why we are again creating struct variable from struct object?

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

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.