Which operator needs to be overridden in order to use std::set in the C++ code?

Question

This is an interview question.

Referring to the sample code, which one of the operators needs to be overridden in order to use std::set<Value>

 #include<iostream>

 class Value
 {
      std::string   s_val;
      int           i_val;
  public:
      Value(std::string s, int i): s_val(s) , i_val(i){}
 };

 // EOF

 /*
 a       operator !=
 b       operator >
 c       operator <=
 d       operator >=
 e       operator <
 */

Actually, I do not understand why an operator needs to be overridden here. "set" does not allow duplicated elements, maybe operator != needs to be overridden ?

Answer 1

You don't have to override any operator, the std::set class template allows you to provide a comparison function as a template parameter. But if you were to provide an operator, the one needed is bool operator<(). This operator has to implement strict weak ordering. See this std::set documentation.

The reason strict weak ordering is used is because set is an ordered container, typically implemented as a self-balancing binary tree. So it is not enough to know whether two elements are the same or not. The set must be able to order them. And the less than operator or the comparator functor are also used to test for element equality.

Answer 2

You need to implement operator< for your type. The implementation must follow strick weak ordering to be able to use with associative containers from Standard library such as std::set and std::map.

Read about:

• Strict Weak Ordering

An example here:

• std map composite key

Answer 3

A set keeps out the duplicates without needing operator= or operator!= by using the notion of equivalence. Two items are equivalent if neither is less than the other:

if (!(a < b || b < a))
    // equivalent!

Answer 4

To speed up the enforcement of no duplicate elements and generally checking if element is in its usually some sort of a tree and only needs operator <. (The only usage of less is enforced by the standard, the rest is just the avarage implementation)