c++ - Why is vector::push_back of local variable not move optimized? -

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in c++11 can use std::vector::push_back in combination std::move avoid copies while inserting elements vector. there section in standard forbids compilers use std::move automatically local variables not used after calls push_back? when filling vectors large elements, huge performance benefit.

i checked following code gcc 7.1.0 , -o3 , version 1 prints move while version 2 prints copy.

#include <iostream> #include <string> #include <vector>  struct s {     s() = default;     s(s const&) { std::cout << "copy" << std::endl; }     s(s &&) { std::cout << "move" << std::endl; }     std::string a;     std::string b; };  void fill_members(s& s) { /*...*/ }  int main() {     std::vector<s> v;     {         s s;          // somehow fill members of s, maybe use function call that.         fill_members(s);          // version 1:         // avoids copy, since there explicit std::move.         v.push_back(std::move(s));          // version 2:         // why dont compilers optimize std::move?         // compiler can see s not used after line.         v.push_back(s);     } }