How to handle class function pointer

Question

I want to be able to use a member variable to hold the address a function to call

I got the error:

cannot convert 'void (TestClass::*)(bool)' to 'void (*)(bool)' in assignment

TestClass.cpp

#include "TestClass.h"

TestClass::TestClass()
{
    _myFctPtr = &setFired;
}

void TestClass::setFired(bool isFired)
{
    _fired = isFired;
}

void TestClass::updateValue()
{
    (_myFctPtr)(true);
}


bool TestClass::getFired()
{
    return _fired;
}

TestClass.h

#pragma once
class TestClass
{
    private:
        bool _fired = false;

    protected:
        void (*_myFctPtr)(bool);

    public:
        TestClass();
        void setFired(bool);
        void updateValue();
        bool getFired();
};

isocpp.org/wiki/faq/pointers-to-members

Michael Bethke
– Michael Bethke

2022-03-31 01:42:37 +00:00
Commented Mar 31, 2022 at 1:42 — Michael Bethke
– Michael Bethke, Commented Mar 31, 2022 at 1:42

Ethan McTague · Accepted Answer · 2022-03-31 01:52:22Z

2

The error:

cannot convert 'void (TestClass::*)(bool)' to 'void (*)(bool)' in assignment

Is showing you that pointers to class member functions have a different type from regular function pointers. This is because, in addition to the regular function arguments and return type, these functions are differentiated by the object they are called on (i.e. their this), which must be provided when invoking them.

Specifically, for a pointer to a function on TestClass which accepts a bool argument as shown here, you need to to define your field as the type void (TestClass::*)(bool):

void (TestClass::*_myFctPtr)(bool);

You can call this like so, assuming you want to invoke the method on this:

((this).*(_myFctPtr))(true);

This article provides additional useful information and advice on better, cleaner ways to invoke pointers to member functions - e.g. using std::invoke for cleaner syntax than what I provided above - but this is the bare minimum to quickly solve your issue.

answered Mar 31, 2022 at 1:52

Ethan McTague

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2 Comments

Charles Plante Over a year ago

Thank your for your quiock answer it helped me find a solution !

Ethan McTague Over a year ago

@CharlesPlante remember to mark a correct solution!

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