Help us to choose minimum required C++ standard for OpenCV 5.0

[vpisarev]

In OpenCV 4.x the minimum required standard is C++ 11. For OpenCV 5.0 we are going to raise the bar. Note that in the subsequent 5.x releases we could raise it up again, but not down. For your reference, please, check C++ standard compliance status in different mainstream compilers (scroll to the bottom to see C++14 and C++17 support status): https://en.cppreference.com/w/cpp/compiler_support

[fengyuentau]

Mainstream compilers (gcc, clang, msvc) have almost complete support of C++17 (Some are missing one or two item though). Also std::string_view is available since C++17 since it is needed in a evolution proposal.

Vote for C++17 🙋

[ShiqiYu]

My only concern about C++17 is some low-cost embedded systems. GCC7 is needed for C++17, and GCC7 was released in 2017. GCC 7.4 is in Ubuntu 18.04. I think very few systems still use an earlier version of Ubuntu 18.04. I think C++17 is possible for OpenCV 5.

[edvn0]

Vote for C++17!

[fspindle]

I'm involved in a number of software developments that are intended to be embedded in computers on board satellites and for which c++14 or later standards are not authorised. I therefore vote in favour of maintaining a minimum c++11 standard.

[LaurentBerger]

Do you ask for a c++11 checkbox?

[mshabunin]

I believe it is possible to define compilation standard by default, but avoid using parts of language and standard library which were not supported or fully implemented. For example, libstdc++ states full support of memory_resource since version 9.1 (https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017), but older compiler, like GCC 7.5.0 (Ubuntu 18.04) already supported --std=c++17 option. We can avoid using memory_resource and remain compatible with older compilers with partial C++17 support.

Compiler versions in some Linux OSes:

• Ubuntu 18.04 - GCC 7.5, Clang 6.0 (EOL Apr 2023)
• Ubuntu 20.04 - GCC 9.4, Clang 10
• Ubuntu 22.04 - GCC 11.4 Clang 14
• Debian 10 - GCC 8.3, Clang 7
• Rocky Linux 8 (CentOS?, RHEL) - GCC 8.5, Clang 16
• NVIDIA Jetson Linux - GCC 9.3 (?)

So we can target C++17, but also check build on Ubuntu 18.04 to make sure older toolchains with incomplete standard support can build OpenCV. Additionally, modern modules can require more features or higher C++ standard and turn themselves off automatically if compiler does not satisfy their requirements. Example: https://github.com/opencv/opencv_contrib/blob/4.x/modules/cvv/CMakeLists.txt#L1

It would be nice to see examples of older toolchains/platforms which are currently used by people to buld OpenCV to take them in account too.

[sabudilovskiy]

Where c++20?

[vpisarev]

@sabudilovskiy, sorry, we only offer to choose from the feasible for us options :) C++ 20 introduces a lot of great stuff, and some of it we could definitely use, like "modules", but not all of the features, including modules, are properly implemented in all mainstream compilers. Many of the features are implemented, but require quite fresh versions of compilers that users may not have installed by default, see the detailed post above.

For example, GCC, the default C++ compiler for Linux distributions, currently declares full support of C++ 17 (whereas C++ 20 support is still partial and is described as "experimental") and the default standard is "gnu++17" (C++ 17 with GCC extensions) since GCC 11: https://gcc.gnu.org/projects/cxx-status.html. C++ 14 (well, gnu++14) has been the default standard from GCC 6 till GCC 10.x inclusive (but, as Maksim pointed out, starting from GCC 8.x it's quite safe to use C++ 17 except for a few missing libstdc++ features).

Let's wait for a few years while compilers and distro's will catch up. Maybe then we will make another leapfrog straight to C++ 23 (or even C++ 26).

[LaurentBerger]

@vpisarev What's opencv team choice?

[Ginkgo-Biloba]

Vote C++17.
Some old gcc has trouble with C++14 constexpr function. When I compiler OpenCV / spdlog with gcc 5.x, it will issue errors like constexpr function call non-constexpr function in protobuf / fmtlib, so I have to delete constexpr / use C++11. Also https://stackoverflow.com/questions/54326901/why-gcc5-4-doesnt-compile-a-constexpr-function-calling-a-non-constexpr-function