Calling C++ member function pointer from a struct

Question

I have found information on calling C++ member function pointers and calling pointers in structs, but I need to call a member function pointer that exists inside of a structure, and I have not been able to get the syntax correct. I have the following snippet inside a method in class MyClass:

void MyClass::run() {
    struct {
        int (MyClass::*command)(int a, int b);
        int id;
    } functionMap[] = {
        {&MyClass::commandRead,  1},
        {&MyClass::commandWrite, 2},
    };

    (functionMap[0].MyClass::*command)(x, y);
}

int MyClass::commandRead(int a, int b) {
    ...
}

int MyClass::commandWrite(int a, int b) {
    ...
}

This gives me:

error: expected unqualified-id before '*' token
error: 'command' was not declared in this scope
(referring to the line '(functionMap[0].MyClass::*command)(x, y);')

Moving those parenthesis around results in syntax errors recommending using .* or ->* neither of which work in this situation. Does anyone know the proper syntax?

http://stackoverflow.com/questions/990625/c-function-pointer-class-member-to-non-static-member-function seems related to this question. — Rudi, Oct 15 '11 at 06:47

score 9 · Accepted Answer · edited Oct 15 '11 at 07:04

9

Use:

(this->*functionMap[0].command)(x, y);

Tested and compiles ;)

edited Oct 15 '11 at 07:04

Mankarse

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answered Oct 15 '11 at 06:51

VoidStar

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Ah perfect! Thanks for the response, and the above answer provides the reasoning. – aaron Oct 15 '11 at 07:14

Dominik Grabiec · Answer 2 · 2011-10-15T06:54:19.880

5

I haven't compiled any code, but just from looking at it I can see you're missing a few things.

Remove the MyClass:: from where you call the function pointer.
Need to pass the this pointer to the functions (if they use any instance data), so that means you need an instance of MyClass to call it.

(After a bit of research) It looks like you need to do something like this (also thanks to @VoidStar):

(this->*(functionMap[0].command)(x, y));

edited Oct 15 '11 at 06:54

answered Oct 15 '11 at 06:49

Dominik Grabiec

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Thanks for the explanation. The below answer works (with the parenthesis including 'this' instead of 'functionMap'. Thank you for the response. – aaron Oct 15 '11 at 07:15
I wasn't sure exactly if it required those or not. Though for that problem I would probably try a different solution, such as just using an if, or if more flexibility is required some sort of Command pattern. – Dominik Grabiec Oct 15 '11 at 07:18

Calling C++ member function pointer from a struct

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