获取错误：C3867 &#39;Template_class<int> ::add&#39;: 非标准语法； 使用带有模板的函数指针时

Question

i am trying to understand the concepts of c++ as i am not very advanced in it. Although i have a descent knowledge , i have absolutely no idea about whats going on here!!!

I am making a program in which I simply have a template_class with a pointer to function which returns the value of function add. I am using Visual c++ and for some reason it returns the following error in main.cpp :-

Error C3867 'Template_class::add': non-standard syntax; use '&' to create a pointer to member`

main.cpp

#include<iostream>
#include<string>
#include"Template_class.h"

using namespace std;

int main()
{
    Template_class <int> t;

    t.retrunOperation(4, 5, t.add);
    cin.get();
}

Template_class.h

#include<iostream>

template<class T>
class Template_class
{
public:
    Template_class()
    {

    }
    ~Template_class()
    {

    }

    T add(T var1 , T var2)
    {
        return var1 + var2;
    }

    void  retrunOperation(T var1 , T var2 , T (*function)(T , T))
    {
        std::cout << (*function)(var1, var2);
    }
};

This may be a invalid question(about which i have no idea) , but as i said i am not an advanced programmer , so please suggest me some solutions

Answer 1

void retrunOperation(T var1 , T var2 , T (*function)(T , T))

The function argument expects a function pointer.

A (regular) function pointer cannot point to a non-static member function. Template_class::add is a non-static member function. Therefore it cannot be pointed by a function pointer.

Most trivial solution is to declare Template_class::add to be a static member function, using the static keyword. Static member functions can be pointed by a function pointer.

---

A name of a non member function will implicitly decay into a pointer to the function, without using the addressof operator ( & ). Non static member functions can be pointed by member function pointers, which are not convertible to (regular) function pointers. However, name of a non static member function does not implicitly decay into a member function pointer. That is what the error message is telling you. If you fixed that, by adding the addressof operator, then you would have been given another error about lack of fitting overload.

Answer 2

The compiler error is telling you that the right way to pass a member function is

t.retrunOperation(4, 5, &Template_class<int>::add);

However, when you fix that, you'll find that retrunOperation() wants a pointer to function rather than a pointer to member function . You'll then need to make add static:

static T add(T var1 , T var2)
{
    return var1 + var2;
}

or (less likely) declare retrunOperation() as taking a pointer to member function:

void  retrunOperation(T var1 , T var2 , T (Template_class<int>::*function)(T , T))

and implement it appropriately.

Answer 3

Non-static member functions have the signature:

Ret Class::name(Type, Type2...);

A pointer to a member function then would look like:

Ret Class::* name(Type, Type...);

So when you say:

t.retrunOperation(4, 5, t.add);

you are trying to give

int(*function)(int, int)

where

int(Template_class<int>::*function)(int , int)

is expected.

Applying the above, the solution to your specific problem would look like so:

template<class T>
class Template_class
{
public:
    T add(T var1 , T var2) { return var1 + var2; }

    void  retrunOperation(T var1 , T var2 , T(Template_class<T>::*function)(T , T)) {}
};

and you can call it like this:

t.retrunOperation(4, 5, &Template_class<int>::add);