How to use struct as key in std::map

Question

I want to use a std::map whose key and value elements are structures.

I get the following error: 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

I understand that I should overload operator < for that case, but the thing is I don't have access to the code of the structure I want to use (GUID structure in VC++).

Here's the code snippet:

//.h

#include <map>
using namespace std;

map<GUID,GUID> mapGUID;


//.cpp

GUID tempObj1, tempObj2;              
mapGUID.insert( pair<GUID,GUID>(tempObj1, tempObj2) );

How to solve this problem?

Bo Persson · Accepted Answer · 2011-03-18 16:35:44Z

10

You can define the comparison operator as a freestanding function:

bool operator<(const GUID & Left, const GUID & Right)
{
    // comparison logic goes here
}

Or, since in general a < operator does not make much sense for GUIDs, you could instead provide a custom comparison functor as the third argument of the std::map template:

struct GUIDComparer
{
    bool operator()(const GUID & Left, const GUID & Right) const
    {
        // comparison logic goes here
    }
};

// ...

std::map<GUID, GUID, GUIDComparer> mapGUID;

edited Mar 18, 2011 at 16:35

Bo Persson

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answered Mar 18, 2011 at 14:26

Matteo Italia

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6 Comments

dragan.stepanovic Over a year ago

when I write the way you said, I get errors, since operator< function should look like bool operator<(GUID& left, GUID& right); Even after this I get compiler error C2804: binary 'operator <' has too many parameters. I don't understand why, cause this function is not placed inside my class containing maps?

Robᵩ Over a year ago

@kobac -- I think that @Matteo meant to say bool operator()(const std::pair<GUID, GUID> & Left, const std::pair<GUID, GUID> & Right)

dragan.stepanovic Over a year ago

The correct answer is bool operator()(const GUID &left, const GUID &right) const

Matteo Italia Over a year ago

Ops, I got a bit messed up with the second one; sorry, now it should be fixed. :S

dragan.stepanovic Over a year ago

Still needs operator(). Please correct it so the people searching for the same problem would have a complete solution. Grazie Matteo.

|

Jerry Coffin · Accepted Answer · 2011-03-18 14:27:49Z

2

Any type you use as a key has to provide a strict weak ordering. You can supply a comparator type as a third template argument, or you can overload operator< for your type.

answered Mar 18, 2011 at 14:27

Jerry Coffin

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1 Comment

underscore_d Over a year ago

upvoted for specifying strict weak ordering. This is important as otherwise a naive attempt at implementing our own operator< probably won't order strictly enough. Mine didn't, until I learned this term elsewhere.

Ralf · Accepted Answer · 2011-03-18 14:22:43Z

0

There is no operator < for GUIDS so you either have to provide the comparison operator or use a different key.

answered Mar 18, 2011 at 14:22

Ralf

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Comments

Eugene Burtsev · Accepted Answer · 2011-03-18 14:25:31Z

0

May be using class inherited from GUID in which you implement operator < would be a workaround for you?

answered Mar 18, 2011 at 14:25

Eugene Burtsev

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