I’d like to do it all by myself..
You should only compare the values of Obj::field2 if the values of Obj::field1 are equal.
The easy-to-understand way:
/* This will meet the requirements of Strict-Weak-Ordering */
if (a.field1 != b.field1) return a.field1 < b.field1;
else return a.field2 < b.field2;
The correct (recommended) way:
The “correct” way of implementing it uses only operator< to compare the fields, the below looks more complicated than it really is.
It will however yield the same result as the easy-to-understand example previously written.
return a.field1 < b.field1 || (
!(b.field1 < a.field1) && a.field2 < b.field2
);
There must be a way of implementing operator< without causing a lot of headache?
C++11
You can use std::tuple from the STL which already have operator< for multiple fields defined, such as in the below example.
#include <utility>
...
inline bool
operator< (Obj const& lhs, Obj const& rhs)
{
return std::tie (lhs.field1, lhs.field2) < std::tie (rhs.field1, rhs.field);
}
C++03
If your compiler doesn’t have support for C++11 yet and you only need to compare two fields from each object you could use std::pair instead.
The reason for std::make_pair is the same as in the previous example using std::tie.
#include <utility>
...
inline bool
operator< (Obj const& lhs, Obj const& rhs)
{
return std::make_pair (lhs.field1, lhs.field2)
< std::make_pair (rhs.field1, rhs.field2);
}
using std::pair will require copies of the members to be created, which in some circumstances is undesirable.
Is this really recommended practise?
See the below question/answers for more information, but to sum it up; the c++11 approach doesn’t cause that much overhead and it’s very simple to implement.