Question

Below I have reproduced a simplified version of a part of my code that gives error when compiling.

testing.cpp

#include <iostream>
#include "../Beta.h"
#include "../Alpha.h"

using namespace std;

int main() {
    cout << "!!!Hello World!!!" << endl; // prints !!!Hello World!!!
    return 0;
}

Alpha.h

#include <vector>

class Alpha
{
    public:

        typedef struct _info{
            int  k;
        } info;

        friend class Beta;
};

Beta.h

   #include <vector>
    
        class Alpha;
        
        class Beta
        {
            public:
            std::vector <Alpha::info*> vecInfo;
        };

When I run g++ testing.cpp I get below error message

In file included from testing.cpp:10:0: ../Beta.h:8:15: error:

incomplete type ‘Alpha’ used in nested name specifier ../Beta.h:8:15:

error: incomplete type ‘Alpha’ used in nested name specifier

../Beta.h:8:27: error: template argument 1 is invalid ../Beta.h:8:27:

error: template argument 2 is invalid

I can see from the forum threads that the first error is because of some kind of cyclic dependency (Error: incomplete type used in nested name specifier). I am unable to see similarity between my code and their code. What am I doing wrong.

Answer 1

You need to include Alpha.h in Beta.h, because the full class definition is needed to have access to Alpha::info. This will not create a cyclic dependency because Alpha.h does not include Beta.h.

// don't forget the include guards!
#ifndef BETA_H_
#define BETA_H_

#include <vector>
#include "../Alpha.h"

class Beta
{
    public:
    std::vector <Alpha::info*> vecInfo;
};

#endif