What is a C++ delegate?

What is the general idea of a delegate in C++? What are they, how are they used and what are they used for?

I'd like to first learn about them in a 'black box' way, but a bit of information on the guts of these things would be great too.

This is not C++ at its purest or cleanest, but I notice that the codebase where I work has them in abundance. I'm hoping to understand them enough, so I can just use them and not have to delve into the horrible nested template awfulness.

These two The Code Project articles explain what I mean but not particularly succinctly:

Member Function Pointers and the Fastest Possible C++ Delegates
The Impossibly Fast C++ Delegates

You have an incredible number of choices to achieve delegates in C++. Here are the ones that came to my mind.

Option 1 : functors:

A function object may be created by implementing operator()

struct Functor
{
     // Normal class/struct members

     int operator()(double d) // Arbitrary return types and parameter list
     {
          return (int) d + 1;
     }
};

// Use:
Functor f;
int i = f(3.14);

Option 2: lambda expressions (C++11 only)

// Syntax is roughly: [capture](parameter list) -> return type {block}
// Some shortcuts exist
auto func = [](int i) -> double { return 2*i/1.15; };
double d = func(1);

Option 3: function pointers

int f(double d) { ... }
typedef int (*MyFuncT) (double d);
MyFuncT fp = &f;
int a = fp(3.14);

Option 4: pointer to member functions (fastest solution)

See Fast C++ Delegate (on The Code Project).

struct DelegateList
{
     int f1(double d) { }
     int f2(double d) { }
};

typedef int (DelegateList::* DelegateType)(double d);

DelegateType d = &DelegateList::f1;
DelegateList list;
int a = (list.*d)(3.14);

Option 5: std::function

(or boost::function if your standard library doesn't support it). It is slower, but it is the most flexible.

#include <functional>
std::function<int(double)> f = [can be set to about anything in this answer]
// Usually more useful as a parameter to another functions

Option 6: binding (using std::bind)

Allows setting some parameters in advance, convenient to call a member function for instance.

struct MyClass
{
    int DoStuff(double d); // actually a DoStuff(MyClass* this, double d)
};

std::function<int(double d)> f = std::bind(&MyClass::DoStuff, this, std::placeholders::_1);
// auto f = std::bind(...); in C++11

Option 7: templates

Accept anything as long as it matches the argument list.

template <class FunctionT>
int DoSomething(FunctionT func)
{
    return func(3.14);
}

A delegate is a class that wraps a pointer or reference to an object instance, a member method of that object's class to be called on that object instance, and provides a method to trigger that call.

Here's an example:

template <class T>
class CCallback
{
public:
    typedef void (T::*fn)( int anArg );

    CCallback(T& trg, fn op)
        : m_rTarget(trg)
        , m_Operation(op)
    {
    }

    void Execute( int in )
    {
        (m_rTarget.*m_Operation)( in );
    }

private:

    CCallback();
    CCallback( const CCallback& );

    T& m_rTarget;
    fn m_Operation;

};

class A
{
public:
    virtual void Fn( int i )
    {
    }
};


int main( int /*argc*/, char * /*argv*/ )
{
    A a;
    CCallback<A> cbk( a, &A::Fn );
    cbk.Execute( 3 );
}

What is a C++ delegate?

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