non-defaulted operator doesn’t generate == and != in C++20

by

This is by design.

[class.compare.default] (emphasis mine)

3 If the class definition does not explicitly declare an ==
operator function, but declares a defaulted three-way comparison
operator function, an == operator function is declared implicitly
with the same access as the three-way comparison operator function.
The implicitly-declared == operator for a class X is an inline
member and is defined as defaulted in the definition of X.

Only a defaulted <=> allows a synthesized == to exist. The rationale is that classes like std::vector should not use a non-defaulted <=> for equality tests. Using <=> for == is not the most efficient way to compare vectors. <=> must give the exact ordering, whereas == may bail early by comparing sizes first.

If a class does something special in its three-way comparison, it will likely need to do something special in its ==. Thus, instead of generating a potentially non-sensible default, the language leaves it up to the programmer.

More Related Contents:

  1. What is the (“spaceship”, three-way comparison) operator in C++?
  2. Why default three-way operator (spaceship ) generates equality operator (==) and user define three-way operator not?
  3. Can I take the address of a function defined in standard library?
  4. What exactly are C++ modules?
  5. Why does same_as concept check type equality twice?
  6. How to use source_location in a variadic template function?
  7. Class type non-type template parameter initialization does not compile
  8. Designated initializers in C++20
  9. How to use c++20 modules with CMake?
  10. Why does aggregate initialization not work anymore since C++20 if a constructor is explicitly defaulted or deleted?
  11. What is `constinit` in C++20?
  12. Does constraint subsumption only apply to concepts?
  13. C++20 constexpr vector and string not working
  14. C++ Concept that requires a member function with an OutputIterator as parameter
  15. Ramifications of C++20 requiring two’s complement
  16. Familiar template syntax for generic lambdas
  17. ‘auto’ as a template argument placeholder for a function parameter
  18. Comparing 3 modern c++ ways to convert integral values to strings
  19. In which access control context are concepts evaluated?
  20. Is C++20 ‘char8_t’ the same as our old ‘char’?
  21. Range TS idioms and the mysterious auto &&
  22. asio How to change the executor inside an awaitable?
  23. Why does Clang 12 refuse to initialize aggregates in the C++20 way?
  24. Is lambdification of a concept an improvement or bad practice?
  25. Why C++ ranges “transform -> filter” calls transform twice for values that match the filter’s predicate?
  26. Why is operator!= removed in C++20 for many standard library types?
  27. How to use C++20’s likely/unlikely attribute in if-else statement
  28. Will consteval allow using static_assert on function arguments?
  29. if constexpr – why is discarded statement fully checked?
  30. if constexpr – why is discarded statement fully checked?

Leave a Comment