How Should I Define an Array of Pointers to Functions in C++?

Question

I'm trying to make an array of functions so I can call functions from the array with an index. I can't seem to figure out the parentheses, asterisks and brackets to create this array of functions. Here is what I have:

void Game::getPawnMoves(int position, bool color, Move ** moveList) {
    ...
}

typedef void (*GetMoveFunction) (int, bool, Move **);
void Game::getLegalMoves(Move ** moveList) {
    GetMoveFunction functions[] = 
    {
        getPawnMoves, 
        getKnightMoves, 
        getBishopMoves,
        getRookMoves,
        getQueenMoves,
        getKingMoves
    };
    ...
}

All of the getPawnMoves, getKnightMoves, and the rest all have the same arguments and all return void. Move is just a struct with two chars and an enum. In this case, if you'd like to try compiling it, you could replace Move ** with int **.

The error I'm getting when I compile is:

Game.cpp:443:5: error: cannot convert ‘Game::getPawnMoves’ from type 
‘void (Game::)(int, bool, Move**) {aka void (Game::)(int, bool, Move_t**)}’ to 
type ‘GetMoveFunction {aka void (*)(int, bool, Move_t**)}’

So for some reason the compiler thinks that GetMoveFunction is void * instead of void, but if I change the typedef to

typedef void (GetMoveFunction) (int, bool, Move **);

I get the error:

Game.cpp:435:28: error: declaration of ‘functions’ as array of functions
  GetMoveFunction functions[] =

I'm honestly super stuck on this one. Thanks so much for your help!

Regular function pointers can't point to non-static member functions. There are member function pointers that can do that. — HolyBlackCat
– HolyBlackCat, Commented Feb 28, 2022 at 18:34
std::vector<std::function<void(int, bool, Move**)>>? Then use lambdas or std::bind or similar tools to use specific objects. — Some programmer dude
– Some programmer dude, Commented Feb 28, 2022 at 18:35
void (*)(int, bool, Move_t**) is the same type GetMoveFunction refers to. You could write (for imho improved readability): using GetMoveFunction = void (*)(int, bool, Move_t**); instead of the typedef and it would mean the exact same thing. The type has nothing to do with void*; the brackets around * change the meaning. — fabian
– fabian, Commented Feb 28, 2022 at 18:36
The first error, the code you didn't show as part of a proper minimal reproducible example would reveal that Game::getPawnMoves is a non-static member function. Therefore, the actual type of that function is void (Game::*)(int, bool, Move **), not void (*)(int, bool, Move **). The second error is just a side-show of misunderstanding the first error and stabbing at a solution. — WhozCraig
– WhozCraig, Commented Feb 28, 2022 at 18:39
Member functions, unless declared static are not normal functions because they have an hidden argument this that refers to the instanced object. — MatG
– MatG, Commented Feb 28, 2022 at 18:39

eerorika · Accepted Answer · 2022-02-28 18:42:07Z

2

How Should I Define an Array of Pointers to Functions in C++?

typedef void (*GetMoveFunction) (int, bool, Move **);

This is a pointer to function.

GetMoveFunction functions[] = 
{
    // ...
};

This is an array of pointers to functions. You've achieved what you asked for in the title of the question.

Game.cpp:443:5: error: cannot convert ‘Game::getPawnMoves’ from type 
‘void (Game::)(int, bool, Move**) {aka void (Game::)(int, bool, Move_t**)}’ to 
type ‘GetMoveFunction {aka void (*)(int, bool, Move_t**)}’

The problem here is that you are trying to initialise the function pointers using non-static member functions. Function pointers cannot point to non-static member functions.

What you probably need is an array of pointers to member functions of Game instead. Another option is an array of type erasing function wrappers (std::function).

answered Feb 28, 2022 at 18:42

eerorika

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1 Comment

ParticlesInCuda Over a year ago

Thanks so much! I'll edit my question with the updated code.

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