Re: std::map lookup function

From:

Carl Barron <cbarron413@adelphia.net>

Newsgroups:

comp.lang.c++.moderated

Date:

30 Apr 2006 13:01:03 -0400

Message-ID:

<290420062206144467%cbarron413@adelphia.net>

In article <20060429113506.58f339a8.mkluwe@gmail.com>, Matthias Kluwe
<mkluwe@gmail.com> wrote:

Hi!

Thorsten Ottosen <thorsten.ottosen@dezide.com>:

Matthias Kluwe wrote:

In my current (toy) project I need to lookup values in a
std::map. As this is constructed from user input, given keys
can't be trusted, and I find myself doing things like

   std::map<...,...>::iterator it = map.find( key );
   if ( it != map.end() ) {
       ...
   }

very often.

[...]

FWIW, The C++0x working paper already contains:

reference at( const Key& );
const_reference at( const Key& ) const;

That leaves me bewildered that the current std::vector has this member,
and the std::map has not, for no reason I can think of at the moment.

    I nor cw 9.6 see anything wrong with the template in fact this works
#include <map>
#include <iostream>

struct Lookup_Error {};

template<typename Map>
typename Map::mapped_type&
lookup( Map & m, const typename Map::key_type v ) {
     Map::iterator it = m.find( v );
     if ( it == m.end() ) throw Lookup_Error();
     return it->second;
}

int main()
{
    std::map<int,int> m;
    for(int i=1;i!=6;++i) m[i]=10*i; // fill the map
    try
    {
       std::cout << lookup(m,2) << '\n';
       std::cout << lookup(m,10) << '\n';
    }
    catch(Lookup_Error)
    {
       std::cout << "item not found\n";
    }
};

yielding
20
item not found

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