C++ Linker Error With Class static constexpr

by

I don’t think this is a bug. If you change the constexpr to const, it still fails, with the exact same error.

You’ve declared S::X, but not defined it anywhere, so there’s no storage for it. If you do anything with it that needs to know the address of it then you’ll need to define it somewhere also.

Examples:

int main() {
      int i = S::X; // fine
      foo<S::X>(); // fine
      const int *p = &S::X; // needs definition
      return std::min(S::X, 0); // needs it also
}

The reason for this is that constexpr can be evaluated at compile time, but it’s not required to be evaluated as such, and can equally happen at runtime. It doesn’t instruct “the compiler to always treat the symbol as an expression”, it hints that it would be sensible and permissible to do so if the compiler felt like it.

More Related Contents:

  1. What’s the difference between constexpr and const?
  2. Undefined reference to static constexpr char[]
  3. Why do constant expressions have an exclusion for undefined behavior?
  4. const vs constexpr on variables
  5. Create N-element constexpr array in C++11
  6. Is is_constexpr possible in C++11?
  7. Is it safe to link C++17, C++14, and C++11 objects
  8. constexpr and initialization of a static const void pointer with reinterpret cast, which compiler is right?
  9. C++11 – static_assert within constexpr function?
  10. Why do we need to mark functions as constexpr?
  11. Is constexpr supported with lambda functions / expressions?
  12. Computing length of a C string at compile time. Is this really a constexpr?
  13. Does constexpr imply inline?
  14. how to initialize a constexpr reference
  15. C++11 constexpr function pass parameter
  16. Constexpr pointer value
  17. Populate An Array Using Constexpr at Compile-time
  18. Is it legal to declare a constexpr initializer_list object?
  19. Static constexpr odr-used or not?
  20. constexpr function parameters as template arguments
  21. Is it possible to use std::string in a constexpr?
  22. Concat two `const char` string literals
  23. Is a constexpr array necessarily odr-used when subscripted?
  24. constexpr not compiling in VC2013
  25. constexpr const vs constexpr variables?
  26. `static constexpr` function called in a constant expression is…an error?
  27. Why can I call a non-constexpr function inside a constexpr function?
  28. Why does constexpr static member (of type class) require a definition?
  29. Why is reinterpret_cast not constexpr?
  30. Is constexpr really needed?

Leave a Comment