Re: How to make insert-and-overwrite into a map

From:

=?iso-8859-1?q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 1 Oct 2007 13:08:12 CST

Message-ID:

<1191258363.663058.324600@g4g2000hsf.googlegroups.com>

On 1 Okt., 16:49, yuval.ro...@gmail.com wrote:

Since std::map::insert doesn't overwrite the mapped value of an
already-existing key, I wonder how to accomplish an overwriting
insert. Using operator[] is not a good idea because it enforces the
mapped value to be default constructible. Checking the boolean value
returned by insert() and if necessary, calling find(), and assign to
the found value, is also not a good idea because it means searching
the map happens twice - both by insert() and find() - which is
inefficient.

Is there another way, or was it just forgotten by the map interface
designers?

It has not been forgotten. If you take a look at insert (with one
argument), you will realize that it also returns std::pair<iterator,
bool>,
which points to the solution you are looking for:

#include <utility>

// Returns whether the key already existed or not
template <typename MapType>
bool put(MapType& m, const typename MapType::value_type& v) {
   std::pair<typename MapType::iterator, bool> result =
     m.insert(v);
   if (!result.second) {
     result.first->second = v.second;
   }
   return result.second;
}

Greetings from Bremen,

Daniel Kr?gler

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