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[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 {// M is the number of placeholderssee below _1; see below _2; ⋮ see below _M;}
The number M of placeholders isimplementation-defined.
All placeholder types meet the Cpp17DefaultConstructible andCpp17CopyConstructible 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;
Placeholders are freestanding items ([freestanding.item]).