header declaration c++ code example

Example 1: what is a header in c++

// sample.h
#pragma once
#include  // #include directive
#include 

namespace N  // namespace declaration
{
    inline namespace P
    {
        //...
    }

    enum class colors : short { red, blue, purple, azure };

    const double PI = 3.14;  // const and constexpr definitions
    constexpr int MeaningOfLife{ 42 };
    constexpr int get_meaning()
    {
        static_assert(MeaningOfLife == 42, "unexpected!"); // static_assert
        return MeaningOfLife;
    }
    using vstr = std::vector;  // type alias
    extern double d; // extern variable

#define LOG   // macro definition

#ifdef LOG   // conditional compilation directive
    void print_to_log();
#endif

    class my_class   // regular class definition,
    {                // but no non-inline function definitions

        friend class other_class;
    public:
        void do_something();   // definition in my_class.cpp
        inline void put_value(int i) { vals.push_back(i); } // inline OK

    private:
        vstr vals;
        int i;
    };

    struct RGB
    {
        short r{ 0 };  // member initialization
        short g{ 0 };
        short b{ 0 };
    };

    template   // template definition
    class value_store
    {
    public:
        value_store() = default;
        void write_value(T val)
        {
            //... function definition OK in template
        }
    private:
        std::vector vals;
    };

    template   // template declaration
    class value_widget;
}

Example 2: what is a header in c++

// my_program.cpp
#include "my_class.h"

using namespace N;

int main()
{
    my_class mc;
    mc.do_something();
    return 0;
}

Example 3: how to include seld declared header file in c++

#include "Employee.h"
//Employee.h should be saved in the same directory.