CODEWITHSUNDEEP
c++functionpointersmember
TNTisCOOL
TNTisCOOL

Reputation: 125

Function Pointer to member function of template class? (C++)

I've been butting my head against this problem in an assignment I've been working on, and can't seem to get it to work at all. I wrote a little test class to demonstrate what I'm trying to do, and hopefully someone can explain what I need to do.

//Tester class
#include <iostream>
using namespace std;

template <typename T>
class Tester
{
    typedef void (Tester<T>::*FcnPtr)(T);

private:
    T data;
    void displayThrice(T);
    void doFcn( FcnPtr fcn );

public:
    Tester( T item = 3 );
    void function();
};

template <typename T>
inline Tester<T>::Tester( T item )
    : data(item)
{}

template <typename T>
inline void Tester<T>::doFcn( FcnPtr fcn )
{
    //fcn should be a pointer to displayThrice, which is then called with the class data
    fcn( this->data );
}

template <typename T>
inline void Tester<T>::function() 
{
    //call doFcn with a function pointer to displayThrice()
    this->doFcn( &Tester<T>::displayThrice );
}

template <typename T>
inline void Tester<T>::displayThrice(T item)
{
    cout << item << endl;
    cout << item << endl;
    cout << item << endl;
}

-and here's main:

#include <iostream>
#include "Tester.h"
using namespace std;

int main()
{
    Tester<int> test;
    test.function();

    cin.get();
    return 0;
}

-and lastly, my compiler errors (VS2010)

    c:\users\name\documents\visual studio 2010\projects\example\example\tester.h(28): error C2064: term does not evaluate to a function taking 1 arguments
1>          c:\users\name\documents\visual studio 2010\projects\example\example\tester.h(26) : while compiling class template member function 'void Tester<T>::doFcn(void (__thiscall Tester<T>::* )(T))'
1>          with
1>          [
1>              T=int
1>          ]
1>          c:\users\name\documents\visual studio 2010\projects\example\example\tester.h(21) : while compiling class template member function 'Tester<T>::Tester(T)'
1>          with
1>          [
1>              T=int
1>          ]
1>          c:\users\name\documents\visual studio 2010\projects\example\example\example.cpp(7) : see reference to class template instantiation 'Tester<T>' being compiled
1>          with
1>          [
1>              T=int
1>          ]

Hopefully, my comments in the Tester class will tell you what I'm trying to do. Thank you for taking the time to look at this!

Upvotes: 6

Views: 8512

Answers (3)

Adam Maras
Adam Maras

Reputation: 26873

You're not calling the member function pointer corrently; it requires the use of a special operator called the pointer-to-member operator.

template <typename T>
inline void Tester<T>::doFcn( FcnPtr fcn )
{
    (this->*fcn)( this->data );
    //   ^^^
}

Upvotes: 11

justin
justin

Reputation: 104708

You need to explicitly add the object you message:

(*this.*fcn)(this->data); // << '*this' in this case

see also the C++ FAQ

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 477494

To call a member function via a pointer-to-member-function plus instance pointer, you need the ->* syntax, minding operator precedence:

(this->*fcn)(data);

Upvotes: 2

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