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

Question

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!

score 2 · Answer 1 · answered Oct 01 '19 at 21:48

2

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?

answered Oct 01 '19 at 21:48

R Sahu

204,454
14
159
270

I added the line `friend void B::template DoSomething(const A& arg);` in classA.h instead of the previous one and uncommented the block in classB.h. I also included "classA.h" on top of classB.h. The compiler barks that in `copy.Operate()` the method is inaccessible! This is regardless of whether I make both of `DoSomething(...)` private or public. Note this compiler error simply gets into conflict with the whole idea of using the friend! – MajinSaha Oct 01 '19 at 21:59
*I also included "classA.h" on top of classB.h* That is going to be a problem -- it make the two .h files circularly dependent on each other. – R Sahu Oct 01 '19 at 22:06
I can't avoid including "classA.h", since otherwise there is no way of knowing what type "A" is inside of B when doing template specialization. – MajinSaha Oct 01 '19 at 22:08
@MajinSaha, of course there is. When the specialization is used in `A`, it will be known. – R Sahu Oct 01 '19 at 22:09
@MajinSaha, just an FYI. I was able to compile and build an executable using five files. I can build it at [ideone.com](https://ideone.com/ciyhe6) using one file too. – R Sahu Oct 01 '19 at 22:12
Well, `identifier "A" is undefined` is what I see if I don't include it! Can you please post all files altogether to make the explanation clear? – MajinSaha Oct 01 '19 at 22:13
@MajinSaha, please see the code at [ideone.com](https://ideone.com/ciyhe6). Hopefully that will help you clear your doubts. – R Sahu Oct 01 '19 at 22:24
I checked the code, but I don't see how that answers my question. I don't see my template specialization that assigns `data = 4`. Please pay attention to commented block in classB.h that I keep emphasizing. It's nowhere to be found in your case. Were you confusing "specialization" with "instantiation" by chance? – MajinSaha Oct 01 '19 at 22:35

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

1 Answers1