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 placeholders see below _1; see below _2; ⋮ see below _M; }
1
#
The number M of placeholders is implementation-defined.
2
#
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.
3
#
Placeholders should be defined as: inline constexpr unspecified _1{};
If they are not, they are declared as: extern unspecified _1;
4
#
Placeholders are freestanding items ([freestanding.item]).

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