c++ - Exception safety of std::vector implementation shipped with Visual Studio 2015 -

consider following program:

#include <iostream> #include <vector> #include <type_traits> struct x {     int state;     x() : state(0) {};     x(int x) : state(x) {};     x(x const& y) noexcept : state(1) {}     x(x&& y) noexcept(false) : state(y.state != 8 ? 2 : throw 1)      { y.state = 0; }     x& operator =(x const&) = default;     x& operator =(x&&) = default; }; int main() {     std::vector<x> a;     a.reserve(2u);     a.emplace_back(6);     a.emplace_back(8);     try {          // ####         std::cout << 'a' << a.front().state << a.back().state;     }     catch (...)      {          std::cout << 'b' << a.front().state << a.back().state;     }     return 0; } 

at position // #### different statements may employed, each giving different output on g++/clang++ versus msvc.

    statement               g++     msvc    expected  1   a.resize(10u, x{});     a11     b08     a11 2   x b; a.push_back(b);    a11     b08     a11 3   a.emplace_back(1);      a11     b08     a11 

so, why expect a11 in cases here?

1: a.resize(10u, x{});

[vector.capacity], 16/17 of n4140 , n4606 void resize(size_type sz, const t& c);

requires: t shall copyinsertable *this.

remarks: if exception thrown there no effects.

thus, if compiler must guarantee full rollback in case of exception possible ("no effects") must either copy elements or can move if move noexcept because moving has immediate effect on elements cannot rolled (which necessary if moving throw in turn not case noexcept move constructors).

therefore, i'd expect a11 output (or b68 if copy constructor throws).

2: x b; a.push_back(b);

3: a.emplace_back(1);

[vector.modifiers], 1 of n4140 , n4606 say:

if exception thrown while inserting single element @ end , t copyinsertable or is_nothrow_move_constructible<t>::value true, there no effects.

again standard requires "no effect" in case of exception when type "copyinsertable" (x copyinsertible). -> move_if_noexcept!

my interpretation of mandates a11 expected output (or in other terms, conformant std::vector shouldn't move elements unless moving noexcept or copying impossible).

i tried find information on vs sl found blog post 'vs 2015 update 2’s stl c++17-so-far feature complete' on vs blog suggests vs standard library implementation "feature complete" don't whether supposed claim standard compliance.

note: doubt i'm first spot vs incorrectly moves elements when should copy them - if has connect issue or so-question, don't refrain pointing it.

so question is: there specific reason behaviour in msvc (i.e. performance implications other copy vs. move or anything) or bug?