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How do I call a C function from C++?

Just declare the C function ``extern "C"'' (in your C++ code) and call it
(from your C or C++ code). For example:
	// C++ code

extern "C" void f(int); // one way

extern "C" { // another way
int g(double);
double h();
};

void code(int i, double d)
{
f(i);
int ii = g(d);
double dd = h();
// ...
}
The definitions of the functions may look like this:
	/* C code: */

void f(int i)
{
/* ... */
}

int g(double d)
{
/* ... */
}

double h()
{
/* ... */
}

Note that C++ type rules, not C rules, are used. So you can't call function declared ``extern "C"''
 with the wrong number of argument. For example:
	// C++ code

void more_code(int i, double d)
{
double dd = h(i,d); // error: unexpected arguments
// ...
}

How do I call a C++ function from C?

Just declare the C++ function ``extern "C"'' (in your C++ code) and
call it (from your C or C++ code). For example:
	// C++ code:

extern "C" void f(int);

void f(int i)
{
// ...
}
Now f() can be used like this:
	/* C code: */

void f(int);

void cc(int i)
{
f(i);
/* ... */
}
Naturally, this works only for non-member functions. If you want to call
 member functions (incl. virtual functions) from C, you need to provide
 a simple wrapper. For example:
	// C++ code:

class C {
// ...
virtual double f(int);
};

extern "C" double call_C_f(C* p, int i) // wrapper function
{
return p->f(i);
}
Now C::f() can be used like this:
	/* C code: */

double call_C_f(struct C* p, int i);

void ccc(struct C* p, int i)
{
double d = call_C_f(p,i);
/* ... */
}
If you want to call overloaded functions from C, you must provide wrappers with distinct names
for the C code to use. For example:
	// C++ code:

void f(int);
void f(double);

extern "C" void f_i(int i) { f(i); }
extern "C" void f_d(double d) { f(d); }
Now the f() functions can be used like this:
	/* C code: */

void f_i(int);
void f_d(double);

void cccc(int i,double d)
{
f_i(i);
f_d(d);
/* ... */
}
Note that these techniques can be used to call a C++ library from C code
even if you cannot (or do not want to) modify the C++ headers.
posted on 2008-07-19 01:39 chatler 阅读(143) 评论(0)  编辑 收藏 引用 所属分类: C++_BASIS

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