Title
Inadvertently inherited copy constructor
Status
cd4
Section
_N4527_.12.9 [class.inhctor]
Submitter
David Krauss

Created on 2014-06-30.00:00:00 last changed 49 months ago

Messages

Date: 2015-10-15.00:00:00

[Adopted at the October, 2015 meeting as P0136R1.]

Consider the following example:

  struct a {
    a() = default;
    a( a const & ) { std::cout << "copy\n"; }
    template< typename t >
    a( t ) { std::cout << "convert\n"; }
  };

  struct b : a {
    using a::a;
  };

  a x;
  b y = x;

The copy constructor is invoked by the inherited constructor template, making it effectively inherited, contrary to the intent of _N4527_.12.9 [class.inhctor] paragraph 3. std::function is affected by this issue.

A kernel of a resolution might be to inherit the copy and move constructors as deleted. Then they will be more specialized than any template, and the user won't get conversion-from-base behavior unless they explicitly declare it. However, reference binding in overload resolution is a potential gap. Something like b::b( a & ) = delete with a non-const parameter would not add safety if it's not chosen.

See also issue 1941.

History
Date User Action Args
2017-02-06 00:00:00adminsetstatus: dr -> cd4
2015-11-10 00:00:00adminsetstatus: drafting -> dr
2014-06-30 00:00:00admincreate