How to access a constexpr string that is defined in a struct?

Question

How to properly access the string "bye" from the struct?

#include <iostream>
static constexpr char hi[] = "hi";
struct S{ static constexpr char bye[] = "bye"; };

int main(int argc, char *argv[]) {
  typedef struct S ST;

  std::cout << hi << std::endl;              // this prints "hi" 
  std::cout << sizeof(ST::bye) << std::endl; // this correctly prints 4
  std::cout << ST::bye << std::endl;         // this does not compile with "undefined reference"
}

I'm working with a c++ framework that has some configuration in this format (in multiply nested structs even) to make its values available during compile time. I'm not deep enough into C++ to grasp the underlying issue here. Also I cannot argue about why this approach on implementing the configuration was chosen and cannot change it.

Does this answer your question? [constexpr static member before/after C++17](https://stackoverflow.com/questions/39646958/constexpr-static-member-before-after-c17) — KamilCuk, May 03 '20 at 07:51
Use the `::` operator to access static members of a class, as in `S::bye` — Michaël Roy, May 03 '20 at 08:02
Thanks KamilCuk! The link showed me how to fix the issue. But what is the underlying reason why 'sizeof' is working and the printing the string isn't? — Unchained, May 03 '20 at 09:39

score 0 · Accepted Answer · answered May 03 '20 at 09:43

This can be made working by redefining the static constexpr outside the struct:

#include <iostream>
static constexpr char hi[] = "hi";
struct S{ static constexpr char bye[] = "bye"; };

constexpr char S::bye[]; // <<< this line was missing

int main(int argc, char *argv[]) {
  typedef struct S ST;

  std::cout << hi << std::endl;              // this prints "hi" 
  std::cout << sizeof(ST::bye) << std::endl; // this correctly prints 4
  std::cout << ST::bye << std::endl;         // Now this prints "bye"
}

Or by compiling using c++17 which makes this obsolete.

How to access a constexpr string that is defined in a struct?

1 Answers1