define header file in c++ code example

Example 1: defining class in other file in c++

//ClassOne.hpp:

class ClassOne
{
public:
  ClassOne(); // note, no function body        
  int method(); // no body here either
private:
  int member;
};
//ClassOne.cpp:

#include "ClassOne.hpp"

// implementation of constructor
ClassOne::ClassOne()
 :member(0)
{}

// implementation of "method"
int ClassOne::method()
{
  return member++;
}
//main.cpp:

#include "ClassOne.hpp" // Bring the ClassOne declaration into "view" of the compiler

int main(int argc, char* argv[])
{
  ClassOne c1;
  c1.method();

  return 0;
}

Example 2: what is a header in c++

// sample.h
#pragma once
#include <vector> // #include directive
#include <string>

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

    template <typename T>  // template declaration
    class value_widget;
}