Delegates in managed C++

• Download demo project (beta 2) - 19 Kb

Introduction

Delegates are .NET's type safe equivalent to function pointers. Delegates go further than this though. Instead of a single delegate having the ability to point to, and invoke, a single function, delegates in .NET give you the ability to have a single delegate point to a list of methods, each which will be called in turn.

Creating a single cast delegate

A single cast delegate is one which points to a single method. To create a delegate you must first declare a delegate type that has the same signature as the methods you wish it to invoke. For instance, if we wished to have a delegate call a function that took a String* as a parameter and returned an int, we might declare it as

__delegate int MyDelegate(String *str);

Single cast delegates are implicitely derived from Delegate. To use a delegate to invoke your methods you must create an instance of the delegate and pass in an object and the method of that object you wish to call.

For instance, suppose we had a managed class

__gc class MyClass 
{
public:
	int MethodA(String *str) 
	{
		Console::WriteLine(S"MyClass::MethodA - The value of str is: {0}", str);
		return str->Length;
	}
}

we could declare an object of type MyClass and a delegate to call the objects methods like

MyClass *pMC = new MyClass();
MyDelegate *pDelegate = new MyDelegate(pMC, &MyClass::MethodA);

To invoke the object's method using the delegate is as simple as calling

pDelegate->Invoke("Invoking MethodA");

This would output:

MyClass::MethodA - The value of str is: Invoking MethodA

Creating a multi cast delegate

Multi cast delegates allow you to chain together methods so that each one will be called in turn when the delegate is invoked. To create a multicast method use the static Delegate::Combine method to combine delegates into a multicast delegate.

When creating multicast delegates in beta 1, you had to declare your delegates you wished to combine as multicast delegates using the __delegate(multicast) directive. This creates a delegate derived from MulticastDelegate. In beta 2 you do not use the (multicast) specifier, and you can use Delegate::Combine your single cast delegates into a multicast delegate.

Below is some sample code showing 2 multicast delegates being combined (beta 2)

// Declare a delegate
__delegate int MyDelegate(String *str);

// Create a simple managed reference class
__gc class MyClass 
{
public:
    int MethodA(String *str) 
    {
        Console::WriteLine(S"MyClass::MethodA - The value of str is: {0}", str);
        return str->Length;
    }

    int MethodB(String* str) 
    {
        Console::WriteLine(S"MyClass::MethodB - The value of str is: {0}", str);
        return str->Length * 2;
    }
};

...

MyClass *pMC = new MyClass();
MyDelegate *pDelegate1 = new MyDelegate(pMC, &MyClass::MethodA);
MyDelegate *pDelegate2 = new MyDelegate(pMC, &MyClass::MethodB);

MyDelegate *pMultiDelegate = 
	static_cast<MyDelegate *>(Delegate::Combine(pDelegate, pDelegate2));

When you invoke the multicast delegate pMultiDelegate it will first call MyClass::MethodA, then MyClass::MethodB. For instance, calling

pMultiDelegate->Invoke("Invoking Multicast delegate");

would result in

MyClass::MethodA - The value of str is: Invoking multicast delegate
MyClass::MethodB - The value of str is: Invoking multicast delegate

Type Safety

Delegates are inherently type safe. You cannot compile calls to a delegate using the wrong parameters, or assign the return value of a delegate to a type that cannot be implicitly cast from the return type of the method the delegate is calling. Because an object and method are passed to the delegates constructor, the compiler has all the information it needs to ensure that errors caused by mismatched parameters and return types do not occur.