Question

I have a struct with some members and I have an implemented operator== for it. Is it safe to implement the operator< with the help of operator==? I want to use this struct in a set, and I want to check that this struct is unique.

struct Data
{
  std::string str1;
  std::string str2;
  std::string str3;
  std::string str4;

  bool operator==(const Data& rhs)
  {
    if (str1 == rhs.str1
     && str2 == rhs.str2
     && str3 == rhs.str3
     && str4 == rhs.str4
       )
      return true;
    else
      return false;
  }

  // Is this ok??
  bool operator<(const Data& rhs)
  {
    return !this->operator==(rhs);
  }
}

So when I insert this struct to a std::set what will happen?

Answer 1

Nope, it’s quite unsafe. The simplest way to implement it is through std::tie.

#include <tuple>
struct Data
{
  std::string str1;
  std::string str2;
  std::string str3;
  std::string str4;

  bool operator<(const Data& rhs) const // you forgot a const
  {
      return 
      std::tie(str1, str2, str3, str4) < 
      std::tie(rhs.str1, rhs.str2, rhs.str3, rhs.str4);
  }
}