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Language: C++
Expertise: Beginner

Feb 20, 1998

Sometimes, an object must be converted into a built-in type (for instance, a string object passed as an argument to C function such as

 strcmp())

:

 
//file Mystring.h

class Mystring {
	char *s;
	int size;
	public:
	Mystring(const char *);
	Mystring();
	//...
};

#include  	//C str- family of functions
#include Mystring.h

void main() {

Mystring str(hello world);

int n = strcmp(str, Hello); //compile time error: str is not of 	
//type const char *

}//end main()

C++ offers an automatic type conversion for such cases. All you have to do is declare a conversion operator in your class definition:

 
class Mystring {  //now with conversion operator
	char *s;
	int size;
	public:
	Mystring(const char *);
	Mystring();
	operator const char * () { return s; } //conversion operator
	//...
};

And all is fine:

 
int n = strcmp(str, Hello); //now OK, automatic conversion to
			//const char *

Important: conversion operator is different than an ordinary overloaded operator: it should not return a value (not even void) and takes no arguments.