The Special Friendship of Templates

29 September 2021

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A friend has unrestricted access to the members of a class. Consequently, friendship should be given wisely. Regarding templates, friendship is special.

templates

Before I write about the friendship rules for templates, I want to present the general rules about friendship.

The friend declaration can be made in any place in the class.
For friendship, the access rights in the class are not considered.
Friendship is not inherited. When a class grants friendship to a class Derived, a from Derived derived class is not automatically a friend to Base.
Friendship is not transitive. When class B is a friend of class A and Class C is a friend of class B, class C is not automatically a friend of class A.

A class or a class template can have friendship to class or class templates, function or function templates, or types.

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General Friendship

A class or a class template can grant friendship to each instance of a class template or function template.

// generalFriendship.cpp

#include <iostream>

template <typename T>                            // (1)
void myFriendFunction(T);

template <typename U>                            // (2)
class MyFriend;

class GrantingFriendshipAsClass {

  template <typename U> friend void myFriendFunction(U);
  template <typename U> friend class MyFriend;

  std::string secret{"Secret from GrantingFriendshipAsClass."};

};

template <typename T>
class GrantingFriendshipAsClassTemplate{

  template <typename U> friend void myFriendFunction(U);
  template <typename U> friend class MyFriend;

  std::string secret{"Secret from GrantingFriendshipAsClassTemplate."};

};

template <typename T>                                // (3)
void myFriendFunction(T){
  GrantingFriendshipAsClass myFriend;
  std::cout << myFriend.secret << '\n';

  GrantingFriendshipAsClassTemplate<double> myFriend1;
  std::cout << myFriend1.secret << '\n';
}

template <typename T>                              // (4)
class MyFriend{
public:
  MyFriend(){
    GrantingFriendshipAsClass myFriend;
    std::cout << myFriend.secret << '\n';

    GrantingFriendshipAsClassTemplate<T> myFriend1;
    std::cout << myFriend1.secret << '\n';
  }
};

int main(){

  std::cout << '\n';

  int a{2011};
  myFriendFunction(a);

  MyFriend<double> myFriend;

  std::cout << '\n';

}

Line (1) and line (2) forward declare the function template myFriendFunction and the class template MyFriend. The function template myFriendFunction is defined in line (3) and the class template MyFriend in line (4). The classes GrantingFriendshipAsClass and GrantingFriendshipAsClassTemplate grant the function template myFriendFunction and the class template MyFriend friendship. Due to the friendship, both templates can directly invoke the private member secrete of the class and the class template.

generalFriendship

There is a pitfall involved in the class template GrantingFriendShipAsClassTemplate. Usually, you call the first type parameter of a template T. When you use - such as in the following code snippet - the same type parameter name for the class template and the function template myFriendFunction or the class template MyFriend, an error occurs. The name T of myFriendFunction or MyFriend shadows the name T of the class template GrantingFriendshipAsClassTemplate.

The following code snippet displays the pitfall.

template <typename T>
class GrantingFriendshipAsClassTemplate{

  template <typename T> friend void myFriendFunction(T);
  template <typename T> friend class MyFriend;

  std::string secret{"Secret from GrantingFriendshipAsClassTemplate."};

};

Special Friendship

A special friendship is a friendship that depends on the type of template parameter.

// specialFriendship.cpp

#include <iostream>

template <typename T> void myFriendFunction(T);
template <typename U> class MyFriend;


class GrantingFriendshipAsClass {

  friend void myFriendFunction<>(int);             // (1)
  friend class MyFriend<int>;                      // (2)

private:
  std::string secret{"Secret from GrantingFriendshipAsClass."};

};

template <typename T>
class GrantingFriendshipAsClassTemplate {

  friend void myFriendFunction<>(int);
  friend class MyFriend<int>;
  friend class MyFriend<T>;                        // (3)

private:
  std::string secret{"Secret from GrantingFriendshipAsClassTemplate."};

};

template <typename T>
void myFriendFunction(T) {
  GrantingFriendshipAsClass myFriend;
  std::cout << myFriend.secret << '\n';             // (4)

  GrantingFriendshipAsClassTemplate<T> myFriend1;
  std::cout << myFriend1.secret << '\n';            // (5)
}

template <typename T>                               // (6)
class MyFriend {                      
public:
  MyFriend() {
    GrantingFriendshipAsClass myFriend;                 
    std::cout << myFriend.secret << '\n';

    GrantingFriendshipAsClassTemplate<int> myFriendInt;  
    std::cout << myFriendInt.secret << '\n';

    GrantingFriendshipAsClassTemplate<T> myFriendT;      
    std::cout << myFriendT.secret << '\n';
  }
};

int main() {

  std::cout << '\n';

  int a{2011};
  myFriendFunction(a);                                    

  MyFriend<int> myFriend;                                 

  std::cout << '\n';

}

The class GrantingFriendshipAsClass grants friendship to the full specialization of the function template myFriendFunction for int (line 1) and the class template MyFriend for int (line 2). The same holds for the class template GrantingFrandshipAsClassTemplate. Lines (3) is special because it grants friendship to the full specialization for MyFriend having the same type parameter as the class template GrantingFrandshipAsClassTemplate. Consequently, the function template myFriendFunction can invoke the secret of the class GrantingFriendshipAsClass when myFriendFunctions is a full specialization for int (line 4) or GrantingFriendshipAsClassTemplate has the same type such as myFriendFunction (line 5). The corresponding argumentation holds for the class template MyFriend (line 6).

specialFriendship

Friend to Types

A class template can also grant its friendship to a type parameter.

// typeFriendship.cpp

#include <iostream>

template <typename T>
class Bank {
    std::string secret{"Secret from the bank."};
    friend T;
};

class Account{
 public:
    Account() {
        Bank<Account> bank;
        std::cout << bank.secret << '\n';   // (1)
    }
};

int main(){

    std::cout << '\n';

    Account acc;

    std::cout << '\n';

}

The class Bank grants friendship to its type parameter T. Consequently, an Account can access the secret of the bank instantiation for Account: Bank<Account> (line 1).

typeFriendship

What's next?

In my next post, I write about one of the more complicated corners of templates: dependent names.

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