What's wrong with const?

Question

What are the known shortfalls of const in C++ and C++0x?

Answer 1

Another problem that has not been mentioned yet is the possibility for a badly designed interface to subvert const (even in the absence of casts).

Example:

class TreeNode {
public:
    TreeNode& getParent() const { return *parent_; }
    TreeNode& getLeft()   const { return *left_;   }
    TreeNode& getRight()  const { return *right_;  }
private:
    //TreeNode has a pointer to the Tree to enable navigation of the tree.
    //Assume that other design constraints mean that this must be a pointer 
    //rather than a reference.
    TreeNode* parent_;
    TreeNode* left_;
    TreeNode* right_;
};
//This function demonstrates the ability for const to be subverted.
TreeNode& remove_const(TreeNode const& toRemoveConstFrom) {
    TreeNode& parent(toRemoveConstFrom.getParent());
    TreeNode& leftChild(parent.getLeft());
    TreeNode& rightChild(parent.getRight());
    return &toRemoveConstFrom == &leftChild ? leftChild : rightChild;
}

The intransitive nature of const means that it is possible to have an interface where a non-const reference to an object can be obtained from a const reference to an object. This is something to be careful of when designing interfaces.