[C++] Function pointer with extra data

Question

I have class which handles packages:

typedef void (*FCPackageHandlerFunction)(FCPackage*);
class FCPackageHandlers{
    ...
    void registerHandler(FCPackage::Type type, FCPackageHandlerFunction handler);
    void handle(FCPackage* package);
    ...
    QHash<FCPackage::Type, FCPackageHandlerFunction> _handlers;
};

Then I have a server class who receive packages. Now I want to register a function who handles the packages. But this function must have a instance of the server for other variables.

So i try this:

struct FCLoginHandler{
    FCServer* server;

    FCLoginHandler(FCServer* server){
        this->server = server;
    }

    void operator()(FCPackage* package){
        std::cout << "Received package: " << package->toString().data() << "\n";
    }
};

...

FCServer::FCServer(){
    _handlers.registerHandle(FCPackage::Login, FCLoginHandler(this));
}

But then I get this error:

error: no matching function for call to 'FCPackageHandlers::registerHandler(FCPackage::Type, FCLoginHandler)'
note: candidates are: void FCPackageHandlers::registerHandler(FCPackage::Type, void (*)(FCPackage*))

Does anybody know the right solution?

Answer 1

You are trying to store a function object in a function pointer, and that's not possible. You should store a std::tr1::function instead:

#include <functional>

typedef std::tr1::function<void(FCPackage*)> FCPackageHandlerFunction;
class FCPackageHandlers{
    ...
    void registerHandler(FCPackage::Type type, FCPackageHandlerFunction handler);
    void handle(FCPackage* package);
    ...
    QHash<FCPackage::Type, FCPackageHandlerFunction> _handlers;
};

There's a similar function class in Boost in case you don't have access to std::tr1 yet.

Also, consider using boost::bind and a regular function to avoid the boilerplate of having to create your own function objects such as FCLoginHandler:

void handle_FC_login(FCServer* server, FCPackage* package)
{
    std::cout << "Received package: " << package->toString().data() << "\n";
    // You can use server if you need it
}


FCServer::FCServer()
{
    _handlers.registerHandle(FCPackage::Login, 
                             std::tr1::bind(&handle_FC_login, this, _1)); 
}

std::tr1::bind is also available in Boost, and if you don't have access to that either you can always use std::bind2nd.

EDIT: Since you can't modify the type of FCPackageHandlerFunction, your best shot might be to add another hash that stores data associated to each function pointer:

typedef void (*FCPackageHandlerFunction)(FCPackage*);
class FCPackageHandlers{
    ...
    void registerHandler(FCPackage::Type type, FCPackageHandlerFunction handler,
                         FCServer * func_data);
    void handle(FCPackage* package);
    ...
    QHash<FCPackage::Type, FCPackageHandlerFunction> _handlers;
    QHash<FCPackageHandlerFunction, FCServer*> _handler_func_data;
};

// The server will be passed by the package handler which will 
// extract it from the _handler_func_data hash
void handle_FC_login(FCServer* server, FCPackage* package)
{
    std::cout << "Received package: " << package->toString().data() << "\n";
}

FCServer::FCServer(){
    _handlers.registerHandle(FCPackage::Login, &handle_FC_login, this );
}

Presumably this is how you'll have to implement FCPackageHandlers::handle:

void FCPackageHandlers::handle(FCPackage * package)
{
    // Query function
    FCPackageHandlerFunction func = _handlers[package->GetType()];

    // Query associated data
    FCServer * server = _handler_func_data[func];

    // Call handler
    func(server, package);
}