Question

I’m working on a Visual C++ 2010 Express console application.

Before I go into detail, the summary here is: How can I make an array/list/vector of functions and call them from that array?

So I’m having a little difficulty with function pointers. I’m writing a ‘Terminal’ class, which in turn has a member class ‘CommandMap’. The purpose of the CommandMap class is to store a vector/array of functions and the strings that represent them in another vector/array. I want the functions to be called (only) when the class calls them from the vector, but it executed only when I added it to the vector and not when trying to call it.

I tried defining a type for it:

typedef void (*CmdCallback)();

I declared a vector to contain them:

vector<string> CmdNames;
vector<CmdCallback> CmdFuncs;

I add them like so:

// Map a new command
bool CommandMap::Map(string name, CmdCallback func)
{
    if (!IsNullOrSpace(name) && func != NULL)
    {
        if (!Exists(name))
        {
            CmdNames.push_back(name);
            CmdFuncs.push_back(func);
            return true;
        }
    }

    return false;
}

And I try calling them like this:

// Get a command callback from its identifier
CmdCallback CommandMap::GetFunc(string name)
{
    int index = IndexOf(name);
    if (index == -1) return NULL;
    else return CmdFuncs.at(index);
}

// If the given string is a command indentifier
// it will invoke the associated callback.
bool CommandMap::Exec(string input)
{
    for each (string id in CmdStrings)
    {
        if (input == id)
        {
            CmdCallback cmd;
            cmd = GetFunc(id);
            cmd();
            return true;
        }
    }

    return false;
}

I tried using this:

CmdCallback SayHello()
{
    cout << "Hello World!" << endl;
    return NULL; // Forces me to return null, guessing since it's
                 // not 'void' but a 'void' pointer it must return something
}

int main(int argc, char *argv[])
{
    App = new Terminal(argc, argv);
    App->Commands->Map("say", SayHello);

    while (!App->ExecComplete)
    {
        App->WaitEnter();
        App->Commands->Exec("say");
        App->WaitEnter();
        App->ExecComplete = true;
    }

    return App->ExitCode;   
}

This works, at first. The function gets called when I try to Map() it though. And when I Exec() “say”, it finds the callback, but when it tries to call it, I get this runtime error, to which I can see no detail other than the option to break or continue. The code it gives me is.

I pretty much want to abandon my method and try a new approach, maybe I’m going the wrong way with the void pointer typedef, and I need to throw a ‘&’ or a ‘*’ somewhere I haven’t like in the Map() argument list. Maybe a vector isn’t the best way to do this either.

Basically, I am asking how I can I make an array of functions that can (and only) be called by referencing them from the array. I’m terrible with callbacks.

Answer 1

If I understand correctly you actually want to declare SayHello as void SayHello() so that a pointer to SayHello has the type void (*)() (i.e. CmdCallback) which is what you need for your vector of functions.