Vector of std::function<>

Question

Say want to store the following:

typedef std::function<void(int)> MyFunctionDecl;

..in a collection:

typedef std::vector<MyFunctionDecl> FunctionVector;
FunctionVector v;

This is possible, but if I want to find something using std::find:

FunctionVector::const_iterator cit = std::find(v.begin(), v.end(), myFunctionDecl);

.. we get an error due to the == operator.

As has been suggested to me in a previous question concerning this, this can be gotten around by encapsulating the function declaration within another class, which provides a == operator:

class Wrapper
{
private:
    MyFunctionDecl m_Func;

public:
    // ctor omitted for brevity

    bool operator == (const Wrapper& _rhs)
    {
         // are they equal?
    }; // eo ==
};  // eo class Wrapper

So what I want to do is somehow generate a hash for "MyFunctionDecl" so that I can properly implement the == operator. I could have some kind of unique identifier, and ask the caller to provide a unique identifier for the delegate, but this seems a bit of a pain and is error prone.

Is there a way that I can do this? So that the same functions will return the same ID for comparative purposes? So far, the only way around it is to dump the notion of using std::function and go back to using fast delegates which support comparisons. But then I lose the ability to use lambdas.

Any help appreciated!

EDIT

Given the answer below, this is what I have come up with... any caveats I might have missed? I'm in the process of putting it through it's paces now:

class MORSE_API Event : boost::noncopyable
{
public:
    typedef std::function<void(const EventArgs&)> DelegateType;
    typedef boost::shared_ptr<DelegateType> DelegateDecl;

private:
    typedef std::set<DelegateDecl> DelegateSet;
    typedef DelegateSet::const_iterator DelegateSet_cit;
    DelegateSet m_Delegates;

public:
    Event()
    {
    };  // eo ctor


    Event(Event&& _rhs) : m_Delegates(std::move(_rhs.m_Delegates))
    {
    };  // eo mtor

    ~Event()
    {
    };  // eo dtor

    // methods
    void invoke(const EventArgs& _args)
    {
        std::for_each(m_Delegates.begin(),
                      m_Delegates.end(),
                      [&_args](const DelegateDecl& _decl) { (*_decl)(_args); });
    };  // eo invoke

    DelegateDecl addListener(DelegateType f)
    {
        DelegateDecl ret(new DelegateType(f));
        m_Delegates.insert(ret);
        return ret;
    };  // eo addListener

    void removeListener(const DelegateDecl _decl)
    {
        DelegateSet_cit cit(m_Delegates.find(_decl));
        if(cit != m_Delegates.end())
            m_Delegates.erase(cit);
    };  // eo removeListener

};  // eo class Event

Answer 1

Have you looked at Boost Signals? It may already be doing what you want to do.

Anyway, a simple way of wrapping the function would be to use a shared_ptr. If you do

typedef std::shared_ptr<std::function<void(int)> > MyFunctionDecl;

and make sure that the function is wrapped immediately inside the shared_ptr when you create it (so that the pointer is unique), pointers can be tested for equality so std::find would work.

For example you can do so with a factory function like

template <class Functor>
MyFunctionDecl createDelegate(Functor f) {
    return MyFunctionDecl(new std::function<void(int)>(f));
}

This way you give an unique identity to the function (its pointer) when you create the delegate.

BTW, I'd use an std::set instead of an std::vector, as both find and erase are logarithmic rather than linear.