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.

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. These alternate representations have not yet been implemented in the C++ compiler, but in future releases may be adopted as reserved words and should not be otherwise used.

Table 6-2 Reserved Words for Operators and Punctuators

and	bitor	not	or	xor
and_eq	compl	not_eq	or_eq	xor_eq
bitand

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.

You can insert an #ifdef statement in your code to conditionally compile C++ or C using the C++ compiler. To do this, use the _ _cplusplus macro:

---

#ifdef __cplusplus 
extern "C" int myfunc(int); // C++ declaration 
#else 
int myfunc();               /* K&R C declaration */ 
#endif 

---

---

Note -

In the past, this macro was c_plusplus, which is no longer accepted.

---

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.