Member Function Pointer Giving multiple errors

Question

I have a member function pointer inside of my MethodPtr class. That's public and is declared like this:

void (Method::*func)() = nullptr;

It's giving me multiple errors and I'm unsure why. These errors are

'func': unknown override specifier

'Method': is not a class or namespace name

'methodptr': unkown override specifier

missing type specifier

syntax error: ")"

syntax error: missing ')' before identifier 'func'

unexpected token(s) preceding ';'

These errors are shown in the error list and show duplicates. Here's my code:

Method.h

#pragma once
#include "Methodptr.h"
#include <iostream>

class Method {
public:
    Method();
    void doSomething();
    MethodPtr methodptr;
};

Method.cpp

#include "Method.h"

Method::Method() { 
        //methodptr.func = &Method::doSomething;
}

void Method::doSomething() {
    std::cout << "Did something!" << std::endl;
}

The program crashes with the constructor expression commented out.

MethodPtr.h

#pragma once
#include "Method.h"

class MethodPtr {
public:
    void (Method::* func)() = nullptr;
};

Main.cpp

#include <iostream>
#include <fstream>
#include "Method.h"

int main() {
    Method method;
    //method.doSomething();
}

At first, I didn't know that member function pointers were a thing. So, I fixed that, and created a separate project to test it. I've continued to get this error since.

I don't want to use lambdas or std::function, unless there is no other way.

Currently, I'm just trying to figure out what I'm doing wrong here. I did get std::function to work when I used std::bind() and std::placeholder along with it.

score 1 · Accepted Answer · answered Dec 18 '22 at 21:14

You need to uncomment the line in the Method constructor or use the member initializer list to initialize methodptr:

Method::Method() : methodptr{&Method::doSomething} {}

and you also need to actually use the member function pointer.

It could look like this:

#include <iostream>

class Method;           // forward declaration

class MethodPtr {
public:
    void (Method::*func)() = nullptr;
};

class Method {
public:
    Method();
    void doSomething();
    void call();             // will use methodptr
    MethodPtr methodptr;
};

Method::Method() : methodptr{&Method::doSomething} {}

void Method::call() {
    // dereferencing the function pointer to call it on `this`:
    (this->*methodptr.func)();
}

void Method::doSomething() {
    std::cout << "Did something!" << std::endl;
}

int main() {
    Method method;
    method.call();
}

Demo

Hey that worked, appreciate it! I'll have to look into dereferencing function pointers a bit more. Specifically dereferencing outside the actual class. Thanks for taking the time out of your day to help me! I was stuck on this for quite a while. I'd upvote you if I could don't have enough rep. — cameronac, Dec 18 '22 at 21:31
@cameronac You're welcome! You can call it form the outside [like this](https://godbolt.org/z/YcMYn87W4). Note that I also added a instance pointer to the `MethodPtr` class and made it into a callable, which simplifies the uses. — Ted Lyngmo, Dec 18 '22 at 21:33

Member Function Pointer Giving multiple errors

1 Answers1