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Subject: Re: [boost] Re : BOOST_FOREACH like macro for map
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-10-20 16:59:00

Pierre Morcello wrote:
> Hi mobi phil,
> > One of the annoyances with std::map is that if you work
> > with const maps, then
> > there is no "one line" code to get an element knowing the
> > key.
> If that is really what you want and you don't care about
> performances, I can propose you for example the following :
> struct ConstMapHelper
> {
> template <class T,class U>
> static const U& getConst(const T& pKey,const
> std::map<T,U>& pMap)
> {
> typedef std::pair<const T,U> TPair;
> BOOST_FOREACH(const TPair& lPair,pMap)
> {
> if(lPair.first==pKey)

That is horribly inefficient: O(N). Try this instead (untested):

   template <class T, class U>
   U const &
   get_value(std::map<T,U> const & _map, T const & _key)
      typename std::map<T,U>::const_iterator it(_map.find(_key));
      if (it != _map.end())
         return it->second;
      throw std::domain_error("key not found");

That is O log(N).

> And then when you want to use it :

   std::string const & s(get_value(lMapConst, "5"));

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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