Question

I’m trying to make a wrapper to the STL map container, in order to add a const method to return the value given the key. In map, operator[] isn’t const, and find() requires dereferencing to get the value (map.find()->second). I’m basing some of my “research” off of Idiomatic C++ for reading from a const map by the way.

The code so far (all inside a single header file):

#include <map>
template <typename K, typename V>
class easymap : public std::map<K, V>
{
    //Constructor
    easymap() : std::map<K, V>() {};

    //The get method
    V get(K key)
    {
        std::map<K, V>::const_iterator iter(find(key));
        return iter != end() ? iter->second : V();
    }
};

When I try to compile this, I get the following errors:

In member function `V easymap::get(K)':
    expected `;' before "iter"
`iter' was not declared in this scope
there are no arguments to `end' that depend on a template parameter, so a declaration of `end' must be available|
(if you use `-fpermissive', G++ will accept your code, but allowing the use of an undeclared name is deprecated)

Does how I’m trying to go about doing this make sense? If so, how do I make this work? If not, how would I go about achieving the effect I’m looking for?

Answer 1

You are missing the template parameters for map, you have to specify typename when declaring the iterator (see here), and for some reason unknown to me (probably a namespace conflict) you have to use this when calling end():

template <typename K, typename V>
class easymap : public std::map<K,V>
{
    //Constructor
    easymap() : std::map<K, V>() {};

    //The get method
    V get(K key)
    {
        typename std::map<K, V>::const_iterator iter(find(key));
        return iter != this->end() ? iter->second : V();
    }
};