c++17: Automatic move from std::unique_ptr to std::shared_ptr

Question

My colleague and I are developing with an updated GCC 8.2 compiler. Previously had been using GCC 4.8 with -std=c++11. I was surprised to find that with GCC 8.2 a unique_ptr is automatically transferred to a shared_ptr. See this minimal example:

#include <iostream>
#include <memory>

using namespace std;

shared_ptr<int>
getMeAnInt()
{
    auto ret = unique_ptr<int>(new int(32));
    return ret;
}


int main()
{
    auto i = getMeAnInt();

    cout << *i << endl;
}

Using GCC 4.8, this fails to compile because an explicit std::move is needed when returning ret to transfer the unique_ptr to a shared_ptr. In GCC 8.2 this compiles and works fine.

Failure with GCC 4.8: https://godbolt.org/z/I4g9dI

Same code, but success with GCC 8.2: https://godbolt.org/z/ZeDajS

I assume this is an intentional change, but can someone explain to me the reasoning behind the change? Up until now I had run under the assumption that the explicit std::move requirement was a valuable compiler enforcement to make explicit that ownership was being transferred.

Wow, I bring two dupes, with one explicitly mentioning the DR that changed this behavior from gcc 5 onwards, and someone reopens. Charming — StoryTeller - Unslander Monica, Dec 31 '18 at 18:57
Here it is again https://stackoverflow.com/questions/17481018/when-is-explicit-move-needed-for-a-return-s — StoryTeller - Unslander Monica, Dec 31 '18 at 18:58
the automatic `move` is only done for return statement on some circumstance (temporary or local variable). — Jarod42, Dec 31 '18 at 19:19

c++17: Automatic move from std::unique_ptr to std::shared_ptr

0 Answers0