C++ include and redefinition of class error

Question

I am currently programming a program which searches song according to diffrent parameters. In my system there are 2 types of songs: lyric and instrumetal. Since i need to put both of them in 1 vector, I have a song class and a LyricsSong & InstrumentalSong subclasses.

So I there is a Song.h file:

#include <stdio.h>
#include <iostream>
#include <string>


class Song
{
public:
    std::string title;
    virtual void print();
    virtual void printSong(std::string query);
};

and there are the instrumental and lyrics subclasses, which are defined this way:

class LyricsSong : public Song
class InstrumentalSong : public Song

both of the include Song.h, and in both of them the class is defines only in the header file.

when I try to run another file which use the both subclasses, and includes:

#include "LyricsSong.h"
#include "InstrumentalSong.h"

(and obviously more cpp libraries), i get the following compilation error:

In file included from /cygdrive/c/Users/Username/Documents/C++ Workshop/ex2/ex2_code/InstrumentalSong.h:16:0,
                 from /cygdrive/c/Users/Username/Documents/C++ Workshop/ex2/ex2_code/songsParser.cpp:26:
/cygdrive/c/Users/Username/Documents/C++ Workshop/ex2/ex2_code/Song.h:6:7: error: redefinition of 'class Song'
 class Song
       ^
In file included from /cygdrive/c/Users/Username/Documents/C++ Workshop/ex2/ex2_code/LyricsSong.h:15:0,
                 from /cygdrive/c/Users/Username/Documents/C++ Workshop/ex2/ex2_code/songsParser.cpp:25:
/cygdrive/c/Users/Username/Documents/C++ Workshop/ex2/ex2_code/Song.h:6:7: error: previous definition of 'class Song'
 class Song
       ^

when:

• lines InstrumentalSong.h:16:0 and LyricsSong.h:15:0 are where i include "Song.h"
• lines songsParser.cpp:25 and songsParser.cpp:26 are where i include InstrumentalSong.h and LyricsSong.h
• line Song.h:6:7: is the defination of Song.h (where it's say's class Song, as showed above).

What should I do? P.S. I do not import any cpp file ever, only header files.

Answer 1

You have to tell preprocessor to include your header files only once. You can achieve by adding #pragma once on the top of all *.h files:

#pragma once

//Your header file's code

It is also a good practice to always begin header files with this line.

Answer 2

They both include 'Song.h' file and preprocessor takes the file content twice. You need to write 'LyricsSong.h' and 'InstrumentalSong.h' file contents inside #ifndef #define and #endif directives. Like this

#ifndef LYRICS_SONG_H
#define LYRICS_SONG_H

your code goes here.
...

#endif