(template metaprogramming) conditional inclusion of the default constructor

[n.torrey.pines]

std:air explicitly defines a default constructor. This makes it
impossible to instantiate it with a type that does not have a default
constructor (or it is private).

I wonder if it would be possible to define std:air in such a way
(using template metaprogramming) that the default constructor is
defined if and only if both of pair's argument types are default-
constructible.

 

[Dave Rahardja]

std:air explicitly defines a default constructor. This makes it
impossible to instantiate it with a type that does not have a default
constructor (or it is private).

I wonder if it would be possible to define std:air in such a way
(using template metaprogramming) that the default constructor is
defined if and only if both of pair's argument types are default-
constructible.

You don't need template metaprogramming for that--you just need to remove the
default constructor from the class definition. Of course then you'll end up
with something /other/ than std:air.

There are many reasons to give "value-like" classes default constructors. One
of which is that you can then have a vector or array of it without explicitly

wouldn't be possible if std:air didn't have a default contsructor.

-dr