Question

When I compile this:

#ifndef BTREE_H
#define BTREE_H
#include <QList>

template <class T, int degree>
class btree
{
public:
    class node
    {
    public :
        node();
    private:
        node* parent;
        QList<T> values;
        QList<node*> children;
    };
public:
    btree();
    void insert(const T& value);
    node* findLeaf(const T& value);
    void performInsertion(const T& value, node& place);
    //
    node* root;
};
#endif // BTREE_H

Implementation of findLeaf is this:

template <class T, int degree>
btree<T,degree>::node* btree<T,degree>::findLeaf(const T &value)
{
    if(root == NULL)
        return root;
}

This error occurs:

 error: need ‘typename’ before ‘btree<T, degree>::Node’
 because ‘btree<T, degree>’ is a dependent scope

Answer 1

The C++ grammar is horrendous, and as such it is not possible, when given a template class, to know whether the ::node you refer to is a variable/constant or a type.

The Standard therefore mandates that you use typename before types to remove this ambiguity, and treats all other usages as if it was a variable.

Thus

template <typename T, int degree>
typename btree<T,degree>::node* btree<T,degree>::findLead(T const& value)
^~~~~~~~

is the correct signature for the definition.