Title
Non-constructed value-initialized objects
Status
c++11
Section
9.4 [dcl.init]
Submitter
Alisdair Meredith

Created on 2005-03-18.00:00:00 last changed 96 months ago

Messages

Date: 2010-08-15.00:00:00

[Voted into WP at August, 2010 meeting.]

Date: 2010-02-15.00:00:00

Proposed resolution (February, 2010):

Change 9.4 [dcl.init] paragraph 7 as follows:

To value-initialize an object of type T means:

  • ...

An object that is value-initialized is deemed to be constructed and thus subject to provisions of this International Standard applying to “constructed” objects, objects “for which the constructor has completed,” etc., even if no constructor is invoked for the object's initialization.

Date: 2006-04-15.00:00:00

Notes from April, 2006 meeting:

There was some concern about whether this wording covered (or needed to cover) cases where an object is “partially constructed.” Another approach might be simply to define value initialization to be “construction.” Returned to “drafting” status for further investigation.

Date: 2005-10-15.00:00:00

Proposed resolution (October, 2005):

Add the indicated words to 9.4 [dcl.init] paragraph 6:

A program that calls for default-initialization or value-initialization of an entity of reference type is ill-formed. If T is a cv-qualified type, the cv-unqualified version of T is used for these definitions of zero-initialization, default-initialization, and value-initialization. Even when value-initialization of an object does not call that object's constructor, the object is deemed to have been fully constructed once its initialization is complete and thus subject to provisions of this International Standard applying to “constructed” objects, objects “for which the constructor has completed execution,” etc.
Date: 2005-03-18.00:00:00

According to the definition of value initialization (9.4 [dcl.init] paragraph 5), non-union class types without user-declared constructors are value-initialized by value-initializing each of their members rather than by executing the (generated) default constructor. However, a number of other items in the Standard are described in relationship to the execution of the constructor:

  • 11.4.7 [class.dtor] paragraph 6: “Bases and members are destroyed in the reverse order of the completion of their constructor.” If a given base or member is value-initialized without running its constructor, is it destroyed? (For that matter, paragraph 10 refers to “constructed” objects; is an object that is value-initialized without invoking a constructor “constructed?”)

  • 14.3 [except.ctor] paragraph 2: “An object that is partially constructed or partially destroyed will have destructors executed for all of its fully constructed subobjects, that is, for subobjects for which the constructor has completed execution...”

  • 6.7.3 [basic.life] paragraph 1: The lifetime of an object begins when “the constructor call has completed.” (In the TC1 wording — “if T is a class type with a non-trivial constructor (11.4.5 [class.ctor]), the constructor call has completed” — the lifetime of some value-initialized objects never began; in the current wording — “the constructor invoked to create the object is non-trivial” — the lifetime begins before any of the members are initialized.)

History
Date User Action Args
2014-03-03 00:00:00adminsetstatus: fdis -> c++11
2011-04-10 00:00:00adminsetstatus: wp -> fdis
2010-11-29 00:00:00adminsetstatus: dr -> wp
2010-08-23 00:00:00adminsetmessages: + msg2914
2010-08-23 00:00:00adminsetstatus: ready -> dr
2010-03-29 00:00:00adminsetstatus: review -> ready
2010-02-16 00:00:00adminsetmessages: + msg2542
2010-02-16 00:00:00adminsetstatus: drafting -> review
2006-04-22 00:00:00adminsetmessages: + msg1332
2006-04-22 00:00:00adminsetstatus: review -> drafting
2005-10-22 00:00:00adminsetmessages: + msg1237
2005-10-22 00:00:00adminsetstatus: open -> review
2005-03-18 00:00:00admincreate