What is the <=> ("spaceship", three-way comparison) operator in C++?

Question

While I was trying to learn about C++ operators, I stumbled upon a strange comparison operator on cppreference.com,^* in a table that looked like this:

"Well, if these are common operators in C++, I better learn them", I thought. But all my attempts to elucidate this mystery were unsuccessful. Even here, on Stack Overflow I had no luck in my search.

Is there a connection between <=> and C++?

And if there is, what does this operator do exactly?

_{* In the meantime cppreference.com updated that page and now contains information about the<=>operator.}

Answer 1

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.