what is a header in c 2b 2b

1// my_class.h
2#ifndef MY_CLASS_H // include guard
3#define MY_CLASS_H
4
5namespace N
6{
7    class my_class
8    {
9    public:
10        void do_something();
11    };
12}
13
14#endif /* MY_CLASS_H */
15

1// sample.h
2#pragma once
3#include <vector> // #include directive
4#include <string>
5
6namespace N  // namespace declaration
7{
8    inline namespace P
9    {
10        //...
11    }
12
13    enum class colors : short { red, blue, purple, azure };
14
15    const double PI = 3.14;  // const and constexpr definitions
16    constexpr int MeaningOfLife{ 42 };
17    constexpr int get_meaning()
18    {
19        static_assert(MeaningOfLife == 42, "unexpected!"); // static_assert
20        return MeaningOfLife;
21    }
22    using vstr = std::vector<int>;  // type alias
23    extern double d; // extern variable
24
25#define LOG   // macro definition
26
27#ifdef LOG   // conditional compilation directive
28    void print_to_log();
29#endif
30
31    class my_class   // regular class definition,
32    {                // but no non-inline function definitions
33
34        friend class other_class;
35    public:
36        void do_something();   // definition in my_class.cpp
37        inline void put_value(int i) { vals.push_back(i); } // inline OK
38
39    private:
40        vstr vals;
41        int i;
42    };
43
44    struct RGB
45    {
46        short r{ 0 };  // member initialization
47        short g{ 0 };
48        short b{ 0 };
49    };
50
51    template <typename T>  // template definition
52    class value_store
53    {
54    public:
55        value_store<T>() = default;
56        void write_value(T val)
57        {
58            //... function definition OK in template
59        }
60    private:
61        std::vector<T> vals;
62    };
63
64    template <typename T>  // template declaration
65    class value_widget;
66}
67

1// my_class.cpp
2#include "my_class.h" // header in local directory
3#include <iostream> // header in standard library
4
5using namespace N;
6using namespace std;
7
8void my_class::do_something()
9{
10    cout << "Doing something!" << endl;
11}
12

1// my_program.cpp
2#include "my_class.h"
3
4using namespace N;
5
6int main()
7{
8    my_class mc;
9    mc.do_something();
10    return 0;
11}
12

1int x; // declaration
2x = 42; // use x
3

1// my_class.h
2namespace N
3{
4    class my_class
5    {
6    public:
7        void do_something();
8    };
9
10}
11