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?

stackoverflow.com/questions/990625/… seems related to this question. — Rudi
– Rudi, Commented Oct 15, 2011 at 6:47

Mankarse · Accepted Answer · 2011-10-15 07:04:31Z

9

Use:

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

Tested and compiles ;)

edited Oct 15, 2011 at 7:04

Mankarse

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answered Oct 15, 2011 at 6:51

VoidStar

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1 Comment

aaron Over a year ago

Ah perfect! Thanks for the response, and the above answer provides the reasoning.

Dominik Grabiec · Accepted Answer · 2011-10-15 06:54:19Z

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, 2011 at 6:54

answered Oct 15, 2011 at 6:49

Dominik Grabiec

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2 Comments

aaron Over a year ago

Thanks for the explanation. The below answer works (with the parenthesis including 'this' instead of 'functionMap'. Thank you for the response.

Dominik Grabiec Over a year ago

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.

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