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
};