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# Default constructors

From cppreference.com
< cpp‎ | language

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`.

### [edit] Syntax

`ClassName` `(` `)` `;` | (1) |
`ClassName` `::` `ClassName` `(` `)` body | (2) |
`ClassName()` `=` `delete` `;` | (3) | (since C++11)
`ClassName()` `=` `default` `;` | (4) | (since C++11)

### [edit] Explanation

1) Declaration of a default constructor
2) Definition of the constructor outside the class body
3) Inhibiting the automatic generation of a default constructor
4) Explicitly forcing the automatic generation of a default constructor

`ClassName` is the identifier of the enclosing class

The default constructors are called when:

- creating objects or arrays of static, thread-local, and automatic storage duration that are declared without an initializer, (T obj; or T arr[10];)
- creating objects of dynamic storage duration when the new-expression is written without an initializer (new T;)
- creating arrays of dynamic storage duration with the expression new T[n]
- creating value-initialized temporary objects with the cast expression T().

### [edit] Implicitly-declared default constructor

If no user-defined 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-defined constructors are present, the user may still force the generation of the implicitly declared constructor with the keyword `default` | (since C++11)

If the implicitly-declared default constructor is not deleted or trivial, it is defined (that is, a function body is generated and compiled) by the compiler, 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.

### [edit] 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 brace-or-equal initializer.(since C++11)
- `T` has a const member without user-defined default constructor or a brace-or-equal initializer(since C++11).
- `T` has a member (without a brace-or-equal 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. | (since C++11)

- `T` is a union and all of its variant members are const.

### [edit] Trivial default constructor

The implicitly-declared 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

`T` has no non-static members with brace-or-equal 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. Objects with trivial default constructors can be created by using reinterpret_cast on any suitably aligned storage, e.g. on memory allocated with std::malloc. All data types compatible with the C language (POD types) are trivially default-constructible.

### [edit] Example

    struct A {
        int x;
        A(int x = 1) : x(x) {} // user-defined default ctor
    };
    struct B : A {
        // B::B() is implicitly-defined, calls A::A()
    };
    struct C {
        A obj;
        // 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 declared as deleted
    };

    int main()
    {
        A a;
        B b;
    //  D d; // compile error
        E e;
    //  F f; // compile error
    }

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