Finding value in unordered_map

I am using Boost unordered_map. I have a key value pair for each entry. How c开发者_运维技巧ould I determine whether a particular value exist in the map? (I don't want to create another unordered_map which stored the value as key and key as value)

Thanks.

How about the following:

typedef std::unordered_map<int,std::string> map_type;
typedef std::unordered_map<int,std::string>::value_type map_value_type;

map_type m;

if (m.end() != find_if(m.begin(),m.end(),[](const map_value_type& vt)
                                           { return vt.second == "abc"; }
                                           ))
   std::cout << "Value found." << std::end;
else
   std::cout << "Value NOT found." << std::end;

Or using an external variable that is captured:

std::string value = "abc";
if (m.end() != find_if(m.begin(),m.end(),[&value](const map_value_type& vt)
                                                 { return vt.second == value; }))
   std::cout << "Value found." << std::end;
else
   std::cout << "Value NOT found." << std::end;

Boost has the Bimap, which is a bidirectional map (ie, keys and values both refer to each other). This sounds more appropriate for your needs than the unordered_map.

You need to iterate over all the elements in the unordered_map and see if the given value exists.

The std::find_if algorithm with a custom predicate can be used to simplify this.