Using std::map where V has no usable default constructor

Question

I have a symbol table implemented as a std::map. For the value, there is no way to legitimately construct an instance of the value type via a default constructo

Answer 1

If the value-type is not default-constructible, then operator[] just won't work for you.

What you can do, though, is to provide free functions that get and set values in a map for convenience.

E.g:

template 
V& get(std::map& m, const K& k)
{
    typename std::map::iterator it = m.find(k);
    if (it != m.end()) {
        return it->second;
    }
    throw std::range_error("Missing key");
}

template 
const V& get(const std::map& m, const K& k)
{
    typename std::map::const_iterator it = m.find(k);
    if (it != m.end()) {
        return it->second;
    }
    throw std::range_error("Missing key");
}

template 
void set(std::map& m, const K& k, const V& v)
{
    std::pair::iterator,bool> result = m.insert(std::make_pair(k, v));
    if (!result.second) {
        result.first->second = v;
    }
}

You might also consider a getter like dict.get(key [, default]) in Python (which returns the provided default if key is not present (but that has a usability problem in that the default always has to be constructed, even if you know that key is in the map).