CODEWITHSUNDEEP
c++comparison
Puppy
Puppy

Reputation: 146910

Implementing other comparison operators in terms of operator< in one call

I've seen that, if you have operator<, you can implement, say, operator>= as !(a < b). However, when I went to the C++ Committee in Bristol, they said that you can implement all the comparison operators this way. Particularly, when considering types which have non-trivial comparison operators (consider an arbitrarily long string, especially if this is Unicode), this can be done with only one call to operator<. I cannot conceive of how this could be done.

How can I implement operator> and operator<= with just one call to operator< and none to the other comparison operators?

Upvotes: 14

Views: 2141

Answers (1)

Casey
Casey

Reputation: 42554

  • a > b == b < a
  • a <= b == !(b < a)
  • a >= b == !(a < b)

It's even possible to implement equality in terms of less than (Kind of abusing my meta-syntax here):

  • (a == b) == (!(a < b) && !(b < a))
  • (a != b) == (a < b || b < a)

Although I wouldn't suggest doing so in practice, since it requires two comparisons and can generally be implemented more efficiently directly.

Upvotes: 20

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