Re: set::find with custom equality operator==

tohava <>
Wed, 3 Feb 2010 08:49:18 CST
On Feb 3, 1:12 am, dragoncoder <> wrote:

Hi all, I have the following code.

// db_operators.h


#include <cstring>

struct ecodb2{
   unsigned long long id;
   char code[21];


inline bool operator==(const ecodb2& lhs,
                        const ecodb2& rhs)
     std::cout << "Some text" << std::endl;
     return (( == || !std::strcmp(lhs.code, rhs.code));


struct Compare
     bool operator()(const ecodb2& lhs,
                     const ecodb2& rhs) const
         bool retval = false;
         if( <
             retval = true;
         else if( ==
             retval = std::strcmp(lhs.code, rhs.code);
         return retval;



// main.cpp

#include <iostream>
#include <cstring>
#include <set>

#include <db_operators.h>

int main()
     std::set<ecodb2, Compare> industry_codes;

     ecodb2 val; = 1;

     std::strncpy(val.code, "GOV", sizeof(val.code));

     std::strncpy(val.code, "ABC", sizeof(val.code));

     std::set<ecodb2>::iterator it = industry_codes.find(val);
     if(it != industry_codes.end())
         std::cout << "Found in set" << std::endl;
         std::cout << "Not found in set" << std::endl;


While running this code, I expect to see Found in set (as either id or
code match meaning a perfect match) and also "Some text" being
printed, becasue set::find should call my custom operator==() function
to find a match but the output I get is "Not found in set" and also
"Some text" is not getting printed.

I don't think it should be possible for 2 variables
of type ecodb2 to be satisfy both x < y and x == y.
This can happen with your code (in the case
where x and y share an id but not a code). My guess
would be that find checks at some point for some x and y
whether x < y, and if this is the case, it assumes that x != y,
which in your example is NOT the case.

I would recommend implementing this using two maps,
one with id as key, and one with code.

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