Default constructors

 

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Technical Specifications

 

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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.

Syntax

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.

Explanation

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

2) Definition of the constructor outside of class definition (the class must contain a declaration (1)). See constructors and member initializer lists for details on the constructor body

3) Deleted default constructor: if it is selected by overload resolution, the program fails to compile.

4) Defaulted default constructor: the compiler will define the implicit default constructor even if other constructors are present.

5) Defaulted default constructor outside of class definition (the class must contain a declaration (1)). Such constructor is treated as user-provided (see below and value initialization).

Default constructors are called during default initializations and value initializations.

Implicitly-declared default constructor

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)

Implicitly-defined default constructor

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)

Deleted implicitly-declared default constructor

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.

T is a union with at least one variant member with non-trivial default constructor, and no variant member of T has a default member initializer. T is a non-union class with a variant member M with a non-trivial default constructor, and no variant member of the anonymous union containing M has a default member initializer.

(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)

Trivial default constructor

The default constructor for class T is trivial (i.e. performs no action) if all of the following is true:

• The constructor is not user-provided (i.e., is implicitly-defined or defaulted on its first declaration)
• T has no virtual member functions
• T has no virtual base classes

T has no non-static members with default initializers.

(since C++11)

• Every direct base of T has a trivial default constructor
• Every non-static member of class type 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.

Defect reports

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

Example

Run this code

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
}