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4225. What should happen when an exception is thrown on resizing std::deque, std::forward_list, or std::list?

Section: 23.3.5.3 [deque.capacity], 23.3.7.5 [forward.list.modifiers], 23.3.11.3 [list.capacity] Status: New Submitter: Jiang An Opened: 2025-03-15 Last modified: 2025-10-17

Priority: 3

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Discussion:

Currently, std::vector and std::inplace_vector's resize functions are specified to have no effects on the container when an exception is throwing on appending. However, such specification seem to be missing for std::deque, std::forward_list, and std::list.

Is such difference intented? Do we want to roll back the status of container when the appending is partially done?

Daniel:

The specific resize exception guarantee for std::vector came from resolving LWG 2033⁽ⁱ⁾ and were later effectively copied to std::inplace_vector because that container's specification should resemble as much as possible that of std::vector.

[2025-10-16; Reflector poll]

Set priority to 3 after reflector poll.

[2025-10-16; Jonathan provides wording]

LWG 4106⁽ⁱ⁾ already fixed this for std::forward_list. For std::list the "If an exception is thrown, there are no effects" wording in 23.3.11.4 [list.modifiers] p2 doesn't apply to std::list::resize because it's in a different subclause (23.3.11.3 [list.capacity]). We can fix that though.

[2025-10-17; Comment from Nevin.]

deque::resize invalidates all iterators if it causes a map reallocation.

Proposed resolution:

This wording is relative to N5014.

1. Modify 23.3.5.3 [deque.capacity] as indicated:

   constexpr void resize(size_type sz);
   

   -1- Preconditions: T is Cpp17MoveInsertable and Cpp17DefaultInsertable into deque.

   -2- Effects: If sz < size() is true, erases the last size() - sz elements from the sequence. Otherwise, appends sz - size() default-inserted elements to the sequence. If an exception is thrown, there is no effect on the container.

   constexpr void resize(size_type sz, const T& c);
   

   -3- Preconditions: T is Cpp17CopyInsertable into deque.

   -4- Effects: If sz < size() is true, erases the last size() - sz elements from the sequence. Otherwise, appends sz - size() copies of c to the sequence. If an exception is thrown, there is no effect on the container.

2. Modify 23.3.11.3 [list.capacity] as indicated:

   constexpr void resize(size_type sz);
   

   -1- Preconditions: T is Cpp17DefaultInsertable into list.

   -2- Effects: If size() < sz is true, appends sz - size() default-inserted elements to the sequence. If sz <= size() is true, equivalent to:

   list<T>::iterator it = begin();
   advance(it, sz);
   erase(it, end());
   

   If an exception is thrown, there is no effect on the container.

   constexpr void resize(size_type sz, const T& c);
   

   -3- Preconditions: T is Cpp17CopyInsertable into list.

   -4- Effects: As if by Equivalent to:

   if (sz < size()
     insert(end(), sz-size(), c);
   else if (sz < size()) {
     iterator i = begin();
     advance(it, sz);
     erase(it, end());
   }
   else
     ;           // do nothing