为什么我们再次从 struct 对象创建 struct 变量？

Question

I am beginner in c++. Here is my doubt why in this program they are again creating struct variable from the previously created struct object ? Here is the example:

typedef struct prog1
{

int a,b;

}*obj1;

int main()
{

obj1 moc=new prog1(); // why again creating object for *obj1 object?
moc->a=10;  // why dont we use obj1 -> a=10;

}

thanks

Answer 1

obj1 is not an object but a type definition, because it is part of the typedef definition. Namely, it is a type of prog1* (a pointer to prog1 ). The obj1 moc declares a variable of this type, ie moc is a pointer to prog1 .

Answer 2

To make it more clear use an alias declaration instead of the typedef definition.

struct prog1
{

int a,b;

};

using obj = struct prog1 *;

So the name obj is an alias for the type struct prog1 * . obj is not a variable.

So in this declaration

obj1 moc; 

there is defined the variable moc with the type obj . This declaration is equivalent to the following declaration

prog1 *moc;

That is there is declared a pointer of the type prog1 * .

Pay attention to that the pointer is not initialized. So it has indeterminate value. As a result the following statement

moc->a=10;

invokes undefined behavior.

Answer 3

you don't need to use typedef before struct. you can directly use prog1 as a type. like this:

struct prog1 {
    int a,b;
} obj1;   //<---create right away the obj1.

int main() {
   prog1 obj2;  //<---another object created.
   prog1 *pObj = new prog1();

   obj1.a = 10;
   obj2.a = 20;
   pObj->a = 30;
   //...
}

or you don't even need the prog1 struct name. like this:

struct {
    int a,b;
} obj1, obj2, *pObj;   //<---obj1 and obj2 already has allocated space for data.

int main() {
   pObj = new prog1();  //<---allocate data space that will be pointed by pObj.

   obj1.a = 10;
   obj2.a = 20;
   pObj->a = 30;
   //...
}