[func.bind.place]
22 General utilities library [utilities]
22.10 Function objects [function.objects]
22.10.15 Function object binders [func.bind]
22.10.15.5 Placeholders [func.bind.place]
namespace std::placeholders { see below _1; see below _2; ⋮ see below _M; }
The number M of placeholders is implementation-defined.
All placeholder types meet the Cpp17DefaultConstructible and Cpp17CopyConstructible requirements, and their default constructors and copy/move constructors are constexpr functions that do not throw exceptions.
It is implementation-defined whether placeholder types meet the Cpp17CopyAssignable requirements, but if so, their copy assignment operators are constexpr functions that do not throw exceptions.
Placeholders should be defined as: inline constexpr unspecified _1{};
If they are not, they are declared as: extern unspecified _1;