abstract (C++ Component Extensions)

The latest version of this topic can be found at abstract (C++ Component Extensions).

The abstract keyword declares either:

• A type can be used as a base type, but the type itself cannot be instantiated.

• A type member function can be defined only in a derived type.

All Platforms

Syntax

  
      class-declaration  
      class-identifier  
      abstract {}  
virtualreturn-typemember-function-identifier() abstract ;  
  

Remarks

The first example syntax declares a class to be abstract. The class-declaration component can be either a native C++ declaration (class or struct), or a C++ extension declaration (ref class or ref struct) if the /ZW or /clr compiler option is specified.

The second example syntax declares a virtual member function to be abstract. Declaring a function abstract is the same as declaring it a pure virtual function. Declaring a member function abstract also causes the enclosing class to be declared abstract.

The abstract keyword is supported in native and platform-specific code; that is, it can be compiled with or without the /ZW or /clr compiler option.

You can detect at compile time if a type is abstract with the __is_abstract(``type``) type trait. For more information, see Compiler Support for Type Traits.

The abstract keyword is a context-sensitive override specifier. For more information about context-sensitive keywords, see Context-Sensitive Keywords. For more information about override specifiers, see How to: Declare Override Specifiers in Native Compilations.

Windows Runtime

For more information, see Ref classes and structs.

Requirements

Compiler option: /ZW

Common Language Runtime

Requirements

Compiler option: /clr

Examples

Example

The following code example generates an error because class X is marked abstract.

// abstract_keyword.cpp  
// compile with: /clr  
ref class X abstract {  
public:  
   virtual void f() {}  
};  
  
int main() {  
   X ^ MyX = gcnew X;   // C3622  
}  

Example

The following code example generates an error because it instantiates a native class that is marked abstract. This error will occur with or without the /clr compiler option.

// abstract_keyword_2.cpp  
class X abstract {  
public:  
   virtual void f() {}  
};  
  
int main() {  
   X * MyX = new X; // C3622: 'X': a class declared as 'abstract'  
                    // cannot be instantiated. See declaration of 'X'}  
  

Example

The following code example generates an error because function f includes a definition but is marked abstract. The final statement in the example shows that declaring an abstract virtual function is equivalent to declaring a pure virtual function.

// abstract_keyword_3.cpp  
// compile with: /clr  
ref class X {  
public:  
   virtual void f() abstract {}   // C3634  
   virtual void g() = 0 {}   // C3634  
};  

See Also

Component Extensions for Runtime Platforms