SPDLOG_USE_STD_FORMAT is not an option in CMake config file

[Krzmbrzl]

The CMake config file should check for whether SPDLOG_USE_STD_FORMAT is defined and if so, set the compile definitions accordingly. At the moment it seems like installing a version of spdlog that was compiled with SPDLOG_USE_STD_FORMAT=OFF forces downstream projects to use libfmt instead of std::format

[tt4g]

What exactly is a downstream project? Does this mean you're importing spdlog as a subproject using CMake?
If so, isn't this an issue with the specification where CMake variables are inherited by the subproject?

[tt4g]

Based on ValeevGroup/SeQuant#384, the problem appears to stem from the system-installed spdlog being built without defining SPDLOG_USE_STD_FORMAT.
CMake variables affect libraries built within the project, but they do not affect libraries found via find_package().
This is because if the macro definitions of built libraries differ from when they were originally built, application binary compatibility cannot be guaranteed.

The issue is avoided by the change of ValeevGroup/SeQuant#386 because the spdlog installed on the system is no longer used, and spdlog is now built as part of your project.

Since SPDLOG_USE_STD_FORMAT is defined in cmake/modules/FindOrFetchSpdlog.cmake, this spdlog uses std::format. However, the spdlog installed on the system continues to use libfmt, so it is expected that an incompatible spdlog will be loaded at runtime, causing a crash.

If an spdlog without SPDLOG_USE_STD_FORMAT defined is detected, either cause the project build to fail or configure your project to support both std::format and libfmt.

[Krzmbrzl]

What exactly is a downstream project?

Any project that uses a given library/project is often referred to as a "downstream" project (vs. "upstream", which refers to the used library).

Does this mean you're importing spdlog as a subproject using CMake?

We're (optionally) using FetchContent to fetch and build spdlog, if no suitable version is found on the system.

If so, isn't this an issue with the specification where CMake variables are inherited by the subproject?

No - I don't think that is related to the current situation.

the problem appears to stem from the system-installed spdlog being built without defining SPDLOG_USE_STD_FORMAT.

Yes

CMake variables affect libraries built within the project, but they do not affect libraries found via find_package().

Not necessarily. Options can (sometimes) be handled in CMake config files - hence my request to do the same here as well.

This is because if the macro definitions of built libraries differ from when they were originally built, application binary compatibility cannot be guaranteed.

This is kinda my question/request: Can spdlog be built in a way makes usage of libfmt optional and if so, there should be an option in the CMake config file to enable or disable use of libfmt.

[tt4g]

Not necessarily. Options can (sometimes) be handled in CMake config files - hence my request to do the same here as well.

Hmm, I don't know. If spdlog was configured to use libfmt at build, then the library depends on libfmt. Even if you remove that dependency in the CMake configuration file, the fact that the library depends on libfmt doesn't change. Right?

This is kinda my question/request: Can spdlog be built in a way makes usage of libfmt optional and if so, there should be an option in the CMake config file to enable or disable use of libfmt.

This option is SPDLOG_USE_STD_FORMAT=ON.

spdlog/CMakeLists.txt

Line 92 in f1d748e

option(SPDLOG_USE_STD_FORMAT "Use std::format instead of fmt library." OFF)

[tt4g]

If this is my misunderstanding and the proposed fix works, please submit PR.

Since the branch that prevents specifying SPDLOG_FMT_EXTERNAL=ON and SPDLOG_USE_STD_FORMAT=ON together already exists in CMakeLists.txt, please remember to make it behave the same way.

spdlog/CMakeLists.txt

Lines 101 to 107 in f1d748e

	if(SPDLOG_USE_STD_FORMAT AND SPDLOG_FMT_EXTERNAL_HO)
	message(FATAL_ERROR "SPDLOG_USE_STD_FORMAT and SPDLOG_FMT_EXTERNAL_HO are mutually exclusive")
	endif()
	if(SPDLOG_USE_STD_FORMAT AND SPDLOG_FMT_EXTERNAL)
	message(FATAL_ERROR "SPDLOG_USE_STD_FORMAT and SPDLOG_FMT_EXTERNAL are mutually exclusive")
	endif()

[Krzmbrzl]

Even if you remove that dependency in the CMake configuration file, the fact that the library depends on libfmt doesn't change. Right?

It's fine if the library depends on libfmt as long as it (also) allows to use std::format. I don't know spdlog's internals though so I can't say whether this is possible.

This option is SPDLOG_USE_STD_FORMAT=ON.

No, there is no option in the CMake config file (the one that gets installed). That's what this request is about.

[tt4g]

That is impossible.
spdlog depends on either libfmt or std::format.
The dependent library is determined at build time, and switching the dependent library at runtime is not possible.

spdlog/include/spdlog/common.h

Lines 145 to 146 in f1d748e

	#ifdef SPDLOG_USE_STD_FORMAT
	namespace fmt_lib = std;

spdlog/include/spdlog/common.h

Lines 174 to 175 in f1d748e

	#else // use fmt lib instead of std::format
	namespace fmt_lib = fmt;

Note that since spdlog v1 supports C++11, libfmt is the default formatting library, and std::format is an option added after C++20 was released.

[Krzmbrzl]

Maybe it would be worth it to make this possible as this makes spdlog more or less unusable for projects using C++20, which don't want to duplicate all formatting implementations for std::format libfmt?

Anyway, thanks for your input.