Sort function for std:sort with & parametr c++

Question

I want to use reference to my instance as a parameter of sort function.

I have vector<CMail> log , in class CMail have function which compare as I want.

And a want to sort log so I have:

bool sortFunction(CMail a, CMail b){
  return (a.CompareByTimeStamp(b) < 0) ? true : false;
}

and then

sort(log.begin(), log.end(), sortFunction);

It works fine. But can I have parameters of function as a reference like this?

bool sortFunction(CMail &a, CMail &b){
...
}

When I did this, my code didn't compile.

How can I do this?

Answer 1

In short: const ness. You need:

bool sortFunction(const CMail& a, const CMail& b){
  return (a.CompareByTimeStamp(b) < 0);
}

Which also means your signature for CompareByTimeStamp must be:

int CompareByTimeStamp(const Cmail& other) const; // (inside class Cmail {...};)
//                      ^ b is const         ^ a is const

See here . This is all because comparing two objects should not change them.