I want to use STL sort with a functor

//
//  This compiles and runs if I comment out the
//  call to sort().
//  I would like to sort on key.  But, I have no clue
//  where to start on the syntax.
//
#include <vector>
#include <algorithm>
#include <iostream>
class keyPLUS {
  public:
     int key;
     int something_else;
} ;

int main() {

  vector<keyPLUS> KeyContainer(2);

  KeyContainer[0].key = 5;
  KeyContainer[0].something_else = 13;

  KeyContainer[1].key = 4;
  KeyContainer[1].something_else = 17;
 
  sort( &KeyContainer[0], &KeyContainer[2],
      what_do_I_put_here );
 
  cout << " [0].key =" << KeyContainer[0].key
       << " [0].something_else =" << KeyContainer[0].something_else << endl ;
  cout << " [1].key =" << KeyContainer[1].key
       << " [1].something_else =" << KeyContainer[1].something_else << endl ;
}

klopterAsked:
Who is Participating?
 
jasonclarkeConnect With a Mentor Commented:
assuming my comment above is sufficient.
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fjfpCommented:
If you look at the prototype for the template function sort in <algorithm>, you'll see that it takes 2 iterators.  It expects the beginning and end of the vector (in this case).

Try:
sort(KeyContainer.begin(), KeyContainer.end());

Hope it helps.
Frank
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jasonclarkeCommented:
> sort(KeyContainer.begin(), KeyContainer.end());

No, that will only work if you have a less than operator defined on the object, which you don't in this case.  You have a couple of options:

i) define a < operator

class keyPLUS {
  public:
     int key;
     int something_else;
     bool operator<(const keyPlus& rhs)
     { return key < rhs.key; // or whatever it should be }
} ;

and then the sort call given above will work (you could also define a non-member function less than operator).

or

ii) define a functor, as you asked originally, the best way is to use a function object:

class keyPlusLessThan
{
   bool operator()(const keyPlus& lhs, const keyPlus& rhs)
   { return lhs.key < rhs.key;  // ? }
};

then the sort call becomes:

sort(keyContainer.begin(), keyContainer.end(), keyPlusLessThan());

you can also do it with a pointer to a function if you want,  but the function object is probably better.  Let me know if you want to see an example with a function pointer.
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klopterAuthor Commented:
Jason is right.  Sort( begin, end ) only works if you have defined operator <.

Ken
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jasonclarkeCommented:
so, is my answer good enough?

or do you need more/different help?
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klopterAuthor Commented:
Jason,
  I meant to accept your answer.  My excuse is that there was a bit of a delay in getting my rejection accepted (busy network I guess) and then I forgot.

Thanks,
  Ken
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jasonclarkeCommented:
No problem, It's sometimes difficult to know what to do around here.  Sometimes completely inappropriate answers get accepted, just because they are answers  and somebody else has added the answer in a comment.

So it seems better to get an answer in if you think it is appropriate...
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klopterAuthor Commented:
Makes sense to me.
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