Default constructors - cppreference.com (original) (raw)

A default constructor is a constructor which can be called with no arguments.

Contents

• 1 Syntax
• 2 Explanation
• 3 Implicitly-declared default constructor
• 4 Implicitly-defined default constructor
• 5 Deleted default constructor
• 6 Trivial default constructor
• 7 Eligible default constructor
• 8 Notes
• 9 Example
• 10 Defect reports
• 11 See also

[edit] Syntax

[edit] Explanation

1. Declaration of a default constructor inside of class definition.

2-4) Definition of a default constructor inside of class definition.

1. The default constructor is explicitly-defaulted.

2. The default constructor is deleted.

5,6) Definition of a default constructor outside of class definition (the class must contain a declaration (1)).

1. The default constructor is explicitly-defaulted.

Default constructors are called during default initializations and value initializations.

[edit] Implicitly-declared default constructor

If there is no user-declared constructor or constructor template for a class type, the compiler will implicitly declare a default constructor as an inline public member of its class.

The implicitly-declared (or defaulted on its first declaration) default constructor has an exception specification as described in dynamic exception specification(until C++17) noexcept specification(since C++17).

[edit] Implicitly-defined default constructor

If the constructor is implicitly-declared(until C++11)the implicitly-declared or explicitly-defaulted default constructor is not defined as deleted(since C++11), it is implicitly-defined by the compiler when odr-used or needed for constant evaluation or when it is explicitly defaulted after its first declaration(since C++11).

An(until C++26)Otherwise, an(since C++26) implicitly-defined default constructor has 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. Class types with an empty user-provided constructor may get treated differently than those with an implicitly-defined default constructor during value initialization.

[edit] Trivial default constructor

The default constructor for class T is trivial if all following conditions are satisfied:

• The constructor is implicitly-declared(until C++11)not user-provided(since C++11).
• T has no virtual member functions.
• T has no virtual base classes.

• Every direct base of 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.

[edit] Eligible default constructor

Triviality of eligible default constructors determines whether the class is an implicit-lifetime type, and whether the class is a trivially copyable type.

[edit] Notes

[edit] Example

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 };   // user declared copy constructor (either user-provided, deleted or defaulted) // prevents the implicit generation of a default constructor   struct G { G(const G&) {} // G::G() is implicitly defined as deleted };   struct H { H(const H&) = delete; // H::H() is implicitly defined as deleted };   struct I { I(const I&) = default; // I::I() is implicitly defined as deleted };   int main() { A a; B b; C c; // D d; // compile error E e; // F f; // compile error // G g; // compile error // H h; // compile error // I i; // compile error }

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also

• constructor
• initialization
  • aggregate initialization
  • constant initialization
  • copy initialization
  • default initialization
  • direct initialization
  • list initialization
  • reference initialization
  • value initialization
  • zero initialization
• new