The C++11 and C++14 standard libraries defined many constexpr global variables like this:
constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
constexpr allocator_arg_t allocator_arg = allocator_arg_t();
constexpr error_type error_ctype = /*unspecified*/;
constexpr defer_lock_t defer_lock{};
constexpr in_place_t in_place{};
constexpr nullopt_t nullopt(/*unspecified*/{});
In C++17, all of these constexpr variables were respecified as inline constexpr
variables. The inline keyword here means the same thing as it does on an inline
function: “This entity might be defined in multiple TUs;
all those definitions are identical; merge them into one definition at link time.”
If you look at the generated code for one of these variables in C++14, you’ll
see something like this (Godbolt):
