added explicit ref-qualifiers for Error::what() and a additional over…

[BestITUserEUW]

…load to allow moving Error::what_ to caller if Error is an rvalue. Adresses #504

[gemini-code-assist]

Summary of Changes

Hello @BestITUserEUW, 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!

This pull request enhances the Error class by refining how its internal error message is accessed. It introduces explicit reference qualifiers to the what() method, providing distinct behaviors for lvalue and rvalue Error objects. Specifically, an rvalue-qualified overload enables the efficient moving of the error message, which can lead to performance improvements by eliminating unnecessary string copies when dealing with temporary Error objects. This change directly addresses issue #504.

Highlights

• Explicit Reference Qualifiers: The what() method in the Error class now explicitly uses an lvalue-reference qualifier (const &) to indicate it operates on lvalue Error objects.
• Rvalue Overload for what(): An additional overload for what() has been introduced with an rvalue-reference qualifier (&&), allowing the internal error message (what_) to be moved out of an rvalue Error object, improving efficiency by avoiding copies.

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[BestITUserEUW]

@liuzicheng1987 Do you want me to setup tests for Error::what() ?

[gemini-code-assist]

Code Review

This pull request introduces ref-qualified overloads for Error::what(), which is a great improvement for allowing moves from rvalue Error objects, enhancing both performance and safety. My review includes a suggestion to add a const && overload to fully support all value categories and prevent a potential compilation failure with const rvalue Error objects.

[gemini-code-assist]

The introduction of lvalue- and rvalue-qualified overloads for what() is a great improvement for both safety and performance. However, the current implementation has a small gap: it doesn't handle const rvalues.

For example, code like this would fail to compile with the current changes:

const rfl::Error make_const_error() { return rfl::Error("an error"); }
// ...
auto msg = make_const_error().what(); // Fails to compile

This is because make_const_error() returns a const rvalue. The && overload of what() is not const-qualified, and the const & overload requires an lvalue, so neither can be called.

To fix this and support const rvalues, you could add a const && overload. This overload would behave like the const & one, returning a const reference to the string, which is safe. This would make the interface for what() complete with respect to value categories.

  const std::string& what() const & { return what_; }
  /// Moves the error message out of Error object and leaves what_ in a moved from state.
  std::string what() && { return std::move(what_); }
  /// Returns the error message from a const rvalue.
  const std::string& what() const && { return what_; }

[liuzicheng1987]

@BestITUserEUW , no, I think that would be overkill. Thanks for your contribution!