error: variable 'boost::pfr::detail::fake_object<Test>' is used but not defined

[sha512sum]

In file included from error.cpp:1:
In file included from /usr/include/boost/pfr.hpp:14:
In file included from /usr/include/boost/pfr/core_name.hpp:17:
In file included from /usr/include/boost/pfr/detail/core_name.hpp:23:
In file included from /usr/include/boost/pfr/detail/core_name20_static.hpp:21:
/usr/include/boost/pfr/detail/fake_object.hpp:20:16: error: variable 'boost::pfr::detail::fake_object' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage
20 | extern const T fake_object;
| ^
error.cpp:7:29: note: used here
7 | (void)boost::pfr::detail::fake_object;
| ^

#include <boost/pfr.hpp>

int main() {
  struct Test {
    int data;
  };
  (void)boost::pfr::detail::fake_object<Test>;
};

[Bela Schaum]

fake_object is intentionally designed to be devoid of linkage, signifying its exclusive purpose for member name retrieval. It serves as a companion to declval<T>() by providing a compile-time "memory address" without actual utilization.

This behavior is not a defect, as long as the object remains encapsulated within the detail namespace.
However, it becomes a bug if the error can be replicated without explicitly using fake_object.

[sha512sum]

#include <boost/pfr.hpp>
int main() {
  struct Test {
    int field;
  };
  static_assert(boost::pfr::get_name<0, Test>() == "field");
};

[Bela Schaum]

Could you please provide details about the operating system, compiler (including version), and toolchain you are using? I am facing challenges in replicating the issue, and automated tests are yielding positive results. It's possible that the issue may be specific to your environment.

[sha512sum]

Gentoo Linux

clang --version
clang version 17.0.4
Target: x86_64-gentoo-linux-musl
Thread model: posix
InstalledDir: /usr/lib/llvm/17/bin
Configuration file: /etc/clang/x86_64-gentoo-linux-musl-clang.cfg

[sha512sum]

Can you show me a test that worked successfully? I didn't find one. Error on g++-13 (Gentoo 13.2.1_p20231014 p10) 13.2.1 20231014:

In file included from /usr/include/boost/pfr/detail/core_name20_static.hpp:21,
                 from /usr/include/boost/pfr/detail/core_name.hpp:23,
                 from /usr/include/boost/pfr/core_name.hpp:17,
                 from /usr/include/boost/pfr.hpp:14,
                 from lu.cpp:1:
/usr/include/boost/pfr/detail/fake_object.hpp:20:16: error: ‘const main()::Test boost::pfr::detail::fake_object<main()::Test>’, declared using local type ‘const main()::Test’, is used but never
defined [-fpermissive]
   20 | extern const T fake_object;

[Bela Schaum]

The limitation says that

The Boost.PFRs extraction of field name has some limitations that depend on a C++ Standard and compiler capabilities:
T should be usable like extern T t;, i.e. has a non-internal linkage.

Function local classes has no external linkage, so it will not work with those structures.

---

The tests can be found here, and the tests are run in GitHub actions, like here

[Bela Schaum]

Hi @denzor200 ,

Is it possible to rename the fake_object or create a pre-checker object with a more meaningful name, such as passed_type_has_no_external_linkage with some comment? This would enhance the clarity of error messages.

[Denis Mikhailov]

Hi, @schaumb ,

Sure, PR will be created in the future

[Denis Mikhailov]

Here is the PR: #146