c ++错误：未在此范围内声明的对象

Question

Could anyone please explain me why I get the following error?

A.cpp: In member function ‘void A::NewObject(int)’:
A.cpp:11: error: ‘list_a’ was not declared in this scope

I tried declaring list_a i various places. Now it is in prog.cpp, in main(). I don't get it why it isn't allegedly in the scope.

My simplified code is below. My idea was to add (NewObject) an object of a class A to a list (Add) while additionally performing some sort of a test (Compare) defined within the A class.

I am a C++ beginner so I'd especially appreciate detailed answers. Thanks in advance. Here are the files:

//A.h

#ifndef A_H
#define A_H

class A
{
private:
    int id;
    int Compare(int, int);
public:
    A()
    {
        id = 0;
    }
    A(int i)
    {
        id = i;
    }
    void NewObject(int i);
};
#endif

//A.cpp

#include "A.h"
#include "List.h"

void A::NewObject(int i)
{   
    list_a->Add(i);
}

int A::Compare(int a, int b)
{
    if ( a>b ) return 1;
    if ( a<b ) return -1;
    else return 0;
}


//List.h

#ifndef LIST_H
#define LIST_H

template<typename T>
class Node
{
public:
    T* dana;
    Node *nxt, *pre;
    Node()
    {
        nxt = pre = 0;
    }
    Node(const T el, Node *n = 0, Node *p = 0 )
    {
        dana = el; nxt = n; pre = p;
    }
};

template<typename T, typename U>
class List
{
public:
    List()
    {
        head = tail = 0;
    }
    void Add(const U);
protected:
    Node<T> *head,*tail;
};

#endif


//List.cpp

#include <iostream>
#include "List.h"

template<typename T, typename U>
void List<T,U>::Add(const U el)
{
    int i = 5;
    Node<T> *hlp = new Node<T>();
    head = hlp;
    if ( Compare(el,i) > i )
        std::cout << "Ok" << std::endl;
}

//prog.cpp
#include "List.h"
#include "A.h"

int main()
{
    int i = 5;
    List<class A, int> *list_a = new List<class A, int>();
    A obj;

    obj.NewObject(i);
}

Answer 1

Well the answer is simple: you have never declared a variable named list_a in class A. Thats it.

Do it that way:

class A
{
private:
    List <int> list_a;
    int id;
    int Compare(int, int);
public:
    A()
    {
        id = 0;
    }
    A(int i)
    {
        id = i;
    }
    void NewObject(int i);
};

Remove the U template parameter from your list class.

Answer 2

When this type of error occurs, there might be a mismatch of class name in the main function. Check whether they are same. Only then, you will get an error like not declared in this scope , which means "they are not declared properly".