The following should work (tested with g++):

// File: heap.hh --------------------------
template <class T>
class heap {
public:a
    virtual void init(T*) = 0;
    virtual void insert(T*) = 0;
    virtual T delete_min() = 0;
};

// File: leftist_tree.hh ------------------
#include "heap.hh"
template <class T>
class leftist_tree : public heap<T> {
private:
    T* root;
public:
    void init(T* ) {}
    void insert(T* ) {}
    T delete_min() {T a; return a;}
};

// File: leftist_node.hh ------------------
class leftist_node {
    int value;
};

// File: leftist_main.cpp -----------------
#include "leftist_tree.hh"
#include "leftist_node.hh"

int main() {
    leftist_tree< leftist_node > mytree;
}

The following should work (tested with g++):

// File: heap.hh --------------------------
template <class T>
class heap {
public:a
    virtual void init(T*) = 0;
    virtual void insert(T*) = 0;
    virtual T delete_min() = 0;
};

// File: leftist_tree.hh ------------------
#include "heap.hh"
template <class T>
class leftist_tree : public heap<T> {
private:
    T* root;
public:
    void init(T* ) {}
    void insert(T* ) {}
    T delete_min() {T a; return a;}
};

// File: leftist_node.hh ------------------
struct leftist_node {
    int value;
};

// File: leftist_main.cpp -----------------
#include "leftist_tree.hh"
#include "leftist_node.hh"

int main() {
    leftist_tree< leftist_node > mytree;
}