Generic Delegates (Visual C++)

 

The latest version of this topic can be found at Generic Delegates (Visual C++).

You can use generic type parameters with delegates. For more information on delegates, see delegate (C++ Component Extensions).

Syntax

[attributes]   
generic < [class | typename] type-parameter-identifiers >  
[type-parameter-constraints-clauses]  
[accessibility-modifiers] delegate result-type identifier   
([formal-parameters]);  

Parameters

attributes (Optional)
Additional declarative information. For more information on attributes and attribute classes, see Attributes.

type-parameter-identifier(s)
Comma-separated list of identifiers for the type parameters.

type-parameter-constraints-clauses
Takes the form specified in Constraints on Generic Type Parameters (C++/CLI)

accessibility-modifiers (Optional)
Accessibility modifiers (e.g. public, private).

result-type
The return type of the delegate.

identifier
The name of the delegate.

formal-parameters (Optional)
The parameter list of the delegate.

Example

The delegate type parameters are specified at the point where a delegate object is created. Both the delegate and method associated with it must have the same signature. The following is an example of a generic delegate declaration.

// generics_generic_delegate1.cpp  
// compile with: /clr /c  
generic < class ItemType>  
delegate ItemType GenDelegate(ItemType p1, ItemType% p2);  

Example

The following sample shows that

• You cannot use the same delegate object with different constructed types. Create different delegate objects for different types.

• A generic delegate can be associated with a generic method.

• When a generic method is called without specifying type arguments, the compiler tries to infer the type arguments for the call.

// generics_generic_delegate2.cpp  
// compile with: /clr  
generic < class ItemType>  
delegate ItemType GenDelegate(ItemType p1, ItemType% p2);  
  
generic < class ItemType>  
ref struct MyGenClass {  
   ItemType MyMethod(ItemType i, ItemType % j) {  
      return ItemType();  
   }  
};  
  
ref struct MyClass {  
   generic < class ItemType>  
   static ItemType MyStaticMethod(ItemType i, ItemType % j) {  
      return ItemType();  
   }  
};  
  
int main() {  
   MyGenClass<int> ^ myObj1 = gcnew MyGenClass<int>();  
   MyGenClass<double> ^ myObj2 = gcnew MyGenClass<double>();  
   GenDelegate<int>^ myDelegate1 =  
      gcnew GenDelegate<int>(myObj1, &MyGenClass<int>::MyMethod);  
  
   GenDelegate<double>^ myDelegate2 =   
      gcnew GenDelegate<double>(myObj2, &MyGenClass<double>::MyMethod);  
  
   GenDelegate<int>^ myDelegate =  
      gcnew GenDelegate<int>(&MyClass::MyStaticMethod<int>);  
}  

Example

The following example declares a generic delegate GenDelegate<ItemType>, and then instantiates it by associating it to the method MyMethod that uses the type parameter ItemType. Two instances of the delegate (an integer and a double) are created and invoked.

// generics_generic_delegate.cpp  
// compile with: /clr  
using namespace System;  
  
// declare generic delegate  
generic <typename ItemType>  
delegate ItemType GenDelegate (ItemType p1, ItemType% p2);  
  
// Declare a generic class:  
generic <typename ItemType>  
ref class MyGenClass {  
public:  
   ItemType MyMethod(ItemType p1, ItemType% p2) {  
      p2 = p1;  
      return p1;  
    }  
};  
  
int main() {  
   int i = 0, j = 0;   
   double m = 0.0, n = 0.0;  
  
   MyGenClass<int>^ myObj1 = gcnew MyGenClass<int>();  
   MyGenClass<double>^ myObj2 = gcnew MyGenClass<double>();   
  
   // Instantiate a delegate using int.  
   GenDelegate<int>^ MyDelegate1 =   
      gcnew GenDelegate<int>(myObj1, &MyGenClass<int>::MyMethod);  
  
   // Invoke the integer delegate using MyMethod.  
   i = MyDelegate1(123, j);  
  
   Console::WriteLine(  
      "Invoking the integer delegate: i = {0}, j = {1}", i, j);  
  
   // Instantiate a delegate using double.  
   GenDelegate<double>^ MyDelegate2 =   
      gcnew GenDelegate<double>(myObj2, &MyGenClass<double>::MyMethod);  
  
   // Invoke the integer delegate using MyMethod.  
   m = MyDelegate2(0.123, n);  
  
   Console::WriteLine(  
      "Invoking the double delegate: m = {0}, n = {1}", m, n);  
}  

Invoking the integer delegate: i = 123, j = 123  
Invoking the double delegate: m = 0.123, n = 0.123  

See Also

Generics