多個結構變量的值是無意的相同

Question

我有一個代碼，像Java那樣做OOP。
我已將界面和實現分隔開，文件名爲demo.h和demo.c。

demo.h

#ifndef DEMO_H 
#define DEMO_H 



typedef struct { 

    /* 
     This is the variable that will be set by setter method 
     and its value will be extracted by getter method. 
     This variable must not be directly accessible by the programmer. 
    */ 
    int num; 


    void (* setNum)(int); // This function will set the value of variable "num". 
    int (* getNum)();  // This function will return the value of variable "num". 
} *Demo; // As objects in java are always called by reference. 



Demo newDemo(); // This function will create an instance of class(struct here) Demo and return. 
/* This is equivalent to: 

     Demo obj = new Demo(); 

    int java. 

    I want my users to create instance of this class(struct here) like this: 

     Demo obj = newDemo(); 

    here in this code. 
*/ 


#endif 

和實現：
demo.c

#include <stdlib.h> 
#include "demo.h" 


Demo demo; /* I have created a global variable so that it is accessible 
       in setNum and getNum functions. */ 



void setNum(int num) { 

    demo->num = num; // This is where the global demo is accessed. 
} 



int getNum(Demo obj) { 

    return demo->num; // This is where the global demo is accessed. 
} 



Demo newDemo() { 

    Demo obj; // This will be the returned object. 


    obj = (Demo)malloc(sizeof(*obj)); /* Allocating separate memory to 
              obj each time this function is called. */ 


    /* Setting the function pointer. */ 
    obj->setNum = setNum; 
    obj->getNum = getNum; 

    /* As obj is at different memory location every time this function is called, 
     I am assigning that new location the the global demo variable. So that each variable 
     of the Demo class(struct here) must have a different object at different memory 
     location. */ 
    demo = obj; 

    return obj; // Finally returning the object. 
} 

這是多麼我已經實現了main功能：

主.c

#include "demo.h" 
#include <stdio.h> 



int main() { 

    void displayData(Demo); 


    Demo obj1 = newDemo(); 
    Demo obj2 = newDemo(); 
    Demo obj3 = newDemo(); 


    obj1->setNum(5); 
    obj2->setNum(4); 
    obj3->setNum(12); 



    displayData(obj1); 
    displayData(obj2); 
    displayData(obj3); 



    return 0; 
} 



void displayData(Demo obj) { 

    int num = obj->getNum(); 

    fprintf(stdout, "%d\n", num); 
} 

在編譯和執行我的Mac書臨：

> gcc -c demo.c 
> gcc main.c demo.o -o Demo 
> ./Demo 

輸出是：

12 
12 
12 

但所需的輸出是：

5 
4 
12 

什麼我做錯了嗎？
請幫忙。
我不希望我的用戶的結構指針作爲參數傳遞爲：

Demo obj = newDemo(); 
obj->setName(obj, "Aditya R.Singh"); /* Creating the program this way was successful as my 
             header file had the declaration as: 

              typedef struct demo { 

               int num; 
               void (* setNum)(struct demo, int); // This is what I don't desire. 
               void (* getNum)(struct demo); // This is what I don't desire. 
              } *Demo;  

             I want to keep it like the way it is in my current 
             demo.h*/ 

/* I don't want to pass obj as an argument. All I want to do this is this way. */ 
obj->setName("Aditya R.Singh"); 

是否有任何可能的方式來做到這一點，得到需要的結果？

請大家幫忙，謝謝！

Answer 1

我完全不知道c++，但在您的代碼中，我認爲demo = obj;是問題所在。 demo是全球性的，對吧？它會被覆蓋，並通過電話newDemo()。

副作用：內存泄漏。