Re: Set with Sort routine

From:

"Heinz Ozwirk" <hozwirk.SPAM@arcor.de>

Newsgroups:

microsoft.public.vc.stl

Date:

Wed, 28 Jun 2006 00:21:04 +0200

Message-ID:

<44a1af52$0$29128$9b4e6d93@newsread4.arcor-online.net>

"Bruce Chastain" <bchastain@XNOSPAMXhyperfeed.com> schrieb im Newsbeitrag
news:%23QOGNsemGHA.5076@TK2MSFTNGP02.phx.gbl...

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;

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:

So the compiler expects a type, but you give it the address of a function.
There are several solutions, but the easiest is to rename "MyCompare" to
"operator<" and replace the definition of database with

    set<MyElem> database.

If you need different sets of MyElem with different sorting, you can also
define MyCompare as a class (or struct), like

struct myCompare
{
    bool operator()(MyElem const& lhs, MyElem const& rhs) const
    {
        return lhs.value < rhs.value;
    }
};

set<MyElem> database( MyCompare );

but then I get this error:

misc2.cpp(31) : error C2664: 'std::set<_Kty>::set(const
std::set<_Kty>::key_compare &)' : cannot convert parameter 1 from 'bool
(const MyElem &,const MyElem &)' to 'const std::set<_Kty>::key_compare &'
       with
       [
           _Kty=MyElem
       ]
       and
       [
           _Kty=MyElem
       ]
       Reason: cannot convert from 'overloaded-function' to 'const
std::set<_Kty>::key_compare'
       with
       [
           _Kty=MyElem
       ]
       No constructor could take the source type, or constructor overload
resolution was ambiguous

There is no constructor in std::set which accepts a pointer to a function.

HTH
Heinz