Template std::function as class member function C++

I would like to store a std::function in a class as a member.

I have troubles with below test code when calling a.callMethod() where the method has been set just before with a.setMethod(). The code works fine if I remove the template.

I have tried to debug with a function callMethodOutsideClass without success.

Is there a better way to manage that ?

#include <iostream>
#include <vector>
#include <functional>

template<typename T>
struct A
{
    A(size_t size, T value) : vec_(size, value), method_(nullptr) {}
    void setMethod(const std::function<int(A<T>&)> method)  { method_ = method; }
    int callMethod()
    {
        if(method_)
            return method_(*this);
        else
            std::cerr << "method not initialized" << std::endl;
        return -1;
    }

    std::vector<int> vec_;
    std::function<int(A<T>& a)> method_;
};

template<typename T>
int callMethodOutsideClass(struct A<T>& a, const std::function<int(A<T>&)> method)
{
    return method(a);
}

template<typename T>
int apple(struct A<T>& a)
{
    a.vec_[0] += 1;
    return 1;
}

template<typename T>
int orange(struct A<T>& a)
{
    a.vec_[0] += 2;
    return 2;
}

int main()
{
    A<int> a(10,4), b(10,4);
    std::cout << callMethodOutsideClass(a, &apple) << std::endl;

    a.setMethod(&orange);
    std::cout << a.callMethod() << std::endl;

    std::cout << a.vec_[0] << std::endl;
}

I currently get the following errors :

Foo6.cpp: In function ‘int main()’:
Foo6.cpp:46:47: error: cannot resolve overloaded function ‘apple’ based on conversion to type ‘std::function<int(A<int>&)>’
  std::cout << callMethodOutsideClass(a, &apple) << std::endl;
                                               ^
Foo6.cpp:48:21: error: no matching function for call to ‘A<int>::setMethod(<unresolved overloaded function type>)’
  a.setMethod(&orange);
                     ^
Foo6.cpp:48:21: note: candidate is:
Foo6.cpp:9:7: note: void A<T>::setMethod(std::function<int(A<T>&)>) [with T = int]
  void setMethod(const std::function<int(A<T>&)> method)  { method_ = method; }
       ^
Foo6.cpp:9:7: note:   no known conversion for argument 1 from ‘<unresolved overloaded function type>’ to ‘std::function<int(A<int>&)>’