Re: Set with Sort routine

From:

"Doug Harrison [MVP]" <dsh@mvps.org>

Newsgroups:

microsoft.public.vc.stl

Date:

Tue, 27 Jun 2006 09:42:57 -0500

Message-ID:

<5fg2a2dukclivdc950llsormrn04a7bfuq@4ax.com>

On Tue, 27 Jun 2006 08:10:22 -0500, "Bruce Chastain"
<bchastain@XNOSPAMXhyperfeed.com> wrote:

I'm trying to create a set with my sort criteria specified, using MS
Developer Studio 7 and I can't get the syntax correct. Everything I try
creates a compile time error. Here's the basics:

#include <set>

using namespace std;

typedef struct {
int value;
} MyElem;

This is C++, so write:

struct MyElem
{
int value;
};

Even in C, you should specify a tag, e.g.

// C only
typedef struct MyElem
{
int value;
} MyElem;

bool MyCompare( const MyElem& elem1, const MyElem& elem2 )
{
return( elem1.value < elem2.value );
}

void main()
{
set<MyElem, MyCompare> database;
}

The last line compiles with an error:

misc2.cpp(30) : error C2923: 'std::set' : 'MyCompare' is invalid as template
argument '#2', type expected
misc2.cpp(23) : see declaration of 'MyCompare'

Any clues? I've also tried:

The error message says "type expected", and that is indeed what you need to
supply:

   set<MyElem, bool(*)(const MyElem&, const MyElem&)> database(MyCompare);

You also have to tell it which function to use, hence the ctor argument.

You have two alternatives:

1. Define operator< for MyElem, and then you will automatically take
advantage of std::less, e.g.

   bool operator<(const MyElem& elem1, const MyElem& elem2)
   {
      return elem1.value < elem2.value;
   }

   set<MyElem> database;

2. Define a class to perform the comparison, e.g.

   struct Comp
   {
      bool operator()(const MyElem& elem1, const MyElem& elem2)
      {
         return elem1.value < elem2.value;
      }
   };

   set<MyElem, Comp> database;

Both these alternatives may help the compiler inline the comparisons, and
they are preferred to using function pointers.

--
Doug Harrison
Visual C++ MVP