Re: std::map lookup function

Carl Barron <>
30 Apr 2006 13:01:03 -0400
In article <>, Matthias Kluwe
<> wrote:


Thorsten Ottosen <>:

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
       std::cout << lookup(m,2) << '\n';
       std::cout << lookup(m,10) << '\n';
       std::cout << "item not found\n";

item not found

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