Get the size of an std::array as r-value

Question

Consider the following snippet of code:

#include<array>
#include<cstdint>

const std::array<int, 3> array{0, 1 , 2};

template<class string_type>
auto parse(string_type&& name) {
        const auto s = std::uint8_t{array.size()};
        return s;
}

While it compiles using gcc 9.3.0 (the default on Ubuntu 20.04), it fails with gcc 11.2.0 (built from sources) with the following error message:

test2.cpp: In function ‘auto parse(string_type&&)’:
test2.cpp:8:47: error: no matching function for call to ‘std::array<int, 3>::size(const std::array<int, 3>*)’
    8 |         const auto s = std::uint8_t{array.size()};
      |                                     ~~~~~~~~~~^~
In file included from test2.cpp:1:
/opt/modules/install/gcc/11.2.0/include/c++/11.2.0/array:176:7: note: candidate: ‘constexpr std::array<_Tp, _Nm>::size_type std::array<_Tp, _Nm>::size() const [with _Tp = int; long unsigned int _Nm = 3; std::array<_Tp, _Nm>::size_type = long unsigned int]’
  176 |       size() const noexcept { return _Nm; }
      |       ^~~~
/opt/modules/install/gcc/11.2.0/include/c++/11.2.0/array:176:7: note:   candidate expects 0 arguments, 1 provided

Running example

Besides the fact that it does not make much sense, I can't find where is the error, can you help me?

Something is wrong with your build of the compiler. ‘auto parse(string_type&&)’: -- rvalue string is actually return value type, gcc somehow treated it as parameter. and in array.size gcc treated implicit this as explicit parameter. — Alex Guteniev
– Alex Guteniev, Commented Oct 27, 2021 at 16:44
It seems a gcc bug. It's also reproducible with a "simplified" snippet. — Bob__
– Bob__, Commented Oct 27, 2021 at 21:06
Yep, problem is that i am not sure on what to search to find similar bug. Opened bug 102980, hopefully is not a duplicate. — a_random_user
– a_random_user, Commented Oct 28, 2021 at 7:37

Intelligent Shade of Blue · Accepted Answer · 2021-10-27 21:56:04Z

2

It appears to be a bug in:

gcc-10: https://godbolt.org/z/95TTv4z9P and
gcc-11: https://godbolt.org/z/KWMs4MMcK

It works fine in:

gcc-9: https://godbolt.org/z/YMqsMjr7x and
clang: https://godbolt.org/z/6Kq9nY7bo

To work around, you can do either this:

const auto s = static_cast<std::uint8_t>(array.size());

or this:

const std::uint8_t s = array.size();

or this (but please don't):

const auto s = std::uint8_t( array.size() );

I would suggest this:

#include<array>
#include<cstdint>

const std::array<int, 3> array{ 0, 1 , 2 };

template<class string_type>
auto parse(string_type&& name)
{
    const std::uint8_t s = array.size();
    return s;
}

Running example

edited Oct 27, 2021 at 21:56

answered Oct 27, 2021 at 19:56

Intelligent Shade of Blue

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2 Comments

a_random_user Over a year ago

Even with std::uint64_t does not work. Btw, I should have the narrowing warning only when the size of the array is greater than 256, is it correct?

Intelligent Shade of Blue Over a year ago

@a_random_user hmm... it seems to be related to it being a function template: godbolt.org/z/847aPboE8

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