“未在此范围内声明”错误的问题

Question

I've got:

error a1 was not declared in this scope

Can somebody please explain why this code causes that?

quiz.h

#ifndef QUIZ_H_
#define QUIZ_H_

    #include "quiz.cpp"   // I deleted this row
                          // and wrote void quiz(int i);


    class A {
    private:
        int player;
    public:
        A(int initPlayer);  // wrote here = 0
        ~A();
        void foo();
    };


    #endif /* QUIZ_H_ */

quiz.cpp

#include "quiz.h"
#include <iostream>
using std::cout;
using std::endl;

A::A(int initPlayer = 0){    // deleted = 0
    player = initPlayer;
}

A::~A(){

}

void A::foo(){
    cout << player;
}

main function

#include "quiz.h"
int main()
{
        quiz(7);
        return 0;
}

quiz function

#include "quiz.h"

void quiz(int i)
{
        A a1(i);
        a1.foo();
}

after my modifications I have an error multiple definition of quiz(int)

Answer 1

不要#include quiz.cpp的quiz.h ！

Answer 2

You should not be including the .cpp file in the header. Remove:

#include "quiz.cpp"

Also, the default value in:

A::A(int initPlayer = 0){

should be in the header file instead.

And edit your question provide the names of all your files, so we can tell you how to compile and link them.

Answer 3

Never include a .cpp file. The header files are used to declare what functions will exist so the compiler knows that you will provide the implementation later (at linking time). All an #include directive does is copy/paste the given file into the source. So in the header file, you've got the contents of your source file before the declarations. The only reason this didn't cause a recursive include was because of the #ifndef tag.

If you want to see what the #include tags are doing to your code, most C++ compilers allow you to only do the preprocessing stage. For gcc, it's just this:

gcc -E main.cpp

And as someone else said, default values go in header files.

Answer 4

I think you have the includes in the wrong places.

quiz.h should not be including the implementation of the class in quiz.cpp

The implementation should be in quiz.cpp as you have For main to know about void quiz(int i) then you need to declare this in a header and include this header in main and the function file (or in this cas just put the function code in the same file as main and before it)