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1340. Why does forward_list::resize take the object to be copied by value?

Section: 23.3.7.5 [forward.list.modifiers] Status: C++11 Submitter: James McNellis Opened: 2010-07-16 Last modified: 2023-02-07

Priority: Not Prioritized

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Discussion:

In N3092 [forwardlist.modifiers], the resize() member function is declared as: 

void resize(size_type sz, value_type c);

The other sequence containers (list, deque, and vector) take 'c' by const reference.

Is there a reason for this difference? If not, then resize() should be declared as: 

void resize(size_type sz, const value_type& c);

The declaration would need to be changed both at its declaration in the class definition at [forwardlist]/3 and where its behavior is specified at [forwardlist.modifiers]/22.

This would make forward_list consistent with the CD1 issue 679(i).

[ Post-Rapperswil ]

Daniel changed the P/R slightly, because one paragraph number has been changed since the issue had been submitted. He also added a similar Requires element that exists in all other containers with a resize member function. He deliberately did not touch the wrong usage of "default-constructed" because that will be taken care of by LWG issue 868(i).

Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

  1. Change [forwardlist]/3, class template forward_list synopsis, as indicated: 
    ...
void resize(size_type sz);
void resize(size_type sz, const value_type& c);
void clear();
...
  2. Change [forwardlist.modifiers]/27 as indicated: 
    	
void resize(size_type sz);
void resize(size_type sz, const value_type& c);

    27 Effects: If sz < distance(begin(), end()), erases the last distance(begin(), end()) - sz elements from the list. Otherwise, inserts sz - distance(begin(), end()) elements at the end of the list. For the first signature the inserted elements are default constructed, and for the second signature they are copies of c.

    28 - Requires: T shall be DefaultConstructible for the first form and it shall be CopyConstructible for the second form.