Getting data out of C++ structs and pointers [duplicate] - c++

Question

In C++ do you always have to initialize a pointer to an object with the new keyword?
Or can you just have this too:
MyClass *myclass;
myclass->DoSomething();
I thought this was a pointer allocated on the stack instead of the heap, but since objects are normally heap-allocated, I think my theory is probably faulty?
Please advice.

Answer 1

No, you can have pointers to stack allocated objects:
MyClass *myclass;
MyClass c;
myclass = & c;
myclass->DoSomething();
This is of course common when using pointers as function parameters:
void f( MyClass * p ) {
p->DoSomething();
}
int main() {
MyClass c;
f( & c );
}
One way or another though, the pointer must always be initialised. Your code:
MyClass *myclass;
myclass->DoSomething();
leads to that dreaded condition, undefined behaviour.

Answer 2

No you can not do that, MyClass *myclass will define a pointer (memory for the pointer is allocated on stack) which is pointing at a random memory location. Trying to use this pointer will cause undefined behavior.
In C++, you can create objects either on stack or heap like this:
MyClass myClass;
myClass.DoSomething();
Above will allocate myClass on stack (the term is not there in the standard I think but I am using for clarity). The memory allocated for the object is automatically released when myClass variable goes out of scope.
Other way of allocating memory is to do a new . In that case, you have to take care of releasing the memory by doing delete yourself.
MyClass* p = new MyClass();
p->DoSomething();
delete p;
Remeber the delete part, else there will be memory leak.
I always prefer to use the stack allocated objects whenever possible as I don't have to be bothered about the memory management.

Answer 3

if you want the object on the stack, try this:
MyClass myclass;
myclass.DoSomething();
If you need a pointer to that object:
MyClass* myclassptr = &myclass;
myclassptr->DoSomething();

Answer 4

First I need to say that your code,
MyClass *myclass;
myclass->DoSomething();
will cause an undefined behavior.
Because the pointer "myclass" isn't pointing to any "MyClass" type objects.
Here I have three suggestions for you:-
option 1:- You can simply declare and use a MyClass type object on the stack as below.
MyClass myclass; //allocates memory for the object "myclass", on the stack.
myclass.DoSomething();
option 2:- By using the new operator.
MyClass *myclass = new MyClass();
Three things will hapen here.
i) Allocates memory for the "MyClass" type object on the heap.
ii) Allocates memory for the "MyClass" type pointer "myclass" on the stack.
iii) pointer "myclass" points to the memory address of "MyClass" type object on the heap
Now you can use the pointer to access member functions of the object after dereferencing the pointer by "->"
myclass->DoSomething();
But you should free the memory allocated to "MyClass" type object on the heap, before returning from the scope unless you want it to exists. Otherwise it will cause a memory leak!
delete myclass; // free the memory pointed by the pointer "myclass"
option 3:- you can also do as below.
MyClass myclass; // allocates memory for the "MyClass" type object on the stack.
MyClass *myclassPtr; // allocates memory for the "MyClass" type pointer on the stack.
myclassPtr = &myclass; // "myclassPtr" pointer points to the momory address of myclass object.
Now, pointer and object both are on the stack.
Now you can't return this pointer to the outside of the current scope because both allocated memory of the pointer and the object will be freed while stepping outside the scope.
So as a summary, option 1 and 3 will allocate an object on the stack while only the option 2 will do it on the heap.

Answer 5

if you want to access a method :
1) while using an object of a class:
Myclass myclass;
myclass.DoSomething();
2) while using a pointer to an object of a class:
Myclass *myclass=&abc;
myclass->DoSomething();

Answer 6

If you have defined a method inside your class as static, this is actually possible.
class myClass
{
public:
static void saySomething()
{
std::cout << "This is a static method!" << std::endl;
}
};
And from main, you declare a pointer and try to invoke the static method.
myClass * pmyClass;
pmyClass->saySomething();
/*
Output:
This is a static method!
*/
This works fine because static methods do not belong to a specific instance of the class and they are not allocated as a part of any instance of the class.
Read more on static methods here: http://en.wikipedia.org/wiki/Static_method#Static_methods

Answer 7

Simple solution for cast pointer to object
Online demo
class myClass
{
public:
void sayHello () {
cout << "Hello";
}
};
int main ()
{
myClass* myPointer;
myClass myObject = myClass(* myPointer); // Cast pointer to object
myObject.sayHello();
return 0;
}