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STL map question

Started by foofightr May 31, 2003 at 4:34 PM 11 replies 2.4k views

Original Post

May 31, 2003 at 4:34 PM

Sorry but I can't make sense of the error VC++ gives me.

typedef std::map<sockaddr_in, client_t *> myMapType;

myMapType myMap;

sockaddr_in sock;
client_t *client = something;
myMap[sock] = client;

I get this error:


c:\program files\microsoft visual studio\vc98\include\functional(86) : error C2784: 'bool __cdecl std::operator <(const class std::multimap<_K,_Ty,_Pr,_A> &,const 
class std::multimap<_K,_Ty,_Pr,_A> &)' : could not deduce template argument for 'const class std::multimap<_K,_Ty,_Pr,_A> &' from 'const struct sockaddr_in'
        c:\program files\microsoft visual studio\vc98\include\functional(86) : while compiling class-template member function 'bool __thiscall std::less::operator ()
(const struct sockaddr_in &,const struct sockaddr_in &) const'

Something about not being able to deduce the sockaddr_in argument. Anyone know why? (the help files are no help) [edited by - foofightr on May 31, 2003 5:37:37 PM] [edited by - foofightr on May 31, 2003 5:43:15 PM]

mfawcett

May 31, 2003 at 4:40 PM

std::map is a template class. You must provide template arguments when declaring an instance of it. The compiler tells you that since you didn't give it a template argument, it can't deduce it for you (compilers don't read minds

).

to declare a std::map that holds integers, you could do:


std::map<int> myMap;

[edited by - mfawcett on May 31, 2003 5:41:17 PM]

edit: although posting here without source or code tags makes those less than/greater than signs go away, so maybe it's something else, but without seeing the actual code I can't say.

[edited by - mfawcett on May 31, 2003 5:42:45 PM]

billybob

May 31, 2003 at 4:42 PM

he did, the forum ate it. quote or edit his post to see what it is

foofightr

May 31, 2003 at 4:42 PM

Yes, the part in brackets was eaten by the board. I hate using source box just for this, but here goes.

see original post that was edited

[edited by - foofightr on May 31, 2003 5:43:34 PM]

Qw3r7yU10p!

May 31, 2003 at 4:44 PM

I think it means you need less-than defined for sockaddr_in


struct sockaddr_in {
    bool lt(const sockaddr_in& rhs);
};

bool operator<(const sockaddr_in& lhs, const sockaddr_in& rhs) {
    return lhs.lt(rhs);
}

should fix it for you

mfawcett

May 31, 2003 at 4:45 PM

std::map's use the less than operator for storing. Does sockaddr_in have an one? If not, you need one for it to be a key.

[edited by - mfawcett on May 31, 2003 5:45:36 PM]

neoztar

May 31, 2003 at 4:46 PM

How is sokcaddr_in defined? If it's a class, it'll need overloaded comparison operators for use with std::map.

EDIT: Argh! beaten...

~nz

// Website // Google // GameDev // NeHe // MSDN // OpenGL Extensions //

[edited by - neoztar on May 31, 2003 5:47:29 PM]

foofightr

May 31, 2003 at 4:47 PM

Argh. sockaddr_in is part of winsock.h, so I can''t just redefine it. Well, I could but it would only compile on my machine.

Oh well, thanks for the quick answers. Will find another solution...

Qw3r7yU10p!

May 31, 2003 at 4:49 PM

wrap it

Mage2k

May 31, 2003 at 9:49 PM

or, store a pointer to it

peace and (trance) out

Mage

Zipster

May 31, 2003 at 11:01 PM

Define a global comparison operator:


bool operator<(const sockaddr_in &l, const sockaddr_in &r)
{
   return (l.someparam < r.someparam);
}

EDIT: This was already suggested. Why didn't you try it?

[edited by - Zipster on May 31, 2003 12:02:46 AM]

Beer Hunter

May 31, 2003 at 11:07 PM

The map template takes three template parameters -- the third is a comparison functor which defaults to std::less, which uses the less-than operator. You can declare your own functor to compare sockaddr_ins, and use that as the third template parameter instead.

Seriema

Jun 1, 2003 at 6:28 AM

exactly as Beer Hunter said!

make a functor

here's a quick one


// You just gotta love the name...

struct SockaddrInCompareLess : public std::binary_function<sockaddr_in,sockaddr_in,bool>
{
public:
  bool operator () (const sockaddr_in& lhs, const sockaddr_in& rhs) const
  {
    return lhs.something < rhs.something;
  }
};

// then when you declare your map just:

typedef std::map<sockaddr_in, client_t *, SockaddrInCompareLess ()> myMapType;

should work like a charm

PS. some prefer to write the operator() as:
result_type operator () (const first_parameter_type& lhs, const second_parameter_type& rhs)
if you're unfamiliar with std::binary_function it's just a simple templated struct that has some typedefs (like those in the alternative op() declaration) which are needed sometimes.

"No lies of sugar can sweeten the sournes of reality"

}+TITANIUM+{ A.K.A. DXnewbie[onMIRC]

[edited by - Seriema on June 1, 2003 7:35:27 AM]

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