Headers in

About Headers

In C++, declarations are often separated from definitions. Declarations are grouped into so-called header files, with the respective implementations placed in source files. You can think of the header files as an API. The header file will tell you what a codebase has to offer without going into the details of how.

Header and Source

The most common file extension for header files is .h. Some projects use .hpp or skip the extension completely.

The definitions are located in a separate .cpp file. To reunite the parts, the source file starts by including the respective header file.

If you want to write a library called "quick_math" that offers a function "super_root" that you want to use often, the files would look like this:

// A file named quick_math.h
#pragma once
namespace quick_math {
    double super_root(double x, int n);
}

// A file named quick_math.cpp
#include "quick_math.h"
#include <cmath>
double quick_math::super_root(double x, int n) {
    while(n) { x = std::sqrt(x), --n;}
    return x;
}

If you need to include a header that is only required by the implementation, the respective #include line is only needed in the source file. Everything that is included in the header is also available in the .cpp file, like the string library in the example below. Attention: the ; is needed after the declaration in the header file, but not after the definition in the source file.

Classes and Headers

Classes can become very complex and their relation to the header / source partition might be confusing. One possible layout is to keep all the implementation details in the source file and all the declarations and member variables in the header:

// A file named robot_flower.h
#if !defined(ROBOT_FLOWER_H)
#define ROBOT_FLOWER_H
#include <string>
namespace robots {
    class Flower {
        private:
            bool needs_water{};
            int size{};
            std::string name{};
        public:
            Flower(std::string name, int size = 0);
            void give_water();
            std::string get_name();
            int get_size();
            void start_next_day();
    };
}
#endif

// A file named robot_flower.cpp
#include "robot_flower.h"
robots::Flower::Flower(std::string name, int size) {this->name = "Robotica " + name; this->size = size;}
void robots::Flower::start_next_day() {if (!needs_water) ++size; needs_water = true;}
std::string robots::Flower::get_name() {return name;}
int robots::Flower::get_size() {return size;}

When the header is used as an API overview, that is where a person would look for information like default values. The size parameter's default of the constructor is therefore handled in the header and not in the implementation. The definitions in the source file are prefixed with the namespace robots and the class type Flower.

Another layout option is a header only library, that does not have a .cpp file at all:

// A file named robot_flower.h
#pragma once
#include <string>
namespace robots {
    class Flower {
        private:
            bool needs_water{};
            int size{};
            std::string name{};
        public:
            Flower(std::string name, int size = 0) {this->name = "Robotica " + name; this->size = size;}
            void give_water() {needs_water = false;}
            std::string get_name() {return name;}
            int get_size() {return size;}
            void start_next_day() {if (!needs_water) ++size; needs_water = true;}
    };
}

Projects might use combinations of these layouts and there is a lot of discussion as to what might be the best fit for each use case.

Include Guards

You may have noticed the #pragma once line in the example header file above. This is called an include guard - and it ensures that the content of the file is included only once during the compilation to avoid errors. There is another, more complex variation of an include guard that starts with #ifndef and ends with #endif, that is detailed below.

int myFunction(int n) {
    if (n < 10) {
        return n;
    } else {
        return myOtherFunction(n / 10);
    }
}

int myOtherFunction(int m) {
    return myFunction(m / 2);
}

int myFunction(int n);          // Forward declaration of myFunction
int myOtherFunction(int m);     // Forward declaration of myOtherFunction

// Definition of myFunction
int myFunction(int n) {
    if (n < 0) {
        return 0;
    } else {
        return myOtherFunction(n - 2);
    }
}

// Definition of myOtherFunction
int myOtherFunction(int m) {
    return myFunction(m / 2);
}

#ifndef MY_HEADER_FILE_H /* any name uniquely mapped to file name */
#define MY_HEADER_FILE_H
// file content
#endif