|(1) default constructor|
Constructs a pair object with its elements value-initialized.
|To value-initialize an object of type T means:|
— if T is a class type (clause 9) with a user-declared constructor (12.1), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);
— if T is a non-union class type without a user-declared constructor, then every non-static data member and base-class component of T is value-initialized;96)
— if T is an array type, then each element is value-initialized;
— otherwise, the object is zero-initialized