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c++objectheap-memorystack-memory

C++ stack variables and heap variables

When you create a new object in C++ that lives on the stack, (the way I've mostly seen it) you do this:

CDPlayer player;

When you create an object on the heap you call new:

CDPlayer* player = new CDPlayer();

But when you do this:

CDPlayer player=CDPlayer();

it creates a stack based object, but whats the difference between that and the top example?

Solution

  • The difference is important with PODs (basically, all built-in types like int, bool, double etc. plus C-like structs and unions built only from other PODs), for which there is a difference between default initialization and value initialization. For PODs, a simple 

    T obj;

    will leave obj uninitialized, while T() default-initializes the object. So 

    T obj = T();

    is a good way to ensure that an object is properly initialized.

    This is especially helpful in template code, where T might either a POD or a non-POD type. When you know that T is not a POD type, T obj; suffices.

    Addendum: You can also write 

    T* ptr = new T; // note the missing ()

    (and avoid initialization of the allocated object if T is a POD).