Cannot create constexpr std::vector

Question

I can create constexpr std::array:

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

It works fine. But I cannot create constexpr vector:

constexpr std::vector<int> vec = {1,2,3,4,5};

It gives me an error:

the type 'const std::vector<int>' of constexpr variable 'vec' is not literal constexpr std::vector<int> vec = {1,2,3,4,5};

Formally, that's because `vector` constructor is not declared `constexpr`. Why is it not so declared? Because `vector` constructor generally needs to allocate memory on the heap, which of course can only be done at run time. — Igor Tandetnik, Oct 20 '15 at 16:36
@Igor Tandetnik so, there are no way to create constexpr vector? — Leo, Oct 20 '15 at 16:42
No there is not. Why would you want to? It makes little sense to me. The whole point of `vector` is its ability to resize dynamically. If you don't need that, just use `std::array` or plain array. — Igor Tandetnik, Oct 20 '15 at 16:46
@Igor Tandetnik. Actually, I am using `Qt` and there are nothing like `std::array` container, so I tried to use `QVector` and `QList` and it does not work. I don't want to mix `Qt` and `stl` containers. So, I guess now I have to — Leo, Oct 20 '15 at 16:57
There are use cases, for example if you have a global array of pair where vector can consist of a limited (but variable) numbers known at compile time. — gast128, Oct 08 '18 at 10:00
@Leo `QVarLengthArray` same as std::array https://doc.qt.io/qt-6/qvarlengtharray.html — , Jun 15 '23 at 10:38

user643011 · Answer 1 · 2021-11-18T19:05:39.387

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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

[edit 2021-11]: both constexpr std::vector and constexpr std::basic_string are now implemented in gcc 12 ( https://en.cppreference.com/w/cpp/compiler_support )

edited Nov 18 '21 at 19:05

answered Oct 14 '18 at 09:00

user643011

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It has gone into C++20, together with `std::string`: https://stackoverflow.com/a/57284740/895245 – Ciro Santilli OurBigBook.com Jul 31 '19 at 06:47
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both `constexpr std::vector` and `constexpr std::basic_string` are implemented in GCC 12 (https://en.cppreference.com/w/cpp/compiler_support) – Desmond Gold Nov 17 '21 at 03:19
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Thanks @DesmondGold. I added this to the original answer. Much appreciated! – user643011 Nov 18 '21 at 19:06
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But constexpr vector doesn't mean that the OP's code has become valid, it is still rejected. – Marc Glisse Nov 18 '21 at 19:14
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OP's code remains invalid because the `constexpr std::vector` is in use **at runtime**. No problem otherwise if `vec` is declared in a `constexpr` function. (Or, use `constexpr std::array`.) – SnzFor16Min Jan 19 '22 at 03:27
The only remaining question is: Why? – Spencer Feb 17 '22 at 14:37

Роман Коптев · Answer 2 · 2020-02-12T21:49:43.337

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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.

edited Feb 12 '20 at 21:49

answered Oct 20 '15 at 20:28

Роман Коптев

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Never say never! Constexpr std::vector is coming in C++2a. – peppe Oct 24 '19 at 19:48

score 22 · Accepted Answer · answered Oct 20 '15 at 16:34

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

answered Oct 20 '15 at 16:34

cdonat

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Actually, it is declared constexpr: c++20 draft: https://wg21.link/std20#lstlisting.22.-2632 and still in current draft sources: https://github.com/cplusplus/draft/blob/6091e264374349c12aa775cd66e5f062d9c7b9e4/source/containers.tex#L5706 it seems, it's just not yet implemented... – T S Sep 10 '21 at 09:36
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It seems, the only compiler, that currently supports it is Visual Studio: https://en.cppreference.com/w/cpp/compiler_support#:~:text=constexpr%20std%3A%3Avector – T S Sep 10 '21 at 10:03

Cannot create constexpr std::vector

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