如何在另一个类中定义模板类变量？

Question

I defined a template class first, then defined another class Node_3, and in that class I defined a variable that its type is class first called Tree_2. Now this worked fine, but when I try to write a function getTree_2 which is supposed to return the Tree_2 class I get an error, I can't understand why... The error I get:

• Invalid arguments ' Candidates are: first < second::Node_2,second::Cmp_node2> *
  Tree_2(second::Cmp_node2) '

Something like this:

template <class element, class compare_fuc>
class first {
   int x;
   compare_fuc cmpFunc;
   // more things here
   public:
         first(compare_fuc cmp): x(0),cmpFunc(cmp) {};
         ~first() {};

         // and more things here
   };

Now I defined another class that is supposed to define a variable from class first type:

class second{

private:

class Node_2{
    int y1;
    int y2;
    public:

    explicit Node_2(int y1,int y2) : y1(y1),y2(y2){};

    ~Node_2() {};
};

class Node_3{
private:
//Cmp_node2 is a class i defined that compares nodes 2
first<Node_2,Cmp_node2>* Tree_2(Cmp_node2);
puclic:
    first<Node_2,Cmp_node2>* getTree_2()
    {
        return Tree_2;  /// here i get the error
    }
};
};

Answer 1

Well, first, you haven't provided a definition of Cmp_node2 ; and you have some typos. But when we fix your code , we see that the problem is that you've mis-defined Type2 , that's all:

<source>: In member function 'first<second::Node_2, second::Cmp_node2>* 
second::Node_3::getTree_2()':

<source>:40:20: error: cannot convert 'second::Node_3::Tree_2' 
from type 'first<second::Node_2, second::Cmp_node2>*(second::Node_3::)(second::Cmp_node2)' 
to type 'first<second::Node_2, second::Cmp_node2>*'

Why do you get this? Well, look at the definition:

first<Node_2,Cmp_node2>* Tree_2(Cmp_node2);

You thought you were defining a member variable of type first<whatever>* , but actually, you defined a member function which takes a Cmp_node2 and returns a first<whatever>* !