Templated friend class in nontemplate class, where friend also uses the class

Question

Background

I have a non templated class, called library and a templated class library_file, which is intended to have a template parameter F for different file types (like std::fstream or QFile and so on) to save/load a library on.

Assumption

So I put a forward declaration of library_file before my definition of library and a friend declaration in the last one. Because, since library_file includes library I would otherwise be in a dependency circle.

Problem

The friend declaration fails with

In file included from /Users/markus/Entwicklung/cmake_Spielwiese/library.cpp:4:
/Users/markus/Entwicklung/cmake_Spielwiese/library.h:12:12: error: C++ requires a type specifier for all declarations
    friend library_file;
    ~~~~~~ ^
/Users/markus/Entwicklung/cmake_Spielwiese/library.h:12:12: error: friends can only be classes or functions
2 errors generated.

Code

/*! @file library.h
 * */

#ifndef CMAKE_CPP_SPIELWIESE_LIBRARY_H
#define CMAKE_CPP_SPIELWIESE_LIBRARY_H

template <typename F>
class library_file;

class library {
    template <typename F>
    friend library_file;

};


#endif //CMAKE_CPP_SPIELWIESE_LIBRARY_H

/*! @file library_file.h
 * */

#ifndef CMAKE_CPP_SPIELWIESE_LIBRARY_FILE_H
#define CMAKE_CPP_SPIELWIESE_LIBRARY_FILE_H

#include "library.h"

template <typename F>
class library_file {
    F file;
    library l;
};


#endif //CMAKE_CPP_SPIELWIESE_LIBRARY_FILE_H

/*! @file main.cpp
 * */
#include <fstream>
#include "library.h"
#include "library_file.h"

int main() {
    library_file<std::fstream> f;
    return 0;
}
/*! @file library.cpp
 * */

#include "library.h"

score 4 · Accepted Answer · edited May 23 '17 at 11:44

4

The correct declaration of a template friend class is :

template<class F> friend class library_file;

see Class template with template class friend, what's really going on here?

edited May 23 '17 at 11:44

Community

1
1

answered Jun 27 '16 at 10:10

Daniel

1,172
14
31

1

Well that's embarrassing that I just forgot the class keyword. Thanks :-) – Superlokkus Jun 27 '16 at 10:12
1

Having someone giving a fresh look to our code is sometimes all we need ! – Daniel Jun 27 '16 at 10:15

score 2 · Answer 2 · answered Jun 27 '16 at 10:12

2

You forgot the class keyword in library_file.h, it compiles just fine after that

#ifndef CMAKE_CPP_SPIELWIESE_LIBRARY_H
#define CMAKE_CPP_SPIELWIESE_LIBRARY_H

template <typename F>
class library_file;

class library {
    template <typename F>
    friend class library_file;

};


#endif //CMAKE_CPP_SPIELWIESE_LIBRARY_H

answered Jun 27 '16 at 10:12

buld0zzr

962
5
10

Thanks too, but Daniel was faster :-) – Superlokkus Jun 27 '16 at 10:24

Templated friend class in nontemplate class, where friend also uses the class

Background

Assumption

Problem

Code

2 Answers2