Question

Is store function pointers with different parameters in a vector of void pointers.

unordered_map<string, vector<void*> > List;

template <typename T>
void Listen(string Name, function<void(T)> Function)
{
    List[Name].push_back(&Function);
}

Then I want to call them, assuming that T is the same type for Fire as used for the Listen.

template <typename T>
void Fire(string Name, T Data)
{
    auto Functions = List[Name];

    for (auto i = Functions.begin(); i != Functions.end(); ++i)
    {
        (function<void(T)>)i)(Data);
    }
}

But I get a compiler error which reads error C2064: term does not evaluate to a function taking 1 arguments in file ...\vc\include\xrefwrap 431 1.

What am I doing wrong?

Answer 1

For one, you’re taking the address of a parameter, here:

List[Name].push_back(&Function);

Then you’re trying to convert an iterator object to a std::function object here:

(function<void(T)>)i)

What you trying to do can be done, like this, although it’s not pretty, to put it mildly:

unordered_map<string, vector<void*> > List;

template <typename T>
void Listen(string Name, function<void(T)> &Function)
{
    List[Name].push_back(&Function);
}

template <typename T>
void Fire(string Name, T Data)
{
    auto Functions = List[Name];

    for (auto i = Functions.begin(); i != Functions.end(); ++i)
    {
      function<void(T)> *ptr = *i;

      (*ptr) (Data);
    }
}

It can break in lot of ways, for example you have no control that the function, registered under some name in Listen is called with the correct argument in Fire – consider calling Listen<int> ("foo", f); and then doing Fire<double> ("foo", 3.14);

Another approach – just pass closures for callbacks:

unordered_map<string, std::vector<function<void()> > > List;

void Listen(string Name, function<void()> Function)
{
    List[Name].push_back(Function);
}

void Fire(string Name)
{
    auto Functions = List[Name];

    for (auto i = Functions.begin(); i != Functions.end(); ++i)
      (*i) ();
}