Why does g++ states, that there is "no matching function for call cbegin(o)" for some valarray o?

Question

Please consider the following code:

using custom_t = std::valarray<unsigned>;
custom_t o;
unsigned acc = std::accumulate(std::cbegin(o), std::cend(o), 0);

g++-5 says

No matching function for call to cbegin(custom_t&)

If I use std::begin(o) and std::end(o) instead, everything works. Is this a compiler bug? The code compiles with Visual Studio 2015.

[Compiles in libc++.](http://coliru.stacked-crooked.com/a/e78b7b445c344d4f) I think that libstdc++ is implementing some kind of SFINAE for types that satisfy the Container concept. Since `std::valarray` is not a Container, the call fails. — David G, Aug 27 '15 at 17:24

Jonathan Wakely · Answer 1 · 2015-08-27T17:34:50.647

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This is a libstdc++ bug, I've just created https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67374

edited Aug 27 '15 at 17:34

answered Aug 27 '15 at 17:13

Jonathan Wakely

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It doesn't make any sense to me, why `std::begin` and `std::end` are specialized for `std::valarray`, but `std::cbegin` and `std::cend` are not. What's the reason? – 0xbadf00d Aug 27 '15 at 17:16
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@0xbadf00d the reason is probably noone thought of doing or even specifying that. Which kind of makes sense, considering valarray is meant to be rather lightweight and not so feature-rich. I must admit I've never used valarray myself, because no library I know can actually make use of the additional aliasing restrictions (is there anything else preferable about valarray?) – Marcus Müller Aug 27 '15 at 17:19
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*"and so doesn't support all the non-member functions for accessing containers."*, I'm wondering why 1. there is non-member function `std::begin` overloaded for a `const` `valarray`, 2. a template function `std::cbegin` from `` is not picked – Piotr Skotnicki Aug 27 '15 at 17:20
I wouldn't be asking without [checking](http://melpon.org/wandbox/permlink/nk9hVzRrwfrSwGkl) – Piotr Skotnicki Aug 27 '15 at 17:27
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@PiotrSkotnicki, it's because `` includes `` before defining the `begin` and `end` overloads for valarrays, so they aren't visible to `cbegin` and can't be called. Which looks like a bug. – Jonathan Wakely Aug 27 '15 at 17:30
@PiotrSkotnicki, bug report created, thanks for making me take a proper look – Jonathan Wakely Aug 27 '15 at 17:35

Marcus Müller · Answer 2 · 2015-08-27T17:08:20.610

-4

There is no

std::cbegin.

Use

unsigned acc = std::accumulate(o.cbegin(), o.cend(), 0);

which feels far more object-oriented.

edited Aug 27 '15 at 17:08

answered Aug 27 '15 at 17:02

Marcus Müller

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the `std::cbegin` has only been introduced in C++14, not in C++11 – Marcus Müller Aug 27 '15 at 17:04
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So, there *is* a `std::cbegin´. I've tagged my question with C++14. – 0xbadf00d Aug 27 '15 at 17:05
Yes, but if you looking for duplicates, http://stackoverflow.com/questions/31038061/support-of-stdcbeginin-c14 pops up which explains that gcc has problems with it, prior to gcc-5.1 (you're using gcc-5, probably) :) cheers! – Marcus Müller Aug 27 '15 at 17:06
You know that your "solution" is not even syntactically correct? – 0xbadf00d Aug 27 '15 at 17:07
There is no Version 5. They started with 5.1. – 0xbadf00d Aug 27 '15 at 17:08
ah shoot. Typo. Thanks! – Marcus Müller Aug 27 '15 at 17:08
hm, is your g++ run with `--std=c++14` (I feel stupid for asking at this point) – Marcus Müller Aug 27 '15 at 17:09
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Unfortunately, `std::valarray` has no member function `cbegin/cend`. So, your *solution* is still wrong. – 0xbadf00d Aug 27 '15 at 17:09
Yes, I compile with `--std=c++14`. – 0xbadf00d Aug 27 '15 at 17:09
@0xbadf00d, in that case, `valarray` seems to violate http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf ' (p.747) Container requirements, which would explain why there's no `std::cbegin` applicable to `valarray`. – Marcus Müller Aug 27 '15 at 17:12
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`valarray` violates the container requirements **because it's not a container** – Jonathan Wakely Aug 27 '15 at 17:14
@JonathanWakely: well, that *would* explain why `std::cbegin` can't get a constant iterator from it. – Marcus Müller Aug 27 '15 at 17:15

Why does g++ states, that there is "no matching function for call cbegin(o)" for some valarray o?

2 Answers2