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Default constructors - cppreference.com

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

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

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

3) The default constructor is explicitly-defaulted.

4) The default constructor is deleted.

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

6) The default constructor is explicitly-defaulted.

Default constructors are called during default initializations and value initializations.

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).

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).

If a default constructor of a union-like class T is trivial, then for each union U that is either T or an anonymous union member of T, if the first variant member (if any) of U has implicit-lifetime type, the default constructor of T begins the lifetime of that member if it is not the active member of its union.

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.

If this satisfies the requirements of a constexpr constructor(until C++23)constexpr function(since C++23), the generated constructor is constexpr.

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.

The implicitly-declared or explicitly-defaulted default constructor for class T is defined as deleted if any of the following conditions is satisfied:

• T has a non-static data member of reference type without a default initializer.
• T is a non-union class and(since C++26) has a non-variant non-static non-const-default-constructible data member of const-qualified type (or possibly multidimensional array thereof) without a default member initializer.

• T is a union and all of its variant members are of const-qualified type (or possibly multidimensional array thereof).
• T is a non-union class and all members of any anonymous union member are of const-qualified type (or possibly multidimensional array thereof).

• Given a class type M, T has a potentially constructed subobject obj of type M (or possibly multidimensional array thereof), and any of the following conditions is satisfied:

• M has a destructor that is deleted or inaccessible from the default constructor, and either obj is non-variant or obj has a default member initializer(since C++26).
• All following conditions are satisfied:

• obj is not a non-static data member with a default initializer.
• obj is not a variant member of a union where another non-static data member has a default initializer(until C++26).
• The overload resolution as applied to find M's default constructor does not result in a usable candidate, or in the case of obj being a variant member, selects a non-trivial function(until C++26).

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.

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.

• T has no non-static members with default initializers.

• Every direct base of T has a trivial default constructor.

• Every non-static member of class type (or array thereof) has a trivial default constructor.

• Either T is a union, or every non-variant non-static member of class type (or array thereof) 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.

A default constructor is eligible if it is either user-declared or both implicitly-declared and definable.

A default constructor is eligible if it is not deleted.

A default constructor is eligible if all following conditions are satisfied:

• It is not deleted.
• Its associated constraints (if any) are satisfied.
• No default constructor whose associated constraints are satisfied is more constrained.

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

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
}

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

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

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