* Noah Roberts:
Alf P. Steinbach wrote:
>* Noah Roberts:
>>NewToCPP wrote:
does the default constructor initialize values?
I have a class as defined below:
class A
{
int i;
char c;
int * iPtr;
};
Does the default constructor initialize the values of i, c & iPtr?
If so, what are the values that they each get initialized to?
It zero initializes them.
No, it does exactly nothing (in this class).
int main()
{
A o; // Default-constructed, members of o are not initialized.
}
That doesn't actually call the default constructor of a POD though.
A o = A();
On the contrary. What you have here is copy construction from a
zero-initialized instance (the expression 'A()' means
default-initialization, 8.5/7, which for a POD is defined as
zero-initialization, 8.5/5). That's not to say that 'A o;' necessarily
actually calls a default constructor: it doesn't affect the outcome
whether you say that there is no default constructor for a POD class (no
call), or that the default constructor does absolutely nothing (call
does nothing), but the standard specifies an implicitly declared default
constructor, 12.1/5, which if it actually exists must do nothing at all.
In particular, if you derive from class A,
struct A { /* as before */ };
struct B: A { int x; int y; B(): x(1), y(2) {} };
B o;
then you have an object 'o' with x and y initialized, and i, c, and iPtr
uninitialized. If class A had a default constructor that did
zero-initialization, then instead you would have i, c and iPtr zeroed.
But that isn't what you get, so the notion of a zeroing automatically
generated default constructor leads to incorrect, dangerous conclusions.
However, with a /conforming/ compiler,
struct B: A { int x; int y; B(): x(1), y(2), A() {} };
should ensure zero-initializion of the A base class sub-object; for a
POD this syntax does not denote a call of the (possibly existing or not)
default constructor, it instead denotes default initialization which is
defined as zero-initialization (references given above).
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
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