don't have to modify compiled C code to link it with C++ code. See The C++ Programming Language, by Margaret A. Ellis and Bjarne Stroustrup, for more specific information on the C++ language.

6.1 Reserved and Predefined Words

TABLE 6-1 shows all reserved keywords in C++ and C, plus keywords that are predefined by C++. Keywords that are reserved in C++ but not in C are shown in boldface.

TABLE 6-1   Reserved Keywords  

asm	do	if	return	typedef
auto	double	inline	short	typeid
bool	dynamic_cast	int	signed	typename
break	else	long	sizeof	union
case	enum	mutable	static	unsigned
catch	explicit	namespace	static_cast	using
char	export	new	struct	virtual
class	extern	operator	switch	void
const	false	private	template	volatile
const_cast	float	protected	this	wchar_t
continue	for	public	throw	while
default	friend	register	true
delete	goto	reinterpret_cast	try

__STDC__ is predefined to the value 0. For example:

#include <stdio.h> main() { #ifdef __STDC__ printf("yes\n"); #else printf("no\n"); #endif #if __STDC__ ==0 printf("yes\n"); #else printf("no\n"); #endif }

produces:

yes yes

The following table lists reserved words for alternate representations of certain operators and punctuators specified in the current ANSI/ISO working paper from the ISO C++ Standards Committee.

TABLE 6-2   C++ Reserved Words for Operators and Punctuators

and	bitor	not	or	xor
and_eq	compl	not_eq	or_eq	xor_eq
bitand

6.2 Creating Generic Header Files

K&R C, ANSI C, and C++ require different header files. To make C++ header files conform to K&R C and ANSI C standards so that they are generic, use the macro _ _cplusplus to separate C++ code from C code. The macro _ _STDC_ _ is defined in both ANSI C and C++. Use this macro to separate C++ or ANSI C code from K&R C code. For more information, see the C++ Programming Guide.

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Note – Early C++ compilers pre-defined the macro c_plusplus, which is no longer supported. Use __cplusplus instead.

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6.3 Linking to C Functions

The compiler encodes C++ function names to allow overloading. To call a C function, or a C++ function "masquerading" as a C function, you must prevent this encoding. Do so by using the extern "C" declaration. For example:

extern "C" { double sqrt(double); //sqrt(double) has C linkage }

This linkage specification does not affect the semantics of the program using sqrt(), but simply causes the compiler to use the C naming conventions for sqrt().

Only one of a set of overloaded C++ functions can have C linkage. You can use C linkage for C++ functions that you intend to call from a C program, but you would only be able to use one instance of that function.

You cannot specify C linkage inside a function definition. Such declarations can only be done at the global scope.