How to write a template wrapper method for other class member functions?

Question

I try to create a templated wrapper for different functions with different parameters. The setup is a class A with the basic implementation of two methods foo and bar. Another class B shall wrap these methods and add new functionality.

The solution from the following link works very well for non-class functions: c++11: Templated wrapper function

But if I try to invoke methods from another class I get an error.

#include <algorithm>
#include <functional>
#include <iostream>

class A
{
public:
    void foo(int x) {
        std::cout << "Foo: " << x << std::endl;
    }

    void bar(int x, float y) {
        std::cout << "Bar: " << x << ", " << y << std::endl;
    }
};

class B
{
public:
    void fooAndMore(int x) {
        foobarWrapper(&A::foo, 1);
    }

    void barAndMore(int x, float y) {
        foobarWrapper(&A::bar, 1, 3.5f);
    }

    template<typename  T, typename... Args>
    void foobarWrapper(T&& func, Args&&... args)
    {
        std::cout << "Start!" << std::endl;
        std::forward<T>(func)(std::forward<Args>(args)...);
        std::cout << "End!" << std::endl;
    }
};

int main()
{
    B b;
    b.fooAndMore(1);
    b.barAndMore(2, 3.5f);
}

I expect something like that:

Start!
Foo: 1
End!
Start!
Bar: 1, 3.5
End!

But instead I get:

error C2064: term does not evaluate to a function taking 1 arguments
note: see reference to function template instantiation 'void B::foobarWrapper<void(__thiscall A::* )(int),int>(T &&,int &&)' being compiled
    with
    [
        T=void (__thiscall A::* )(int)
    ]

error C2064: term does not evaluate to a function taking 2 arguments
note: see reference to function template instantiation 'void B::foobarWrapper<void(__thiscall A::* )(int,float),int,float>(T &&,int &&,float &&)' being compiled
    with
    [
        T=void (__thiscall A::* )(int,float)
    ]

Any idea how to solve this?

Answer 1

Try this,

#include <algorithm>
#include <functional>
#include <iostream>

class A
{
public:
    void foo(int x) {
        std::cout << "Foo: " << x << std::endl;
    }

    void bar(int x, float y) {
        std::cout << "Bar: " << x << ", " << y << std::endl;
    }
};

class B
{
public:
    void fooAndMore(int x) {
        foobarWrapper(&A::foo, x);
    }

    void barAndMore(int x, float y) {
        foobarWrapper(&A::bar, x, y);
    }

    template<typename  T, typename... Args>
    void foobarWrapper(T func, Args&&... args)
    {
        std::cout << "Start!" << std::endl;

        auto caller = std::mem_fn( func); // Newly added lines
        caller( A(), args...);  // Newly added line

        std::cout << "End!" << std::endl;
    }
};

int main()
{
    B b;
    b.fooAndMore(1);
    b.barAndMore(2, 3.5f);
}

output:

Start!
Foo: 1
End!
Start!
Bar: 2, 3.5
End!

See this link for more details std::mem_fn

Answer 2

The simplest fix is to make the member functions of class A to be static. (See online)

class A
{
public:
    static void foo(int x) {
    ^^^^^^
        std::cout << "Foo: " << x << std::endl;
    }

    static void bar(int x, float y) {
    ^^^^^^
        std::cout << "Bar: " << x << ", " << y << std::endl;
    }
};

Otherwise, you need to pass an instance of class A to call its member functions in the foobarWrapper function. Using lambdas you can pack them to the callable func and pass to the foobarWrapper.

(See online)

class B
{
public:
    void fooAndMore(const A& a_obj, int x) {
        foobarWrapper([&]() { return a_obj.foo(x); });
        //            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^Args captured to the lambda
    }

    void barAndMore(const A& a_obj, int x, float y) {
        foobarWrapper([&]() { return a_obj.bar(x, y); });
        //            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  Args captured to the lambda
    }

    template<typename  T>
    void foobarWrapper(T&& func)   // no Args needed any more (@credits Jarod42)
    {
        std::cout << "Start!" << std::endl;
        std::forward<T>(func)();   // simply call the func
        std::cout << "End!" << std::endl;
    }
};

int main()
{
    B b;
    b.fooAndMore(A{}, 1);       // pass a temporary A object
    b.barAndMore(A{}, 2, 3.5f);
}