C++ – Meaning of >?= Operator

c++g++operators

Looking through this C++ BigInt library and found the BigInt.cpp file. At the top there is a a comment at the top about compatibility:

This class was written for the g++ compiler and uses some of the g++ extensions (like "long double" and the ">?=" operator).

What does that >?= operator do? I can't find a reference to it anywhere else.

Best Answer

It's a GCC extension that was removed in GCC version 4.2 and later.

The equivalent of a >?= b is a = max(a,b);

There is also a very similar operator a <?= b which means the same as a = min(a, b);.

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C++ – Understanding the <=> (Spaceship) Operator in C++

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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C++ – Understanding the <=> (Spaceship) Operator in C++

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