How to use Standard Library function in include file

Question

func.h

void in(){}

func.cpp

void in(){  
printf("HelloWorld");
}

main.cpp

#include "iostream"
#include "func.h"
int main(){
    in();
}

error C3861: 'printf': identifier not found

Help me to solve this problem, thanks

Answer 1

Your source file func.cpp should #include <cstdio> or perhaps #include <stdio.h> to declare printf() before you use it. With <cstdio>, you get to use namespace std, so you might write using namespace::std; after the #include line, or you might use std:: as a prefix to the function name.

You also need #include "func.h" in func.cpp.

func.cpp — Option 1

#include <cstdio>
#include "func.h"

void in()
{  
    std::printf("HelloWorld");
}

func.cpp — Option 2

#include <cstdio>
#include "func.h"
using namespace std;

void in()
{  
    printf("HelloWorld");
}

func.cpp — Option 3

#include <stdio.h>
#include "func.h"

void in()
{  
    printf("HelloWorld");
}

Option 1 is probably the preferred one.

func.h

Also, the header should declare the function, not define it, unless you really want an empty function body for the function — in which case you don't need the source file func.cpp at all.

void in();

Usually, a header should be protected against multiple inclusion. This time, it won't hurt, but it is a good idea to learn good habits:

#ifndef FUNC_H_INCLUDED
#define FUNC_H_INCLUDED

void in();

#endif /* FUNC_H_INCLUDED */