试图用模板编译嵌套类？

Question

Basically what I want is to have a global variable GLOBAL_WRAPPER_TYPE within a class so that I can store any data type into it. So far I have the code below which may have its faults but it compiles as long as you don't try to return anything from WRAPPED .

I also wondered if my set function works properly and do I need to overload the = operator to retrieve the template type T from the WRAPPED class. I was having trouble getting the "extraction" of the GLOBAL_WRAPPER_TYPE to work and I don't know for sure if the set() function inside the WRAPPED class even does the job. It compiles but runtime errors might occur (haven't run it yet).

I did get a compiler error with the code...

nested_class_global_template.cpp|44|error: invalid use of nonstatic data member 'WRAPPER<NODE>::GLOBAL_WRAPPER_TYPE'|

Due to line 62 NODE temp = WRAPPING.INSIDE.get(); but the code does compile without that line, I'm just unsure the set() function works.

And finally I wanted to know how NODE* declared as a pointer would screw everything up seeing that its not static?

Heres the code.

#include <iostream>

using namespace std;

class NODE
{
    int data;

    public:

    NODE(){}
    ~NODE(){}
    NODE(int data)
    {
        this->data = data;
    }
    void print()
    {
        std::cout<<"data: "<<this->data<<std::endl;
    }
};
template <class T>
class WRAPPER
{
    public:

     T GLOBAL_WRAPPER_TYPE;

    WRAPPER(){}
    ~WRAPPER(){}

    class WRAPPED
    {
        public:

        WRAPPED(){}
        ~WRAPPED(){}
        void set(T GLOBAL_WRAPPER_TYPE)
        {
            GLOBAL_WRAPPER_TYPE = GLOBAL_WRAPPER_TYPE;
        }
        T& get()
        {
            return GLOBAL_WRAPPER_TYPE;
        }
        WRAPPED& operator=(const WRAPPED &INSIDE)
        {
            GLOBAL_WRAPPER_TYPE = INSIDE.GLOBAL_WRAPPER_TYPE;

            return *this;
        }
    };

    WRAPPED INSIDE;
};
int main()
{
    WRAPPER<NODE> WRAPPING;
    WRAPPING.INSIDE.set(NODE(99));


    NODE temp = WRAPPING.INSIDE.get();

    return 0;
}

EDITED this one works!

    #include <iostream>

using namespace std;

class NODE
{
        int data;

    public:

        NODE(){}
        ~NODE(){}
        NODE(int data)
        {
            this->data = data;
        }
        void print()
        {
            std::cout<<"data: "<<this->data<<std::endl;
        }
};
template <class T>
class WRAPPER
{
    public:

        static T GLOBAL_WRAPPER_TYPE;

    WRAPPER(){}
    ~WRAPPER(){}

        class WRAPPED
        {
            public:

            WRAPPED(){}
            ~WRAPPED(){}
            void set(T GLOBAL_WRAPPER_TYPE)
            {
                WRAPPER::GLOBAL_WRAPPER_TYPE = GLOBAL_WRAPPER_TYPE;
            }
            T& get()
            {
                return GLOBAL_WRAPPER_TYPE;
            }
            WRAPPED& operator=(const WRAPPED &INSIDE)
            {
                GLOBAL_WRAPPER_TYPE = INSIDE.GLOBAL_WRAPPER_TYPE;

                return *this;
            }
        };

        WRAPPED INSIDE;
};
template <class T>
T WRAPPER<T>::GLOBAL_WRAPPER_TYPE;
int main()
{
    WRAPPER<NODE> WRAPPING;
    WRAPPING.INSIDE.set(NODE(99));
    NODE temp = WRAPPING.INSIDE.get();
    temp.print();

    return 0;
}

Answer 1

GLOBAL_WRAPPER_TYPE is a non-static member of WRAPPER but it's not a member of WRAPPED , you can't return WRAPPER::GLOBAL_WRAPPER_TYPE from WRAPPED.

You can either make WRAPPER::GLOBAL_WRAPPER_TYPE static.

Note: Your set function sets GLOBAL_WRAPPER_TYPE to itself, not set to WRAPPER::GLOBAL_WRAPPER_TYPE

  void set(T GLOBAL_WRAPPER_TYPE)
  {
        GLOBAL_WRAPPER_TYPE = GLOBAL_WRAPPER_TYPE;  // Doesn't do what you think!!!
  }

Update from comment, you could make GLOBAL_WRAPPER_TYPE static member:

template <class T>
class WRAPPER
{
    public:    
     static T GLOBAL_WRAPPER_TYPE;
    //......
};

template <class T>
T WRAPPER<T>::GLOBAL_WRAPPER_TYPE;

Now you could access WRAPPER::GLOBAL_WRAPPER_TYPE directly.

 void set(T GLOBAL_WRAPPER_TYPE)
 {
       WRAPPER::GLOBAL_WRAPPER_TYPE = GLOBAL_WRAPPER_TYPE;
 }

Note: your variable names are very hard to read, better choose something better. Find a good C++ coding style guilde. :)