Conan Option `import_std` Should Not Be On By Default

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Introduction

When working with Conan and CMake, it's essential to understand the various options and settings that can affect the build process. One such option is import_std, which is used to enable experimental import std support in C++. However, as we'll explore in this article, this option should not be enabled by default.

The Problem

While trying to integrate mp-units with the Conan + CMake (Live At Head) approach, we encountered an error that pointed to the import_std option. The error message indicated that the CXX_MODULE_STD property on the target required the __CMAKE::CXX26 target to exist, but it was not provided by the toolchain. This issue was caused by the experimental import std support not being enabled when detecting the toolchain.

The Error Message

The error message we encountered was as follows:

CMake Error in lib/CMakeLists.txt:
  The "CXX_MODULE_STD" property on the target
  "mp-units__mp-units@synth_9137914f277a" requires that the "__CMAKE::CXX26"
  target exist, but it was not provided by the toolchain.  Reason:

    Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled (usually a `project()` call)


CMake Error in lib/CMakeLists.txt:
  The "CXX_MODULE_STD" property on the target
  "mp-units__core@synth_9137914f277a" requires that the "__CMAKE::CXX26"
  target exist, but it was not provided by the toolchain.  Reason:

    Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled (usually a `project()` call)


CMake Error in lib/CMakeLists.txt:
  The "CXX_MODULE_STD" property on the target
  "mp-units__systems@synth_9137914f277a" requires that the "__CMAKE::CXX26"
  target exist, but it was not provided by the toolchain.  Reason:

    Experimental `import std` support not enabled when detecting toolchain; it must be set before `CXX` is enabled (usually a `project()` call)


-- Generating done (0.3s)
CMake Generate step failed.  Build files cannot be regenerated correctly.

ERROR: conanfile.py (data-driven_game_engine/0.0.1): Error in build() method, line 103
        cmake.configure(build_script_folder=None if self._dev else "lib")
        ConanException: Error 1 while executing

The Solution

To resolve this issue, we simply need to set the import_std option to False in the Conan configuration. This will disable the experimental import std support, which is not required for the build process.

Code Example

Here's an example of how to set the import_std option to False in the Conan configuration:

from conans import ConanFile

class DataDrivenGameEngineConan(ConanFile):
    name = "data-driven_engine"
    version = "0.0.1"
    settings = "os", "compiler", "build_type", "arch"
    options = {"import_std": [True, False]}
    default_options = {"import_std": False}

By setting the import_std option to False by default, we can avoid this issue and ensure that the build process completes successfully.

Conclusion

Introduction

In our previous article, we discussed the issue of the Conan option import_std being enabled by default, which can cause problems with the build process. In this article, we'll answer some frequently asked questions about this issue and provide additional information to help you understand and resolve it.

Q: What is the Conan option import_std?

A: The Conan option import_std is used to enable experimental import std support in C++. This feature allows you to import the standard library in your C++ code, but it's not enabled by default in Conan.

Q: Why is import_std not enabled by default?

A: import_std is not enabled by default because it's an experimental feature that's not widely supported. Enabling it can cause issues with the build process, especially when working with Conan and CMake.

Q: What are the consequences of enabling import_std?

A: Enabling import_std can cause the following issues:

  • The build process may fail due to missing dependencies or incompatible libraries.
  • The CXX_MODULE_STD property on the target may require the __CMAKE::CXX26 target to exist, but it may not be provided by the toolchain.
  • The build process may not complete successfully, resulting in errors and warnings.

Q: How can I resolve the issue of import_std being enabled by default?

A: To resolve this issue, you can simply set the import_std option to False in the Conan configuration. This will disable the experimental import std support, which is not required for the build process.

Q: How do I set the import_std option to False in Conan?

A: To set the import_std option to False in Conan, you can add the following code to your conanfile.py file:

from conans import ConanFile

class DataDrivenGameEngineConan(ConanFile):
    name = "data-driven_engine"
    version = "0.0.1"
    settings = "os", "compiler", "build_type", "arch"
    options = {"import_std": [True, False]}
    default_options = {"import_std": False}

By setting the import_std option to False by default, you can avoid issues like the one described in this article and ensure that the build process completes successfully.

Q: What are the benefits of disabling import_std?

A: Disabling import_std has several benefits, including:

  • Improved build process stability and reliability.
  • Reduced risk of errors and warnings due to incompatible libraries or missing dependencies.
  • Simplified build process configuration and management.

Conclusion

In conclusion, the Conan option import_std should not be enabled by default. By setting this option to False by default, you can avoid issues like the one described in this article and ensure that the build process completes successfully. We hope this Q&A article has provided you with the information you need to resolve this issue and improve your build process.