Question

I have a templated class which looks as follows (assume a using namespace std for brevity):

template <typename Type, typename Compare = less<Type>>
struct weighted_base
{
    typedef typename set<pair<Type, double>, set_compare<Type, Compare>> mapped_type;
    map<Type, mapped_type, Compare> backing_map;
    ...
};

where set_compare is defined as:

template <typename Type, typename Compare>
struct set_compare
{
     bool operator()(const pair<Type, double>& a, 
                     const pair<Type, double>& b)
     {
         return Compare(a.first, b.first);
     }
};

That is, the map takes keys of type Type to std::set<std::pair<Type, double>> values. This has some problems when I use a method such as:

void insert(const Type& from, const Type& to, double weight)
{
    //...
    mapped_type& adj_nodes = backing_map[from];
    adj_nodes.insert(make_pair(to, weight));
} 

The problem is that within the set, when it goes to call set_compare, it has a type of const Type&, not Type. Thus, assuming it’s std::less, in this case it will be trying to call less<int>::less(const int& a, const int& b) which fails.

Is there some way of fixing it so that both contains can use (effectively) the same comparison function here?

Answer 1

Compare is the type of a binary functor, so you probably need

return Compare()(a.first, b.first);

where Compare() is a temporary, defualt constructed, Compare instance. For example, if you substitute Compare for std::less<int>,

std::cout << std::boolalpha;
std::cout << std::less<int>(5,50) << "\n"; // Error!
std::cout << std::less<int>()(5,50) << "\n"; // OK

Your error is likely to give you many compiler errors, some of which could send you down the wrong track. I suggest fixing it first, then seeing if the code behaves as you expect it to.