[question] How to validate cppstd and compiler for header-only packages?

[uilianries]

Using the new validate approach recommended by the migration guide, we should use self.info.settings instead of self.settings as in the past. Also, for header-only projects, we should consider self.info.clear() instead of self.info.header_only()

However, we have cases where we need to check the cppstd or even the compiler version to be sure that's is enough, for instance:

from conan import ConanFile
from conan.errors import ConanInvalidConfiguration
from conan.tools.files import get, copy
from conan.tools.build import check_min_cppstd
from conan.tools.scm import Version
from conan.tools.layout import basic_layout

import os

required_conan_version = ">=1.52.0"

class PackageConan(ConanFile):
    settings = "os", "arch", "compiler", "build_type"
    no_copy_source = True

    @property
    def _minimum_cpp_standard(self):
        return 17

    @property
    def _compilers_minimum_version(self):
        return {
            "Visual Studio": "15.7",
            "msvc": "191",
            "gcc": "7",
            "clang": "7",
            "apple-clang": "10",
        }

    def layout(self):
        basic_layout(self, src_folder="src")

    def package_id(self):
        self.info.clear()

    def validate(self):
        if self.info.settings.get_safe("compiler.cppstd"):
            check_min_cppstd(self, self._minimum_cpp_standard)
        minimum_version = self._compilers_minimum_version.get(str(self.info.settings.compiler), False)
        if minimum_version and Version(self.info.settings.get_safe("compiler.version")) < minimum_version:
            raise ConanInvalidConfiguration(f"{self.ref} requires C++{self._minimum_cpp_standard}, which your compiler does not support.")

    def source(self):
        get(self, **self.conan_data["sources"][self.version],
            destination=self.source_folder, strip_root=True)

    def build(self):
        pass

    def package(self):
        copy(self, pattern="LICENSE", dst=os.path.join(self.package_folder, "licenses"), src=self.source_folder)
        copy(self, pattern="*.h", dst=os.path.join(self.package_folder, "include"), src=os.path.join(self.source_folder, "include"))

    def package_info(self):
        self.cpp_info.bindirs = []
        self.cpp_info.frameworkdirs = []
        self.cpp_info.libdirs = []
        self.cpp_info.resdirs = []

This is a very regular header-only recipe from CCI. The important thing here is: it requires C++17, and at least some new compiler version.

When using the new self.info.clear, then self.info.settings.compiler will no longer exist when running conan create which is fine, since we are only copying files to the package. However, when running conan install to get that pre-built package, the same occurs, because the package ID is unique, so the user will not check that validate and eventually will fail during the compilation due an older and incompatible compiler.

So the question is, how to validate the compiler in this case?

• I've read the CONTRIBUTING guide.

[danimtb]

I have created a PR to the docs clarifying this case. As there still some uncertainties around how to manage cppstd, using self.settings in validate() is still the recommended way to do it. See conan-io/docs#2778

[memsharded]

Will be closed by #12555

[memsharded]

Closed by #12555, will be in beta.6

And for 1.55 the approach is: use self.settings in validate(), not self.info.settings