Is empty struct defined by C++ standard?

Question

Is there any std::empty struct or something similar or do I need to define my own:

struct empty{}; 

This can be used very nice in combination with std::conditional or other new std features and I wonder if the standard defines it or not.

Answer 1

There is a section to add this kind of construct as part of the Variant proposal (n4542).

After being voted on,

What do we want to call the “empty_t” stand-in type?
empty_t 4
empty 4
one_t 1
blank 6
blank_t 7
monostate 7

Runoff:
blank* 3
monostate 8

the agreed upon name would be: std::monostate.

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It would be defined the following way:

// 2.?, Explicitly default-constructed alternative struct monostate {}; bool operator<(const monostate&, const monostate&) constexpr { return false; } bool operator>(const monostate&, const monostate&) constexpr { return false; } bool operator<=(const monostate&, const monostate&) constexpr { return true; } bool operator>=(const monostate&, const monostate&) constexpr { return true; } bool operator==(const monostate&, const monostate&) constexpr { return true; } bool operator!=(const monostate&, const monostate&) constexpr { return false; }