CODEWITHSUNDEEP
c++objectmemory-managementdynamic
Vikas Verma
Vikas Verma

Reputation: 3706

Dynamic initialization of object with constructor Vs. new in c++?

What is difference between these two statements with respect to dynamic memory initialization of object :-

class_name *obj = new class_name;

class_name obj = class_name();

Upvotes: 0

Views: 2589

Answers (5)

ur.
ur.

Reputation: 2947

The first example works roughly like this:

  • operator new is called and memory allocated on the heap
  • the default constructor for class_name is called and thus the object pointed to is initialized.

The second example:

  • a temporary object A is created (right hand side of the assignment).
  • an object B is created (default constructor; left hand side)
  • the contents of A is overwritten by that of object B - this done by calling the assignment operator
  • A becomes destroyed.

Edit: Actually, what I've written for the second example above, is not right (maybe it's not wrong, too). I don't know the standard here, but with VC12 (optimizations off) the code doesn't create an temporary object. It just calls the default constructor for an object on the stack. And the code doesn't compile if there's a private assignment operator - as James writes in his comment.

Upvotes: 3

James Kanze
James Kanze

Reputation: 153929

The first defines a pointer to class_name, dynamically allocates an object of that type, and initializes the pointer with its address. The lifetime of the object itself is until you explicitly delete it. If the class_name has a default constructor, that is called; otherwise, the object is not initialized.

The second defines an instance of a class_name; formally, it constructs a temporary of that type, using the default constructor if one is defined, or zero initialization if not, and copy constructs the defined instance from that. The compiler is allowed to elide this copy construction, however (and all that I know of do), and construct the defined object directly. (It must still verify that there is an accessible copy constructor, however.) The lifetime of the defined object is until it goes out of scope.

In general, we avoid the first form, unless we really need the explicit lifetime.

Upvotes: 2

Cheers and hth. - Alf
Cheers and hth. - Alf

Reputation: 145279

Hopefully you have related the assignment/question text correctly. It's subtle. The "dynamic" has nothing to do with the dynamic allocation in the first statement.

The () parenthesis ensures a C++03 value initialization, which for a POD type means zeroing. Without it (first statement), if the class is a POD its members needs not be initialized. I would have to check the standard for the more general case, but I think, now that you know what the question is about, that's something you can do on your own.

Upvotes: 1

Ferenc Deak
Ferenc Deak

Reputation: 35408

This:

class_name *obj = new class_name;

creates an object on the heap, you will need to delete it via delete to avoid memory leaks.

This:

class_name obj = class_name();

creates an object on the stack, the object will be deleted when it goes out of scope.

Upvotes: 1

Blaz Bratanic
Blaz Bratanic

Reputation: 2279

First is dynamically allocated. Second is statically allocated, but value initialized.

Upvotes: 1

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