Using GUID as the key for std::map

[Faisal]

I've a map which uses GUID as the key.

#include <map>
#include <string>
#include <unknwn.h>

void main()
{
std::map<GUID, std::string> map;

GUID d1;
std::string szName = "test";

map[d1] = szName;
}

But when compiling this code in VS2008 shows following errors

c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not
deduce template argument for 'const
std::basic_string<_Elem,_Traits,_Alloc> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\string(150) : see declaration of 'std::operator <'
1> c:\program files\microsoft visual studio 9.0\vc\include
\functional(142) : while compiling class template member function
'bool std::less<_Ty>::operator ()(const _Ty &,const _Ty &) const'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const _Elem *,const
std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce
template argument for 'const _Elem *' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\string(140) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::basic_string<_Elem,_Traits,_Alloc> &,const
std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce
template argument for 'const std::basic_string<_Elem,_Traits,_Alloc>
&' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\string(130) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const std::_Tree<_Traits>
&,const std::_Tree<_Traits> &)' : could not deduce template argument
for 'const std::_Tree<_Traits> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include\xtree
(1466) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::reverse_iterator<_RanIt> &,const std::reverse_iterator<_RanIt2>
&)' : could not deduce template argument for 'const
std::reverse_iterator<_RanIt> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\xutility(2262) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::_Revranit<_RanIt,_Base> &,const std::_Revranit<_RanIt2,_Base2>
&)' : could not deduce template argument for 'const
std::_Revranit<_RanIt,_Base> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\xutility(2072) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const std::pair<_Ty1,_Ty2>
&,const std::pair<_Ty1,_Ty2> &)' : could not deduce template argument
for 'const std::pair<_Ty1,_Ty2> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\utility(99) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2676: binary '<' : 'const GUID' does not define this
operator or a conversion to a type acceptable to the predefined
operator
1


How can i solve this?

[Giovanni Dicanio]

Post by Faisal
I've a map which uses GUID as the key.

[...]

Post by Faisal
But when compiling this code in VS2008 shows following errors

I would suggest you to define an operator< for GUID's, e.g.

<code>

inline bool operator<( const GUID & lhs, const GUID & rhs )
{
return ( memcmp( &lhs, &rhs, sizeof(GUID) ) > 0 ? true : false );
}

</code>


Giovanni

[Giovanni Dicanio]

Post by Giovanni Dicanio
inline bool operator<( const GUID & lhs, const GUID & rhs )
{
return ( memcmp( &lhs, &rhs, sizeof(GUID) ) > 0 ? true : false );

Probably this should better be:

return ( memcmp( &lhs, &rhs, sizeof(GUID) ) < 0 ? true : false );

i.e. '<' instead of a '>' considering the semantics of memcmp and the fact
that we are overloading operator<.

Giovanni

[Victor Bazarov]

Post by Giovanni Dicanio

Post by Giovanni Dicanio
inline bool operator<( const GUID & lhs, const GUID & rhs )
{
return ( memcmp( &lhs, &rhs, sizeof(GUID) ) > 0 ? true : false );

return ( memcmp( &lhs, &rhs, sizeof(GUID) ) < 0 ? true : false );
i.e. '<' instead of a '>' considering the semantics of memcmp and the
fact that we are overloading operator<.

Say, what prevents *you* from writing

return memcmp(&lhs, &rhs, sizeof(GUID)) < 0;

? Why do you feel the need for the op ?: and 'true' and 'false'? And
what's with the parentheses? And why stop at just one? Keep going:

return ((((((memcmp(..)<0)?true:false)?true:false)?true:false)));

The compiler can take it.

V

[Giovanni Dicanio]

Post by Victor Bazarov
Say, what prevents *you* from writing
return memcmp(&lhs, &rhs, sizeof(GUID)) < 0;
? Why do you feel the need for the op ?: and 'true' and 'false'? And
what's with the parentheses?

