A default constructor is a constructor which can be called with no arguments (either defined with an empty parameter list, or with default arguments provided for every parameter). A type with a public default constructor is DefaultConstructible.
class_name ( ) ; | (1) | |
class_name :: class_name ( ) body | (2) | |
class_name() = delete ; | (3) | (since C++11) |
class_name() = default ; | (4) | (since C++11) |
class_name :: class_name ( ) = default ; | (5) | (since C++11) |
Where class_name must name the current class (or current instantiation of a class template), or, when declared at namespace scope or in a friend declaration, it must be a qualified class name.
Default constructors are called during default initializations and value initializations.
If no user-declared constructors of any kind are provided for a class type (struct
, class
, or union
), the compiler will always declare a default constructor as an inline public
member of its class.
If some user-declared constructors are present, the user may still force the automatic generation of a default constructor by the compiler that would be implicitly-declared otherwise with the keyword default . | (since C++11) |
The implicitly-declared (or defaulted on its first declaration) default constructor has an exception specification as described in dynamic exception specification (until C++17)exception specification (since C++17).
If the implicitly-declared default constructor is not defined as deleted, it is defined (that is, a function body is generated and compiled) by the compiler if odr-used, and it has exactly the same effect as a user-defined constructor with empty body and empty initializer list. That is, it calls the default constructors of the bases and of the non-static members of this class. If this satisfies the requirements of a constexpr constructor, the generated constructor is constexpr
. (since C++11).
If some user-defined constructors are present, the user may still force the automatic generation of a default constructor by the compiler that would be implicitly-declared otherwise with the keyword default . | (since C++11) |
The implicitly-declared or defaulted default constructor for class T
is undefined (until C++11)defined as deleted (since C++11) if any of the following is true:
T
has a member of reference type without a default initializer. (since C++11) T
has a const
member without user-defined default constructor or a default member initializer (since C++11). T
has a member (without a default member initializer) (since C++11) which has a deleted default constructor, or its default constructor is ambiguous or inaccessible from this constructor. T
has a direct or virtual base which has a deleted default constructor, or it is ambiguous or inaccessible from this constructor. T
has a direct or virtual base which has a deleted destructor, or a destructor that is inaccessible from this constructor.
| (since C++11) |
T
is a union
and all of its variant members are const
. If no user-defined constructors are present and the implicitly-declared default constructor is not trivial, the user may still inhibit the automatic generation of an implicitly-defined default constructor by the compiler with the keyword delete . | (since C++11) |
The default constructor for class T
is trivial (i.e. performs no action) if all of the following is true:
T
has no virtual member functions T
has no virtual base classes
| (since C++11) |
T
has a trivial default constructor A trivial default constructor is a constructor that performs no action. All data types compatible with the C language (POD types) are trivially default-constructible. Unlike in C, however, objects with trivial default constructors cannot be created by simply reinterpreting suitably aligned storage, such as memory allocated with std::malloc
: placement-new is required to formally introduce a new object and avoid potential undefined behavior.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
CWG 2084 | C++11 | default member initializers have no effect on whether a defaulted default constructor of a union is deleted | they prevent the defaulted default constructor from being defined as deleted |
struct A { int x; A(int x = 1): x(x) {} // user-defined default constructor }; struct B: A { // B::B() is implicitly-defined, calls A::A() }; struct C { A a; // C::C() is implicitly-defined, calls A::A() }; struct D: A { D(int y): A(y) {} // D::D() is not declared because another constructor exists }; struct E: A { E(int y): A(y) {} E() = default; // explicitly defaulted, calls A::A() }; struct F { int& ref; // reference member const int c; // const member // F::F() is implicitly defined as deleted }; int main() { A a; B b; C c; // D d; // compile error E e; // F f; // compile error }
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