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Issue with set comparators

Started by Premandrake Nov 29, 2002 at 2:23 PM 5 replies 1.2k views

Original Post

Nov 29, 2002 at 2:23 PM

Okay, first the issue is I want to use a set with a custom comparator, which is normally fine. Normally something like this would work fine:

  
struct CompFunc
{
  bool operator()(const int *p1,const int *p2)
  {
    // Do stuff with p1 and p2

  }
};

std::set<int*,CompFunc> tada;

However, what I''m looking to do is inside my comparison function I need to use data from a class to do the comparison. That proved to be more complicated that I thought. I ended up doing something like this:

  
struct CompFunc
{
  bool operator()(const int *p1,const int *p2)
  {
    // Do stuff with p1 and p2

  }
  static int dataNeeded;
};

int CompFunc::dataNeeded;

CompFunc::dataNeeded = value;
std::set<int*,CompFunc> tada;

This feels wrong to me, but I''m having a tough time coming up with a nice encapsulated way to do this. Any suggestions?

Kylotan

Mod

3,487

Kylotan

Dec 1, 2002 at 8:45 PM

  
struct CompFunc
{
  CompFunc(int data) : dataNeeded(data) {}

  bool operator()(const int *p1,const int *p2)
  {
    // Do stuff with p1 and p2

  }

private:
  int dataNeeded;
};

CompFunc comparison(value);
std::set<int*,comparison> tada;

Premandrake

Author

Premandrake

Dec 2, 2002 at 2:50 PM

Ah! Thank you very much, that solves my problems very well

sark

Dec 2, 2002 at 3:45 PM

quote:
Original post by Kylotan

    
struct CompFunc
{
  CompFunc(int data) : dataNeeded(data) {}

  bool operator()(const int *p1,const int *p2)
  {
    // Do stuff with p1 and p2

  }

private:
  int dataNeeded;
};

CompFunc comparison(value);
std::set<int*,comparison> tada;

That wont work. "comparison" is a variable which is only known at runtime - std::set requires a type for its comparison functor.

Kylotan

Mod

3,487

Kylotan

Dec 2, 2002 at 5:38 PM

joanusdmentia

Dec 2, 2002 at 6:20 PM

well, if you know what your value will be at compile-time you could just use a template parameter:

  
template<unsigned int dataNeeded>
struct CompFunc
{
  bool operator()(const int *p1,const int *p2)
  {
    // Do stuff with p1 and p2

  }
};

std::set< int*,CompFunc<value> > tada;

Premandrake

Author

Premandrake

Dec 2, 2002 at 9:33 PM

Indeed, well done Sark, you can also tell I didn't test that out

. I gave up on it actually as I was unable to arrive at a solution that worked across threads, that has now been remedied though.

Joanusdmentia, unfortunately the values are not known at compile time (and are not integral types also), so template parameters do not solve the problem, thanks for the suggestion though!

Thanks again Kylotan, I've actually implemented that now and it works great.

Edit: wow, bad spelling again...

[edited by - premandrake on December 2, 2002 10:37:47 PM]

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