Maybe just personal coding style :)

I could prefer:

return ( <some expression> );

instead of:

return <some expression>;

...Like there are people who love using Hungarian notation, people who
dislike it, people who use braces like this:

if (...) {
...
} else {
....
}

other who likes (like me):

if (...)
{
...
}
else
{
...
}

:)


Giovanni

[Stephen Howe]

inline bool operator<( const GUID & lhs, const GUID & rhs )
{
return memcmp(&lhs, &rhs, sizeof(GUID) < 0;
}

should do

S

[Victor Bazarov]

Post by Giovanni Dicanio
inline bool operator<( const GUID & lhs, const GUID & rhs )
{
return memcmp(&lhs, &rhs, sizeof(GUID) < 0;
}
should do

A closing paren is missing. :-)

V

[Wenwei Peng]

在 2009年11月19日星期四 UTC+8下午9:27:17，Faisal写道：

Post by Faisal
I've a map which uses GUID as the key.
#include <map>
#include <string>
#include <unknwn.h>
void main()
{
std::map<GUID, std::string> map;
GUID d1;
std::string szName = "test";
map[d1] = szName;
}
But when compiling this code in VS2008 shows following errors
c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not
deduce template argument for 'const
std::basic_string<_Elem,_Traits,_Alloc> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\string(150) : see declaration of 'std::operator <'
1> c:\program files\microsoft visual studio 9.0\vc\include
\functional(142) : while compiling class template member function
'bool std::less<_Ty>::operator ()(const _Ty &,const _Ty &) const'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const _Elem *,const
std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce
template argument for 'const _Elem *' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\string(140) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::basic_string<_Elem,_Traits,_Alloc> &,const
std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce
template argument for 'const std::basic_string<_Elem,_Traits,_Alloc>
&' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\string(130) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const std::_Tree<_Traits>
&,const std::_Tree<_Traits> &)' : could not deduce template argument
for 'const std::_Tree<_Traits> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include\xtree
(1466) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::reverse_iterator<_RanIt> &,const std::reverse_iterator<_RanIt2>
&)' : could not deduce template argument for 'const
std::reverse_iterator<_RanIt> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\xutility(2262) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const
std::_Revranit<_RanIt,_Base> &,const std::_Revranit<_RanIt2,_Base2>
&)' : could not deduce template argument for 'const
std::_Revranit<_RanIt,_Base> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\xutility(2072) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2784: 'bool std::operator <(const std::pair<_Ty1,_Ty2>
&,const std::pair<_Ty1,_Ty2> &)' : could not deduce template argument
for 'const std::pair<_Ty1,_Ty2> &' from 'const GUID'
1> c:\program files\microsoft visual studio 9.0\vc\include
\utility(99) : see declaration of 'std::operator <'
1>c:\program files\microsoft visual studio 9.0\vc\include\functional
(143) : error C2676: binary '<' : 'const GUID' does not define this
operator or a conversion to a type acceptable to the predefined
operator
1
How can i solve this?

Title: The core of the core of the big data solutions -- Map
Author: pengwenwei
Email:
Language: c++
Platform: Windows, linux
Technology: Perfect hash algorithm
Level: Advanced
Description: Map algorithm with high performance
Section MFC c++ map stl
SubSection c++ algorithm
License: (GPLv3)

Download demo project - 1070 Kb
Download source - 1070 Kb

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Download:
https://sourceforge.net/projects/pwwhashmap/files

Applications:
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There are many tables to query and set in the router DHCP ptotocol,such as IP,Mac ,and all these are completed by map.But until now,all map are using STL liabrary,its performance is very low,and using the Hash map has error probability,so it can only use multi router packet dispersion treatment.If using pwwHashMap, you can save at least ten sets of equipment.

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Background to this article that may be useful such as an introduction to the basic ideas presented:
http://blog.csdn.net/chixinmuzi/article/details/1727195