CODEWITHSUNDEEP
c++templatesfriendtemplate-specialization
MajinSaha
MajinSaha

Reputation: 198

How to make a specialized function template a friend to some class?

I'm trying to find a way to make a function that is a friend to a given class. That function is another class' method and is a specialization of a template. Without specialization, I have the following compiling code in Visual Studio:

ClassA.h:

#pragma once
#include "classB.h"

class A
{
private:
    int data;
    void Operate();
public:
    A();
    ~A();
    template<class T> friend void B::DoSomething(const T& arg);
};

ClassB.h:

#pragma once

class B
{
private:
    int data;
    template<typename T> void DoSomething(const T& arg)
    {
        T copy = arg;
        copy.Operate();
        data = 3;
    };
/*
    template<> void DoSomething(const A& arg)
    {
        A copy = arg;
        copy.Operate();
        data = 4;
    };
*/

public:
    B();
    ~B();
};

ClassA.cpp:

#include "classA.h"

A::A()
{
    data = 1;
}

A::~A()
{
}

void A::Operate()
{
    data = 2;
}

ClassB.cpp:

#include "classB.h"

B::B()
{
    data = 1;
}

B::~B()
{
}

How do I specialize the template and make it a friend instead of the entire template? If that is possible, where do I place it then? Do I need forward declarations anywhere? Which headers would I need to include, etc.?

I tried to uncomment the block in classB.h and add #include "classA.h" on top of it. I also tried to replace the line template<class T> friend void B::DoSomething(const T& arg); in classA.h with something like template<> friend void B::DoSomething(const A& arg);. Nothing helped. It refuses to compile.

I would appreciate any insight!

Upvotes: 0

Views: 73

Answers (1)

R Sahu
R Sahu

Reputation: 206697

To make B::DoSomething<int> a friend of A, use

friend void B::template DoSomething<int>(const int& arg);

To make B::DoSomething<A> a friend of A, use

friend void B::template DoSomething<A>(const A& arg);

Please note that in order to be able to do that, DoSomething has to be a public member of B.

Further reading: Where and why do I have to put the "template" and "typename" keywords?

Upvotes: 2

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