Converting constructor

 

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A constructor that is not declared with the specifier explicit and which can be called with a single parameter(until C++11) is called a converting constructor.

Unlike explicit constructors, which are only considered during direct initialization (which includes explicit conversions such as static_cast), converting constructors are also considered during copy initialization, as part of user-defined conversion sequence.

It is said that a converting constructor specifies an implicit conversion from the types of its arguments (if any) to the type of its class. Note that non-explicit user-defined conversion function also specifies an implicit conversion.

Implicitly-declared and user-defined non-explicit copy constructors and move constructors are converting constructors.

Example

Run this code

struct A
{
    A() { }         / converting constructor (since C++11)  
    A(int) { }      / converting constructor
    A(int, int) { } / converting constructor (since C++11)
};
 
struct B
{
    explicit B() { }
    explicit B(int) { }
    explicit B(int, int) { }
};
 
int main()
{
    A a1 = 1;      / OK: copy-initialization selects A::A(int)
    A a2(2);       / OK: direct-initialization selects A::A(int)
    A a3{4, 5};    / OK: direct-list-initialization selects A::A(int, int)
    A a4 = {4, 5}; / OK: copy-list-initialization selects A::A(int, int)
    A a5 = (A)1;   / OK: explicit cast performs static_cast, direct-initialization
 
/  B b1 = 1;      / error: copy-initialization does not consider B::B(int)
    B b2(2);       / OK: direct-initialization selects B::B(int)
    B b3{4, 5};    / OK: direct-list-initialization selects B::B(int, int)
/  B b4 = {4, 5}; / error: copy-list-initialization selected an explicit constructor
                   /        B::B(int, int)
    B b5 = (B)1;   / OK: explicit cast performs static_cast, direct-initialization
    B b6;          / OK, default-initialization
    B b7{};        / OK, direct-list-initialization
/  B b8 = {};     / error: copy-list-initialization selected an explicit constructor
                   /        B::B()
 
    [](...){}(a1, a4, a4, a5, b5); / may suppress "unused variable" warnings
}

See also

• copy assignment
• copy constructor
• copy elision
• default constructor
• destructor
• explicit
• initialization
  • aggregate initialization
  • constant initialization
  • copy initialization
  • default initialization
  • direct initialization
  • initializer list
  • list initialization
  • reference initialization
  • value initialization
  • zero initialization
• move assignment
• move constructor
• new

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