Class A
{
};
What is the difference between A a , A* a and A* a = new A().
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closed as "not a real question". The given code isn't C++.Johannes Schaub - litb– Johannes Schaub - litb2010-04-17 10:55:09 +00:00Commented Apr 17, 2010 at 10:55
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2 Answers
A a;
Creates an instance of an A that lives on the stack using the default constructor.
A *a;
Is simply a uninitialized pointer to an A. It doesn't actually point to an A object at this point, but could. An initialized pointer (in this case, set to NULL) would look like so:
A *a = 0;
The difference here is that a null pointer does not point to any object while an uninitialized pointer might point anywhere. Initializing your pointers is a good practice to get into lest you find yourself wondering why your program is blowing up or yielding incorrect results.
Similarly, you don't want to attempt to dereference either a NULL pointer or an uninitialized pointer. But you can test the NULL pointer. Testing an uninitialized pointer yields undetermined and erroneous results. It may in fact be != 0 but certainly doesn't point anywhere you intend it to point. Make sure you initialize your pointers before testing them and test them before you attempt to dereference them.
A a = new A();
should be written as
A *a = new A();
and that creates a new A object that was allocated on the heap. The A object was created using the default constructor.
Where a default constructor is not explicitly written for a class, the compiler will implicitly create one though I don't believe the standard does not specify the state of data members for the object implicitly instantiated. For a discussion about implicit default constructors, see Martin York's response to this SO question.
5 Comments
Tyler McHenry
It's probably worth noting that
A a calls the default constructor too, if you're going to mention that about the heap-allocated version.
KeithB
Its also worth noting that
A *a; creates an uninitialized pointer. The only thing that you can do with it is assign a value to it (meaning, no comparisons or other uses). It might be worth adding a fourth example, A *a = 0, in which case a can be used in comparisons.
AnT stands with Russia
The last statement is incorect. The object
A() is created with value-initialization. It doesn't use any contructors of A (in this case). Expression A() will use the default contructor only when it is explicitly declared by user. Otherwise, the initialization proceeds without using the constructor.
itsmatt
@AndreyT - Agreed - there will be an implicit construction done by the compiler if there isn't a default constructor explicitly defined for the class. The assumption I made here was that there was one - despite the fact that OP's class is empty - because there typically is one defined. The distinction is a good one to make and I thank you for pointing that out. I'll add a note to the answer.
Johannes Schaub - litb
Consider:
struct A { ~A() { } int const a; };. Then new A() is perfectly valid (and initializes a to zero). But if it would use the default constructor like in A a; or new A it would be ill-formed (because of the const member). So it makes a big difference whether the default constructor is used or not.A a declares an instance of A named a
A *a declares a pointer to a A class
A *a = new A() allocates space for a on the heap and calls the proper constructor (if no constructor is specified, it performs default initialization).
For further information about the last form see http://en.wikipedia.org/wiki/New_%28C%2B%2B%29
answered Apr 16, 2010 at 11:08
Federico klez Culloca
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6 Comments
Andreas Brinck
Although the last one should have been
A* a = ...
Stack Overflow is garbage
The last one doesn't just allocate space for
a on the heap. It also initializes the A object, and declares a pointer to it.
Sachin
@Jalf, how about A a, where does it allcate memory . i can see constructor gets implicitly called when we put A a .
Stack Overflow is garbage
@Sachin: wherever it is declared. If a class contains a member declared in this way, it increases the class object's size enough to hold an additional
A. If it is declared in a function, it is put on the call stack.Sachin
@jalf, Does it mean unless I use new the memory gets allocated on stack rather than on heap.
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