求教undefined reference to

[happypetersun]

用g++编译一下代码时出现这样的问题：
undefined reference to `Tree<int>::Tree()'

1 #include <stdlib.h>
2 #include <iostream>
3 using namespace std;
4
5 template <typename T>
6 struct CSTnode
7 {
8 T data;
9 struct CSTnode* firstchild;
10 struct CSTnode* nextsibling;
11 struct CSTnode* parent;
12 };
13
14 template <class T>
15 class Tree
16 {
17 public:
18 Tree();
19 virtual ~Tree();
20 virtual void getParent() const=0;
21 virtual void getFirstChild() const=0;
22
23 };
24
25 template <class T>
26 class csTree: public Tree<T>
27 {
28 public:
29 csTree();
30 ~csTree();
31 void getParent() const;
32 void getFirstChild() const;
33
34 private:
35 struct CSTnode<T>* root;
36 };
37
38 template <class T>
39 csTree<T>::csTree():Tree<T>()
40 {
41 root=new CSTnode<T>;
42 root->firstchild=NULL;
43 root->nextsibling=NULL;
44 }
45
46 template <class T>
47 csTree<T>::~csTree()
48 {
49 }
50 template <class T>
51 void csTree<T>::getParent() const
52 {
53 cout<<""<<endl;
54 }
55 template <class T>
56 void csTree<T>::getFirstChild() const
57 {
58 cout<<""<<endl;
59 }
60
61
62 int main()
63
64 {
65 csTree<int> cs;
66 return 1;
67 }

[xuzhuo]

没找到定义

[happypetersun]

各位能说得具体点吗？

[ouyh12345]

Tree();
19 virtual ~Tree();

这两个函数都没实现

[自信男孩]

提示是未定义的行为……