Cannot create constexpr std::vector

Question

I can create constexpr std::array:

constexpr std::array values {1,2,3,4,5};

It works fine. But I cannot create

Answer 1

AFAIK The initlializer_list constructor of std::vector<> is not declared constexpr.

Answer 2

std::vector is not constexpr. There is a proposal to make std::vector constexpr: https://github.com/ldionne/wg21/blob/master/generated/p1004r1.pdf

There is a whole talk about the upcoming C++20/23 changes: https://youtu.be/CRDNPwXDVp0?t=3080

So check again with C++20.

[edit]: constexpr std::vector has been approved for C++20! https://www.reddit.com/r/cpp/comments/au0c4x/201902_kona_iso_c_committee_trip_report_c20/

[edit 2019-10]: gcc trunk (with --std=c++2a flag) has started to implement constexpr new (a prerequisite for constexpr vector). See: https://youtu.be/FRTmkDiW5MM?t=372

Answer 3

For c++ version at least prior C++2a:

std::vector uses a dynamic memory allocation. Operator new can't be used in constexpr methods, thus std::vector will never be constexpr, constexpr constructor can't be declared for it. std::array doesn't use dynamic memory allocation, it is allocated in stack. It has no any problem with rules of creation constexpr objects and can be constexpr.