Constructors and explicit conversion functions in direct initialization
Section [over.match.copy]
Edward Catmur

Created on 2012-09-18.00:00:00 last changed 90 months ago


Date: 2013-04-15.00:00:00

[Moved to DR at the April, 2013 meeting.]

Date: 2012-10-15.00:00:00

Proposed resolution (October, 2012):

Change the second bullet of [over.match.copy] paragraph 1 as follows:

...Assuming that “cv1 T” is the type of the object being initialized, with T a class type, the candidate functions are selected as follows:

  • The converting constructors ( [class.conv.ctor]) of T are candidate functions.

  • When the type of the initializer expression is a class type “cv S”, the non-explicit conversion functions of S and its base classes are considered. When initializing a temporary to be bound to the first parameter of a constructor that takes a reference to possibly cv-qualified T as its first argument, called with a single argument in the context of direct-initialization of an object of type “cv2 T, explicit conversion functions are also considered. Those that are not hidden within S and yield a type whose cv-unqualified version is the same type as T or is a derived class thereof are candidate functions. Conversion functions that return “reference to X” return lvalues or xvalues, depending on the type of reference, of type X and are therefore considered to yield X for this process of selecting candidate functions.

Date: 2012-09-18.00:00:00

Issue 899 added wording to the second bullet of [over.match.copy] paragraph 1 permitting use of explicit conversion functions when calling a copy constructor in a direct-initialization context. Issue 1087 addressed the problem in the earlier resolution that the phrase “copy constructor” did not include move constructors and template constructors. However, the term “copy constructor” implicitly (and correctly) restricted the constructor parameter to be the constructor's class, i.e., the class of the object being directly initialized. The new phrasing, “a constructor that takes a reference to possibly cv-qualified T as its first argument,” incorrectly assumed that T referred to the type of the object being initialized; however, that is not the case, and a converting constructor from T to the type of the object being initialized is inadvertently permitted. The wording needs a further tweak to restore the intended context.

Date User Action Args
2014-03-03 00:00:00adminsetstatus: drwp -> cd3
2013-10-14 00:00:00adminsetstatus: dr -> drwp
2013-05-03 00:00:00adminsetmessages: + msg4418
2013-05-03 00:00:00adminsetstatus: ready -> dr
2012-11-03 00:00:00adminsetmessages: + msg4066
2012-11-03 00:00:00adminsetstatus: open -> ready
2012-09-18 00:00:00admincreate