What is the (“spaceship”, three-way comparison) operator in C++?

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This is called the three-way comparison operator.

According to the P0515 paper proposal:

There’s a new three-way comparison operator, <=>. The expression a <=> b returns an object that compares <0 if a < b, compares >0 if a > b, and compares ==0 if a and b are equal/equivalent.

To write all comparisons for your type, just write operator<=> that
returns the appropriate category type:

  • Return an _ordering if your type naturally supports <, and we’ll efficiently generate <, >, <=, >=, ==, and !=;
    otherwise return an _equality, and we’ll efficiently generate
    == and !=.

  • Return strong if for your type a == b implies f(a) == f(b) (substitutability, where f reads only comparison-salient state
    accessible using the nonprivate const interface), otherwise return
    weak.

The cppreference says:

The three-way comparison operator expressions have the form

lhs <=> rhs   (1)

The expression returns an object that

  • compares <0 if lhs < rhs
  • compares >0 if lhs > rhs
  • and compares ==0 if lhs and rhs are equal/equivalent.

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