Question

If I have a project with

main .cpp 
Knife .h and .cpp
Cucumber .h and .cpp 

and I want to use Knife’s members in Cucumber, does it matter whether I use the following:

#include "Knife.h"
#include <iostream>
using namespace std;

in Cucumber.h or Cucumber.cpp (assume that Cucumber.cpp already has an include for Cucumber.h)?

Answer 1

My recommendation is to minimize the number of files included in the header files.
So, if I have the choice, I prefer to include in the source file.

When you modify a header file, all the files that include this header file must be recompiled.

So, if cucumber.h includes knife.h, and main.cpp includes cucumber.h, and you modify knife.h, all the files will be recompiled (cucumber.cpp, knife.cpp and main.cpp).

If cucumber.cpp includes knife.h and main.cpp includes cucumber.h, and you modify knife.h, only cucumber.cpp and knife.cppwill be recompiled, so your compilation time is reduced.

If you need to use knife in cucumber you can proceed like this:

// Cucumber.hpp
#ifndef CUCUMBER_HPP
#define CUCUMBER_HPP

class Knife;

class Cucumber
{
public :
///...
    private :
    Knife* myKnife
}
#endif

//Cucumber.cpp
#include "Cucumber.hpp"
#include "Knife.hpp

// .. your code here

This “trick” is called “forward declaration”. That is a well-known trick of C++ developers, who want to minimize compilation time.