Added support for exception-free TOML; resolves #436

[liuzicheng1987]

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[gemini-code-assist]

Hello @liuzicheng1987, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

Summary of Changes

Hello team, Gemini here to provide a quick summary of this pull request. This PR, authored by liuzicheng1987, aims to add support for parsing TOML strings without relying on C++ exceptions. This is a valuable addition for users who compile their projects with exceptions disabled, often for performance or embedded system constraints. The changes primarily involve modifying the TOML reading logic to use conditional compilation based on whether TOML_EXCEPTIONS is defined, switching between a try-catch block and checking the result object returned by the underlying TOML parsing library.

Highlights

• Exception-Free TOML Parsing: The main goal is to allow users to parse TOML configuration without requiring C++ exceptions to be enabled in their build environment.
• Conditional Compilation: The implementation uses #if TOML_EXCEPTIONS to provide two different code paths for parsing TOML strings, one for exception-enabled builds and one for exception-disabled builds.
• Error Handling: In the exception-free path, the code now checks the result of the ::toml::parse function and returns an error object if parsing fails, instead of throwing an exception.
• Function Signature Update: The read function template taking a std::string_view is marked noexcept when exceptions are disabled, reflecting that it will not throw.

Changelog

• include/rfl/toml/read.hpp
  • Modified the read function template that takes a std::string_view to use conditional compilation (#if TOML_EXCEPTIONS).
  • Added a try-catch block for the exception-enabled path.
  • Implemented error checking on the result object for the exception-free path.
  • Marked the function as noexcept in the exception-free path.

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[gemini-code-assist]

Code Review

This pull request aims to add support for exception-free TOML reading by introducing a noexcept specifier to the rfl::toml::read function and handling potential exceptions from the underlying toml-plus-plus parser. This is a valuable improvement for users who prefer or require exception-free APIs.

The core idea of using conditional compilation (#if TOML_EXCEPTIONS) and try-catch blocks is sound. However, there are critical concerns regarding the noexcept correctness that need to be addressed to prevent potential std::terminate calls.

Summary of Findings

• noexcept correctness in TOML_EXCEPTIONS path: The catch block for when TOML_EXCEPTIONS is defined is too narrow (std::runtime_error). It should be widened to const std::exception& to prevent std::terminate if other standard exceptions (like std::bad_alloc) occur during TOML parsing or subsequent processing within the try block.
• noexcept correctness in non-TOML_EXCEPTIONS path: The path for when TOML_EXCEPTIONS is not defined lacks exception handling for the read<T, Ps...> call. If this call throws any C++ exception (e.g., std::bad_alloc), it will violate the noexcept contract of the main function, leading to std::terminate. A try-catch(const std::exception&) block should be added.

Merge Readiness

The pull request addresses an important aspect of API design by aiming for exception-free TOML operations. However, due to the critical issues identified regarding noexcept correctness, which could lead to program termination, I recommend that these changes be addressed before merging. Once these concerns are resolved, the PR will be in a much better state. I am not authorized to approve pull requests, so please ensure further review and approval by authorized maintainers after addressing the feedback.