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Derived classes

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Revision as of 13:21, 9 February 2014 by Cubbi (Talk | contribs)

Any class type (whether declared with class-key class or struct) may be declared as derived from one or more base classes which, in turn, may be derived from their own base classes, forming an inheritance hierarchy.

The list of base classes is provided in the base-clause of the class declaration syntax. The base-clause consists of the character : followed by a comma-separated list of one or more base-specifiers.

attr(optional) access-specifier(optional) virtual-specifier(optional) class-or-decltype
attr(C++11) - optional sequence of any number of attributes
access-specifier - one of private, public, or protected
virtual-specifier - the keyword virtual

virtual-specifier and access-specifier may appear in any order

The last base-specifier in a base-clause may be a pack expansion.

If access-specifier is omitted, it defaults to public for classes declared with class-key struct and to private for classes declared with class-key class.

struct Base {
    int a, b, c;
};
// every object of type Derived includes Base as a subobject
struct Derived : Base {
    int b;
};
// every object of type Derived2 includes Derived and Base as subobjects
struct Derived2 : Derived {
    int c;
};

The classes listed in the base-clause are direct base classes. Their bases are indirect base classes. The same class cannot be specified as a direct base class more than once, but the same class can be both direct and indirect base class.

Each direct and indirect base class is present, as base class subobject, within the object representation of the derived class at implementation-defined offset. Empty base classes usually do not increase the size of the derived object due to empty base optimization. The constructors of base class subobjects are called by the constructor of the derived class: arguments may be provided to those constructors in the member initializer list.

Contents

Virtual base classes

For each distinct base class that is specified virtual, the most derived object contains only one base class subobject of that type, even if the class appears many times in the inheritance hierarchy (as long as it is inherited virtual every time).

struct B { int n; };
class X : public virtual B {};
class Y : virtual public B {};
class Z : public B {};
// every object of type AA has one X, one Y, one Z, and two B's:
// one that is the base of Z and one that is shared by X and Y
struct AA : X, Y, Z {
    void f() {
        X::n = 1; // modifies the virtual B subobject's member
        Y::n = 2; // modifies the same virtual B subobject's member
        Z::n = 3; // modifies the non-virtual B subobject's member
 
        std::cout << X::n << Y::n << Z::n << '\n'; // prints 223
    }
};

An example of an inheritance hierarchy with virtual base classes is the iostreams hierarchy of the standard library: std::istream and std::ostream are derived from std::ios using virtual inheritance. std::iostream is derived from both std::istream and std::ostream, so every instance of std::iostream contains a std::ostream subobject, a std::istream subobject, and just one std::ios subobject (and, consequently, one std::ios_base).

All virtual base subobjects are initialized before any non-virtual base subobject, so only the most derived class calls the constructors of the virtual bases in its member initializer list:

struct B {
    int n;
    B(int x) : n(x) {}
};
struct X : virtual B { X() : B(1) {} };
struct Y : virtual B { Y() : B(2) {} };
struct AA : X, Y     { AA() : B(3), X(), Y() {} };
 
// the default constructor of AA calls the default constructors of X and Y
// but those constructors do not call the constructor of B because B is a virtual base
AA a; // a.n == 3
// the default constructor of X calls the constructor of B
X x; // x.n == 1

The are special rules for unqualified name lookup for class members when virtual inheritance is involved (sometimes referred to as the rules of dominance), see lookup

Member access

Member name lookup

Unqualified and qualified name lookup rules for class members are detailed in name lookup.

See also