bazel: set c++ flags for all project depending on os/compiler

Question

Recently I wanted to migrate all my projects to bazel and finally I am building them with command setting c++ standard manually for different operations systems:

bazel build //... --cxxopt=-std=c++17 --compilation_mode opt
bazel build //... --cxxopt=/std:c++17 --compilation_mode opt

I would set it in .bzl file but setting standard is different for linux and windows, so I will anyway need to override it with --cxxopt=/std:c++17 for MSVC. I tried to add global variable to .bzl file and load it for all projects, e.g.:

# variables.bzl
COPTS = ["-std=c++17"]

This works (but as I mentioned before it will anyway require changing for different platforms). Then I've tried:

# variables.bzl
COPTS = select({
        "//tools/cc_target_os:windows": ["/std:c++17"],
        "//conditions:default": ["-std=c++17"],
})

But it lead to error:

ERROR: path/to/project/BUILD:2:1: no such package 'tools/cc_target_os': BUILD file not found on package path and referenced by '//project:smth'

Is it possible to set c++ flags to all projects depending on platform (compiler)? Unfortunately, I could not find any working example. Could you please help me?

Answer 1

Use @bazel_tools//src/conditions:windows instead.

# variables.bzl
COPTS = select({
        "@bazel_tools//src/conditions:windows": ["/std:c++17"],
        "//conditions:default": ["-std=c++17"],
})