node/convert: support conversion for std::string_view

[tchaikov]

as the key is not always nul terminated, it'd be handy to use a std::string_view when a key is expected.

before this change, we would have FTBFS if the key is a string_view when looking up a node in a map:

/usr/include/yaml-cpp/node/detail/impl.h: In instantiation of ‘bool YAML::detail::node::equals(const T&, YAML::detail::shared_memory_holder) [with T = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’:
/usr/include/yaml-cpp/node/detail/impl.h:122:26:   required from ‘YAML::detail::node* YAML::detail::node_data::get(const Key&, YAML::detail::shared_memory_holder) const [with Key = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’
/usr/include/yaml-cpp/node/detail/node_ref.h:64:55:   required from ‘YAML::detail::node* YAML::detail::node_ref::get(const Key&, YAML::detail::shared_memory_holder) const [with Key = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’
/usr/include/yaml-cpp/node/detail/node.h:125:53:   required from ‘YAML::detail::node* YAML::detail::node::get(const Key&, YAML::detail::shared_memory_holder) const [with Key = std::basic_string_view<char>; YAML::detail::shared_memory_holder = std::shared_ptr<YAML::detail::memory_holder>]’
/usr/include/yaml-cpp/node/impl.h:392:71:   required from ‘const YAML::Node YAML::Node::operator[](const Key&) const [with Key = std::basic_string_view<char>]’
../../src/config/base_config.cc:20:23:   required from here
/usr/include/yaml-cpp/node/detail/impl.h:93:25: error: incomplete type ‘YAML::convert<std::basic_string_view<char> >’ used in nested name specifier
   93 |   if (convert<T>::decode(Node(*this, pMemory), lhs)) {
      |       ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

after this change, a full specialization for convert<std::string_view is added so that we can lookup a node using key of std::string_view, like

string_view key = "yet_another_key";
auto value = node[key];

tested with

$ cmake -B build -D CMAKE_CXX_STANDARD=20 -D YAML_CPP_BUILD_TESTS=ON -G Ninja
$ cmake --build build yaml-cpp-tests
$ ctest --test-dir build --output-on-failure

Signed-off-by: Kefu Chai tchaikov@gmail.com

[tchaikov]

@jbeder hi Jesse, could you please help review this change at your convenience?

[tchaikov]

@jbeder hello Jesse, ping for reviews.

[jbeder]

Can you sync to head so it can run the tests? Otherwise LGTM.

[tchaikov]

@jbeder sure. thank you for your review! rebased and repushed.

[tchaikov]

[0 / 1] [Prepa] BazelWorkspaceStatusAction stable-status.txt
ERROR: D:/a/yaml-cpp/yaml-cpp/BUILD.bazel:14:11: Compiling src/binary.cpp failed: (Exit 1): vc_installation_error_x64.bat failed: error executing command (from target //:yaml-cpp) external\local_config_cc\vc_installation_error_x64.bat /nologo /DCOMPILER_MSVC /DNOMINMAX /D_WIN32_WINNT=0x0601 /D_CRT_SECURE_NO_DEPRECATE /D_CRT_SECURE_NO_WARNINGS /bigobj /Zm500 /EHsc /wd4351 /wd4291 ... (remaining 20 arguments skipped)

The target you are compiling requires Visual C++ build tools. 
Bazel couldn't find a valid Visual C++ build tools installation on your machine. 

Visual C++ build tools seems to be installed at C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC 
But Bazel can't find the following tools: 
    VCVARSALL.BAT, cl.exe, link.exe, lib.exe, ml64.exe 
for x64 target architecture 

Please check your installation following https://bazel.build/docs/windows#using 

my guess is that this failure is an occurrence of https://github.com/bazelbuild/bazel/issues/18592 .

[jbeder]

Yeah, I think you're right. Bad timing :)

It looks like they're working on a fix, so once they release it, we can rerun and then I'll merge this PR. (I hesitate to merge it with a broken test even if it's almost certainly unrelated.)

[tchaikov]

sure. let's keep an eye on the update on this bazel issue!

[tchaikov]

@jbeder hi Jesse, it seems that https://github.com/bazelbuild/bazel/issues/18592 has been fixed. could you please give a green light to the workflow, so we can verify the change?

[0xFireWolf]

Thanks for your work, but I found an issue after upgrading to 0.8.0.

MSVC initializes __cplusplus with a value of 199711L, if users don't specify the option Zc:__cplusplus explicitly. As a result, your template specialization is disabled by default on Windows. I wonder whether it would be better to use the feature-test macro __cpp_lib_string_view` instead. What's your opinions? @tchaikov @jbeder

#if __cpp_lib_string_view >= 201606L
// ...
#endif

[tchaikov]

__cpp_lib_string_view

@0xFireWolf indeed. but instead of assuming the existence of __cpp_lib_string_view, i'd prefer checking its existence instead, see #1222

[tchaikov]

Thanks for your work, but I found an issue after upgrading to 0.8.0.

MSVC initializes __cplusplus with a value of 199711L, if users don't specify the option Zc:__cplusplus explicitly. As a result, your template specialization is disabled by default on Windows. I wonder whether it would be better to use the feature-test macro __cpp_lib_string_view` instead. What's your opinions? @tchaikov @jbeder

#if __cpp_lib_string_view >= 201606L
// ...
#endif

@0xFireWolf yaml-cpp 0.6.0 and up requires C++11 and up. i don't think we are able to use C++98 with the master HEAD. how do you test yaml-cpp in your project?

[0xFireWolf]

How do you test yaml-cpp in your project?

@tchaikov My project uses C++20 and is compiled by GCC 10+, Clang 13+, and VS2019+.

[tchaikov]

@0xFireWolf so why do you care about C++standard of 199711L?

[0xFireWolf]

Shall we keep the conversation in one place?

[tchaikov]

yeah. ahh, but kind of late. agreed, we should have used an issue for the discussion. but i am fine either place: here or https://github.com/jbeder/yaml-cpp/pull/1222 .