Why C++ STL containers use "less than" operator< and not "equal equal" operator== as comparator?
c++, comparator, dictionary, operator-overloading
Solution
`std::map<K,D>` needs to be able to sort. By default is uses `std::less<K>`, which for non-pointers uses `<`1.
Using the rule that you demand the least you can from your users, it synthesizes "equivalence" from `<` when it needs it (`!(a<b) && !(b<a)` means `a` and `b` are equivalent, ie, neither is less than the other).
This makes it easier to write classes to use as key components for a `map`, which seems like a good idea.
There are `std` containers that use `==` such as `std::unordered_map`, which uses `std::hash` and `==`. Again, they are designed so that they require the least from their users -- you don't need full ordering for `unordered_` containers, just equivalence and a good `hash`.
As it happens, it is really easy to write a `<` if you have access to `<tuple>`.
struct Address {
long m_IPv4Address;
bool isTCP;
bool operator< (const Address& o) const {
return
std::tie( m_IPv4Address, isTCP )
< std::tie( o.m_IPv4Address, o.isTCP );
}
};
which uses `std::tie` defined in `<tuple>` to generate a proper `<` for you. `std::tie` takes a bunch of data, and generates a `tuple` of references, which has a good `<` already defined.
1 For pointers, it uses some comparison that is compatible with `<` where `<` behaviour is specified, and behaves well when `<` does not. This only really matters on segmented memory model and other obscure architectures.
Problem
While implementing a comparator operator inside a custom class for `std::map`, I came across this question and couldn't see anywhere being asked. Apart from the above question, also interested to know in brief, how `operator<` would work for `std::map`. Origin of the question: ``` struct Address { long m_IPv4Address; bool isTCP; bool operator< (const Address&) const; // trouble }; ```