gcc-4.9.x using c++1y - error: call to non-constexpr function

[th0ma7]

Please provide the following information

libtorrent version (or branch): using 2.0.6

platform/architecture: any (armv7, v8, ppc, x86_64, i686)

compiler and compiler version: gcc-4.9.4 and gcc-4.9.3

please describe what symptom you see, what you would expect to see instead and
how to reproduce it.

Synology Linux NAS using DSM-v6.x uses an older gcc-4.9.x (while newer DSM-7.x uses gcc-7.5). Issue is only occurring on DSM-6 with older gcc using -std=c++1y option to achieve c++14 "equivalence".

error is the following to which I hope there may be a workaround it for this older compiler c++1y:

../../include/libtorrent/file_storage.hpp:238:78: error: call to non-constexpr function ‘const _Tp& std::min(const _Tp&, const _Tp&) [with _      Tp = long int]’

See full log for more details: build-aarch64-6.1.log

Thnx in advance!

[NotTsunami]

Changing constexpr to const for the affected line should work (See documentation for [std::min](https://en.cppreference.com/w/cpp/algorithm/min)):

libtorrent/include/libtorrent/file_storage.hpp

Line 236 in e9bbf16

static constexpr std::int64_t max_file_size = (std::min)(

I'm wondering if the issue lies in c++1y being an earlier state of c++14.

See Arvid's comment. It's still a constant expression, this is likely a GCC issue.

[th0ma7]

I created a new v2-gcc49-c++1y branch and gave that a shot:

236                 static const std::int64_t max_file_size = (std::min)(
237                         (std::int64_t(1) << 48) - 1
238                         , std::int64_t((std::numeric_limits<int>::max)() / 2) * default_block_size);

And still getting that same error:

      ../../include/libtorrent/file_storage.hpp:238:78: error: call to non-constexpr function ‘const _Tp& std::min(const _Tp&, const _Tp&) [with _Tp = long int]’

My C++ programming are quite far behind me but isn't line 238 , std::int64_t((std::numeric_limits<int>::max)() / 2) * default_block_size); character 78 be right before * ?

EDIT: Just noticed, character 78 closes the (std::min)(

[arvidn]

the calculation of the constant has to be done at compile time, as a constrexpr value. As I'm sure you can see, that is a constant expression. However, it seems GCC 4.9 had not yet marked min() as a constexpr function. You could implement your own min() that is constexpr in that same header to see if that solves it. But I suspect this is not the only thing preventing libtorrent from building on GCC 4.9

[th0ma7]

Closing this issue as I ended-up using libtorent 1.2.x for GCC-4.9+ builds and version 2.x for newer Synology Linux DSM which comes with GCC-7.5. Getting version 2 to build on such old GCC was just too much of a pain (and probably not worth the effort as libtorrent 1.x is still maintained).

Thnx.