abstract class
Defines an abstract type which cannot be instantiated, but can be used as a base class.
Syntax
A pure virtual function is a virtual function whose declarator has the following syntax:
declarator virt-specifier(optional) = 0
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Here the sequence = 0 is known as pure-specifier, and appears either immediately after the declarator or after the optional virt-specifier (override or final).
pure-specifier cannot appear in a member function definition.
struct Base { virtual int g(); virtual ~Base() {} }; struct A : Base { // OK: declares three member virtual functions, two of them pure virtual int f() = 0, g() override = 0, h(); // OK: destructor can be pure too ~A() = 0; // Error: pure-specifier on a function definition virtual int b()=0 {} };
An abstract class is a class that either defines or inherits at least one function for which the final overrider is pure virtual.
Explanation
Abstract classes are used to represent general concepts (for example, Shape, Animal), which can be used as base classes for concrete classes (for example, Circle, Dog).
No objects of an abstract class can be created (except for base subobjects of a class derived from it) and no non-static data members of an abstract class can be declared.
Abstract types cannot be used as parameter types, as function return types, or as the type of an explicit conversion (note this is checked at the point of definition and function call, since at the point of function declaration parameter and return type may be incomplete)
Pointers and references to an abstract class can be declared.
struct Abstract { virtual void f() = 0; // pure virtual }; // "Abstract" is abstract struct Concrete : Abstract { void f() override {} // non-pure virtual virtual void g(); // non-pure virtual }; // "Concrete" is non-abstract struct Abstract2 : Concrete { void g() override = 0; // pure virtual overrider }; // "Abstract2" is abstract int main() { // Abstract a; // Error: abstract class Concrete b; // OK Abstract& a = b; // OK to reference abstract base a.f(); // virtual dispatch to Concrete::f() // Abstract2 a2; // Error: abstract class (final overrider of g() is pure) }
The definition of a pure virtual function may be provided (and must be provided if the pure virtual is the destructor): the member functions of the derived class are free to call the abstract base's pure virtual function using qualified function id. This definition must be provided outside of the class body (the syntax of a function declaration doesn't allow both the pure specifier = 0 and a function body).
Making a virtual call to a pure virtual function from a constructor or the destructor of the abstract class is undefined behavior (regardless of whether it has a definition or not).
struct Abstract { virtual void f() = 0; // pure virtual virtual void g() {} // non-pure virtual ~Abstract() { g(); // OK: calls Abstract::g() // f(); // undefined behavior Abstract::f(); // OK: non-virtual call } }; // definition of the pure virtual function void Abstract::f() { std::cout << "A::f()\n"; } struct Concrete : Abstract { void f() override { Abstract::f(); // OK: calls pure virtual function } void g() override {} ~Concrete() { g(); // OK: calls Concrete::g() f(); // OK: calls Concrete::f() } };