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.

Syntax

class-declaration class-identifier abstract {}
virtual return-type member-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 (C++ Component Extensions).

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

For more information, see Ref classes and structs.

Compiler option: /ZW

Compiler option: /clr

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
};
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