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added explicit ref-qualifiers for Error::what() and a additional over… #505

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Sep 29, 2025
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added explicit ref-qualifiers for Error::what() and a additional over… #505

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4 changes: 3 additions & 1 deletion include/rfl/Result.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,9 @@ class Error {
Error& operator=(Error&&) = default;

/// Returns the error message, equivalent to .what() in std::exception.
const std::string& what() const { return what_; }
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_); }
Comment on lines +33 to +35
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high

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_; }


private:
/// Documents what went wrong
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