Re: std::sort
On Nov 26, 2:31 pm, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:
Jeff Schwab wrote:
Would std::sort ever compare an object with itself?
I don't believe the Standard prohibits that.
I'm not talking
about two distinct, equal-valued objects, but rather this == &that.
The container being sorted is a std::vector.
Objects are never compared using '=='. They are compared using '<'
or the functor provided.
The standard sort algorithm defaults to using std::less. Since we are
sorting a container of pointers, we provide our own comparison
routine, which internally compares the results of the dereferenced
pointers. The pointers themselves are first checked for equality,
since an object is always equal to itself. This is very common, basic
stuff, and I assume you already know it.
I've never seen this, but a coworker says he is. NB: I can't post
sample code that reproduces the issue, nor do I claim any bug in the
STL implementation (GCC 3.4.2). I'm just hoping a definitive answer
resides in one of the brains who frequent this group.
Tell your coworker to provide you with proof.
No need. The following example took ~ 2 min. to write and debug.
Note the explicit operator== to which I referred earlier. I'm using a
POD object type here (int) for simplicity.
#include <iostream>
template<typename T>
bool
deref_less(T* p, T* q)
{
if (p == q) {
std::clog << "warning: object at " << p << " compared to
itself.\n";
}
return *p < *q;
}
#include <algorithm>
#include <vector>
int
main()
{
typedef int Object;
std::vector<Object*> v;
for (int i = 0; i < 1000; ++i) {
v.push_back(new Object(i));
}
std::sort(v.begin(), v.end(), deref_less<Object>);
// cleanup ...
return 0;
}