Annex C (informative) Compatibility [diff]

Change: Complexity of size() member functions now constant.

Rationale: Lack of specification of complexity of size() resulted in divergent implementations with inconsistent performance characteristics.

Effect on original feature: Some container implementations that conform to C++ 2003 may not conform to the specified size() requirements in this revision of C++.

Adjusting containers such as std​::​list to the stricter requirements may require incompatible changes.

Change: Requirements change: relaxation.

Rationale: Clarification.

Effect on original feature: Valid C++ 2003 code that attempts to meet the specified container requirements may now be over-specified.

Code that attempted to be portable across containers may need to be adjusted as follows:

• (2.1)
  not all containers provide size(); use empty() instead of size() == 0;
• (2.2)
  not all containers are empty after construction (array);
• (2.3)
  not all containers have constant complexity for swap() (array).

Change: Requirements change: default constructible.

Rationale: Clarification of container requirements.

Effect on original feature: Valid C++ 2003 code that attempts to explicitly instantiate a container using a user-defined type with no default constructor may fail to compile.

Change: Signature changes: from void return types.

Rationale: Old signature threw away useful information that may be expensive to recalculate.

Effect on original feature: The following member functions have changed:

• (4.1)
  erase(iter) for set, multiset, map, multimap
• (4.2)
  erase(begin, end) for set, multiset, map, multimap
• (4.3)
  insert(pos, num, val) for vector, deque, list, forward_list
• (4.4)
  insert(pos, beg, end) for vector, deque, list, forward_list

Valid C++ 2003 code that relies on these functions returning void (e.g., code that creates a pointer to member function that points to one of these functions) will fail to compile with this revision of C++.

Change: Signature changes: from iterator to const_iterator parameters.

Rationale: Overspecification.

Effect on original feature: The signatures of the following member functions changed from taking an iterator to taking a const_iterator:

• (5.1)
  insert(iter, val) for vector, deque, list, set, multiset, map, multimap
• (5.2)
  insert(pos, beg, end) for vector, deque, list, forward_list
• (5.3)
  erase(begin, end) for set, multiset, map, multimap
• (5.4)
  all forms of list​::​splice
• (5.5)
  all forms of list​::​merge

Valid C++ 2003 code that uses these functions may fail to compile with this revision of C++.

Change: Signature changes: resize.

Rationale: Performance, compatibility with move semantics.

Effect on original feature: For vector, deque, and list the fill value passed to resize is now passed by reference instead of by value, and an additional overload of resize has been added.

Valid C++ 2003 code that uses this function may fail to compile with this revision of C++.