why doesn't c++ uses RVO when returning local std::stringstream?

Question

I've read lots of info about rvalue and returning local variables in C++ >= 11. From what I understood is that "just return by value, do not use move/forward and do not add && to method signature and the compiler will optimize it for you".

Okay, I want it happen:

#include <sstream>

std::stringstream GetStream() {
    std::stringstream s("test");
    return s;
}

auto main() -> int {
    auto s = GetStream();
}

I get the nice

error: use of deleted function ‘std::basic_stringstream<char>::basic_stringstream(const std::basic_stringstream<char>&)’
     return s;

error. I don't get it, why it tries the copy constructor? Shan't it use move constructor with all the nice things from c++11 here? I use "--std=c++14".

This compile [without error](http://ideone.com/7aOvLQ). Which compiler are you using? — Holt, Mar 21 '16 at 13:54
My crystal ball tells me you are using GCC 4.9, which doesn't support move semantics for `stringstream`. If you want to use C++14 then you should be using a compiler that supports it. — Jonathan Wakely, Mar 21 '16 at 13:55
@erip, there were three clues: I recognized the error message as one of GCC's; the first GCC version to support the "-std=c++14" option was 4.9.0; and I know I added move semantics to `stringstream` for GCC 5; so it must be GCC 4.9.x :) — Jonathan Wakely, Mar 21 '16 at 14:00
I just like how it looks, obviously I don't use it where there are code conventions, but what's wrong in having a little fun) — Scott Tiger, Mar 21 '16 at 14:23

Scott Tiger · Accepted Answer · 2016-03-21T14:15:25.357

9

okay, this is a bug in my version of gcc (4.9.3). appears to be fixed in >= 5. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54316

edited Mar 21 '16 at 14:15

answered Mar 21 '16 at 13:54

Scott Tiger

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As the reporter of 54316, glad you found it ;) – Johan Lundberg Mar 21 '16 at 18:23

why doesn't c++ uses RVO when returning local std::stringstream?

1 Answers1