what is the difference btw heap and stack?

What is the difference between creating an instance of a class in heap or in stack.

Code:

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        MyClass *clss = new MyClass(...);
        clss->someMethod();
  and
        MyClass clss(...);
        clss.someMethod();

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One of them is a pointer while the other is an instance of the class...

Both are instances of the class. It's just that in the case of the heap, you also need to allocate a pointer on the stack to hold the address of the instance.

The difference is in object lifetime. Objects on the stack are destroyed as soon as they go out of scope. Objects on the heap must be explicitely destroyed.