c++ - Why has the std::vector::resize signature been changed in C++11?

Question

What are the reasons behind the change in std::vector::resize from the pre-C++11:

void resize( size_type count, T value = T() );

to the compatible C++11 form:

void resize( size_type count );
void resize( size_type count, const value_type& value);

Answer 1

Paragraph C.2.12 of the Annex C (Compatibility) to the C++11 Standard specifies:

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 International Standard.

The old resize() function was copy-constructing new elements from value. This makes it impossible to use resize() when the elements of the vector are default-constructible but non-copyable (you may want to move-assign them later on). This explains the "Compatibility with move semantics" rationale.

Moreover, it may be slow if you do not want any copy to occur, just new elements to be default-constructed. Also, the value parameter is passed by value in the C++03 version, which incurs in the overhead of an unnecessary copy (as mentioned by TemplateRex in his answer). This explains the "Performance" rationale.