abstract class

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

C++ language

General topics
Preprocessor Comments Keywords	ASCII chart Escape sequences History of C++
Flow control
Conditional execution statements
if statement	switch statement
Iteration statements
for loop range-for loop (C++11)	while loop do-while loop
Jump statements
continue statement break statement	goto statement return statement
Functions
function declaration
lambda function declaration
function template
inline specifier
exception specifications (deprecated)
noexcept specifier (C++11)
Exceptions
throw expression	try-catch block
Namespaces
namespace declaration	namespace aliases
Types
fundamental types compound types enumeration types	union types function types
decltype specifier (C++11)
Specifiers
cv specifiers
storage duration specifiers
constexpr specifier (C++11)
auto specifier (C++11)
alignas specifier (C++11)
Initialization
default initialization value initialization zero initialization copy initialization direct initialization	aggregate initialization list initialization (C++11) constant initialization reference initialization

Literals
integer literals character literals floating-point literals string literals	boolean literals nullptr (C++11) user-defined (C++11)
Expressions
value categories order of evaluation	operators operator precedence
alternative representations
Utilities
Types
typedef declaration
type alias declaration (C++11)
attributes (C++11)
Casts
implicit conversions
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-style and functional cast
Memory allocation
new expression	delete expression
Classes
class declaration this pointer access specifiers	friend specifier initializer lists
Class-specific function properties
virtual function override specifier (C++11) final specifier (C++11)	explicit (C++11) static
Special member functions
default constructor copy constructor move constructor (C++11)	copy assignment move assignment (C++11) destructor
Templates
class template
function template
template specialization
parameter packs (C++11)
Miscellaneous
Inline assembly

 

Defines an abstract type which cannot be instantiated, but can be used as a base class.

Syntax

pure virtual function is a virtual function whose declarator has the following syntax:

declarator virt-specifier(optional) = 0

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), which can be used as base classes for concrete classes (for example, Circle).

No objects of an abstract class can be created. Abstract types cannot be used as parameter types, as function return types, or as the type of an explicit conversion. 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(); // okay, calls Abstract::g()
        // f(); // undefined behavior!
        Abstract::f(); // okay, 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(); // okay, calls Concrete::g()
        f(); // okay, calls Concrete::f()
    }
};

See also

• virtual