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Reference

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object
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std::allocator_arg

constexpr allocator_arg_t allocator_arg = allocator_arg_t();
Allocator arg
Value used as first argument to the constructor of certain classes to specify that the next argument is an allocator object.

The value is a compile-time constant that carries no state, and is merely used to disambiguate between constructor signatures.

allocator_arg_t is an empty class.