delegate (C++ Component Extensions)
The latest version of this topic can be found at delegate (C++ Component Extensions).
Declares a type that represents a function pointer.
All Runtimes
Both the Windows Runtime and common language runtime support delegates.
Remarks
delegate is a context-sensitive keyword. For more information, see Context-Sensitive Keywords.
To detect at compile time if a type is a delegate, use the __is_delegate() type trait. For more information, see Compiler Support for Type Traits.
Windows Runtime
C++/CX supports delegates with the following syntax.
Syntax
access
delegate
return-type
delegate-type-identifier
(
[ parameters ]
)
Parameters
access
(optional) The accessibility of the delegate, which can be public (the default) or private. The function prototype can also be qualified with the const or volatile keywords.
return-type
The return type of the function prototype.
delegate-type-identifier
The name of the declared delegate type.
parameters
(Optional) The types and identifiers of the function prototype.
Remarks
Use the delegate-type-identifier to declare an event with the same prototype as the delegate.
Requirements
Compiler option: /ZW
Common Language Runtime
The common language runtime supports delegates with the following syntax.
Syntax
access
delegate
function_declaration
Parameters
access
(optional) The accessibility of the delegate outside of the assembly can be public or private. The default is private. Inside a class, a delegate can have any accessibility.
function_declaration
The signature of the function that can be bound to the delegate. The return type of a delegate can be any managed type. For interoperability reasons, it is recommended that the return type of a delegate be a CLS type.
To define an unbound delegate, the first parameter in function_declaration should be the type of the this pointer for the object. For more information, see Unbound Delegates.
Remarks
Delegates are multicast: the "function pointer" can be bound to one or more methods within a managed class. The delegate keyword defines a multicast delegate type with a specific method signature.
A delegate can also be bound to a method of a value class, such as a static method.
A delegate has the following characteristics:
It inherits from System::MulticastDelegate.
It has a constructor that takes two arguments: a pointer to a managed class or NULL (in the case of binding to a static method) and a fully qualified method of the specified type.
It has a method called
Invoke, whose signature matches the declared signature of the delegate.
When a delegate is invoked, its function(s) are called in the order they were attached.
The return value of a delegate is the return value from its last attached member function.
Delegates cannot be overloaded.
Delegates can be bound or unbound.
When you instantiate a bound delegate, the first argument shall be an object reference. The second argument of a delegate instantiation shall either be the address of a method of a managed class object, or a pointer to a method of a value type. The second argument of a delegate instantiation must name the method with the full class scope syntax and apply the address-of operator.
When you instantiate an unbound delegate, the first argument shall either be the address of a method of a managed class object, or a pointer to a method of a value type. The argument must name the method with the full class scope syntax and apply the address-of operator.
When creating a delegate to a static or global function, only one parameter is required: the function (optionally, the address of the function).
For more information on delegates, see
Requirements
Compiler option: /clr
Examples
Example
The following example shows how to declare, initialize, and invoke delegates.
// mcppv2_delegate.cpp
// compile with: /clr
using namespace System;
// declare a delegate
public delegate void MyDel(int i);
ref class A {
public:
void func1(int i) {
Console::WriteLine("in func1 {0}", i);
}
void func2(int i) {
Console::WriteLine("in func2 {0}", i);
}
static void func3(int i) {
Console::WriteLine("in static func3 {0}", i);
}
};
int main () {
A ^ a = gcnew A;
// declare a delegate instance
MyDel^ DelInst;
// test if delegate is initialized
if (DelInst)
DelInst(7);
// assigning to delegate
DelInst = gcnew MyDel(a, &A::func1);
// invoke delegate
if (DelInst)
DelInst(8);
// add a function
DelInst += gcnew MyDel(a, &A::func2);
DelInst(9);
// remove a function
DelInst -= gcnew MyDel(a, &A::func1);
// invoke delegate with Invoke
DelInst->Invoke(10);
// make delegate to static function
MyDel ^ StaticDelInst = gcnew MyDel(&A::func3);
StaticDelInst(11);
}
Output
in func1 8
in func1 9
in func2 9
in func2 10
in static func3 11