Question

I have custom binary tree class that holds values of template type T (it could be value or pointer). Each value is encapsulated with number (this number is used for searching in tree). I want to have an std::map inside my tree class fo fast O(1) access to objects without numbers.

template <typename T>
stuct BSTNode 
{
  T value;
  int searchValue;
}

template <typename T>
class BST 
{
  BSTNode<T> * root;
  std::map<T, BSTNode<T>> cache;
  //... etc.
}

Example: I have class instance a inserted in tree under value n. Now I want to get the node associated with this a. I cannot search the tree, because I don’t have n. So I want to use a, and from std::map get node = map[a]. Now I can do node->n.

How can I achieve this? I can override compare method of std::map:

bool operator()(const void * s1, const void * s2) const

But it doesn’t work for value and pointer at the same time: cannot convert parameter 1 from const double to const void *.

Answer 1

Make a traited comparator:

template <typename T>
struct NodeComp
{
    bool operator<(T const & lhs, T const & rhs) const
    {
        return lhs < rhs;
    }
};

template <typename U>
struct NodeComp<U *>
{
    bool operator<(U * lhs, U * rhs) const
    {
        return *lhs < *rhs;
    }
};

Now your map can be defined like so:

template <typename T>
class BST 
{
    BSTNode<T> * root;
    std::map<T, BSTNode<T>, NodeComp<T>> cache;
}