当我使用STL集时，我应该重载==运算符吗？

Question

当我想使用STL集来存储C ++中的自定义对象时，有很多人说：你应该重载<operator，但如果我想使用find（）方法，我认为它可能会使用==运算符来实现它。任何人都可以给我一些指示？

Answer 1

The Standard Library set::find uses equivalence instead of equality to find values. You don't need to provide operator == , just operator < (or whatever comparison operator you specified for set , std::less being the default).

If you are wondering how your element would be found, then assuming the default ordering find(x) would return the element e for which:

!( x < e || e < x )

Answer 2

Since std::set s require something that specifies a strict weak ordering . operator== is insufficient for this task.

You should overload operator< only if it makes sense for the class. If it doesn't, far better would be utilizing the fact that std::set has as a second template parameter Compare . Hence, defining a comparison struct/function and passing this as the 2nd parameter of your set is another, generally preferable option.

The final option is to specialize std::less for your type. For example:

namespace std
{ 
    template <>
    struct less<CustomClass>
    { ... };
}

Answer 3

Most Standard Library algorithms and containers use operator< (or a comparison function you provide such that it returns true if the lhs element is smaller) for ordering and searching. Algorithms for use with unordered containers will use operator==.

For example, std::lower_bound() will either return the first element matching the search criteria from a sorted container, the largest element still smaller than the search term if the exact term is not found, or an iterator to the end of the container if no element is larger than what you searched for. It does this with operator<, no other operator is required. It takes a number of operations in the order of log(n).

Every other comparison operator (>, >=, <=), with the exception of != and ==, can be derived from operator<.

std::find, however, requires that the type used be Equality Comparable. You can find the reference here: http://en.cppreference.com/w/cpp/algorithm/find

So if you're working with ordered elements, you need operator<. If you are working with unordered elements, you need operator==.

Answer 4

set , map , multiset and multimap use only the comparison function you give them, which defaults to std::less , which is generally operator< . The unordered version in C++11 have a more complex protocol; they don't need an order comparison (they're unordered ) but they need both equality and hash.

This is also true of all of the standard algorithms which involve ordering ( sort , nth_element , lower_bound , binary_search , etc.). However, the find algorithm (and other similar algorithms, such as count , search , mismatch , etc., do require an equality function, defaulting to operator== . No standard library algorithm requires both.

Since you specifically asked about the find member function, the answer would be that the set and all member functions will work fine with just order comparison.

Still, it's almost always a good idea to define all comparison operators if you're going to define operator< .