User's Guide

Although the string literal following the extern keyword in a linkage directive is
implementation-dependent, all implementations must support C and C++ string literals. Refer to
linkage specifications in The C++ Programming Language, Third Edition for more information.
Differences in Argument Passing Conventions
When your C++ code calls functions written in C, ensure that the called C functions do not use
function prototypes that suppress argument widening. If they do, your C++ code passes arguments
wider than the expectations of your C code.
The main() Function
When you mix C++ modules with C modules, the overall control of the program must be written
in C++. The main function should appear in a C++ module, rather than in a C module. There are
two exceptions:
1. C++ programs and libraries, including HP-supplied libraries, without any global class objects
containing constructors or destructors.
2. C++ programs and libraries, including HP-supplied libraries, without static objects.
Examples: HP aC++ Calling HP C
The following examples show a C++ program, calling_c.C that calls a C function, get_name.
The C++ program contains a main function.
//************************************************************
// This is a C++ program that illustrates calling a function *
// written in C. It calls the get_name() function, which is *
// in the “get_name.c” module. The object modules generated *
// by compiling the “calling_c.C” module and by compiling *
// the “get_name.c” module must be linked to create an *
// executable file. *
//************************************************************
#include <iostream.h>
#include “string.h”
//************************************************************
// declare the external C module
extern “C” char* get_name();
class account
{
private:
char* name; // owner of the account
protected:
double balance; // amount of money in the account
public:
account(char* c) // constructor
{ name = new char [strlen(c) +1];
strcpy(name,c);
balance = 0; }
void display()
{ cout << name << “ has a balance of “
<< balance << “\n”; }
};
int main()
{
account* my_checking_acct = new account (get_name());
// send a message to my_checking_account to display itself
my_checking_acct->display();
}
The following example shows the module get_name.c. This function is called by the C++ program.
/****************************************************/
/* This is a C function that is called by main() in */
/* a C++ module, “calling_c.C”. The object */
/* modules generated by compiling this module and */
190 Mixing C++ with Other Languages