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NewToCPP
*nix forums beginner
Joined: 08 Dec 2005
Posts: 31
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 Posted: Thu Jul 20, 2006 3:37 pm Post subject:
does the default constructor initialize values?
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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?
thanks. |
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roberts.noah@gmail.com
*nix forums Guru
Joined: 11 Sep 2005
Posts: 795
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| Posted: Thu Jul 20, 2006 3:40 pm Post subject:
Re: does the default constructor initialize values?
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NewToCPP wrote:
| Quote: | 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?
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It zero initializes them.
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Alf P. Steinbach
*nix forums Guru
Joined: 09 Mar 2005
Posts: 1855
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| Posted: Thu Jul 20, 2006 3:44 pm Post subject:
Re: does the default constructor initialize values?
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* Noah Roberts:
| Quote: | 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.
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No, it does exactly nothing (in this class).
int main()
{
A o; // Default-constructed, members of o are not initialized.
}
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail? |
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Rolf Magnus
*nix forums Guru
Joined: 21 Feb 2005
Posts: 1236
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| Posted: Thu Jul 20, 2006 3:46 pm Post subject:
Re: does the default constructor initialize values?
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NewToCPP wrote:
| Quote: | does the default constructor initialize values?
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That depends.
| Quote: | I have a class as defined below:
class A
{
int i;
char c;
int * iPtr;
};
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Note that this class is quite useless, because all the members are private.
| Quote: | Does the default constructor initialize the values of i, c & iPtr?
|
In this case, the default constructor will be generated by the compiler. It
does nothing, so no, the values won't get initialized. |
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roberts.noah@gmail.com
*nix forums Guru
Joined: 11 Sep 2005
Posts: 795
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| Posted: Thu Jul 20, 2006 3:52 pm Post subject:
Re: does the default constructor initialize values?
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Alf P. Steinbach wrote:
| Quote: | * 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.
}
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That doesn't actually call the default constructor of a POD though.
A o = A(); |
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Frederick Gotham
*nix forums Guru
Joined: 09 Jun 2006
Posts: 502
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| Posted: Thu Jul 20, 2006 4:36 pm Post subject:
Re: does the default constructor initialize values?
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NewToCPP posted:
| Quote: | class A
{
int i;
char c;
int * iPtr;
};
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That is equivalent to:
struct A {
private:
int i;
char c;
int *p;
};
Let's remove the "private" as it just complicates the example. So we have:
struct A {
int i;
char c;
int *p;
};
int main()
{
A obj1; /* Contains garbage */
A obj2 = {}; /* Each member gets its default value */
A obj3 = A(); /* Each member gets its default value */
A static obj4; /* Each member gets its default value */
}
--
Frederick Gotham |
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Alf P. Steinbach
*nix forums Guru
Joined: 09 Mar 2005
Posts: 1855
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| Posted: Thu Jul 20, 2006 6:01 pm Post subject:
Re: does the default constructor initialize values?
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* Noah Roberts:
| Quote: | 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();
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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.
Q: What is the most annoying thing on usenet and in e-mail? |
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NewToCPP
*nix forums beginner
Joined: 08 Dec 2005
Posts: 31
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| Posted: Thu Jul 20, 2006 7:23 pm Post subject:
Re: does the default constructor initialize values?
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Does it initialize if the members are public?
Rolf Magnus wrote:
| Quote: | NewToCPP wrote:
does the default constructor initialize values?
That depends.
I have a class as defined below:
class A
{
int i;
char c;
int * iPtr;
};
Note that this class is quite useless, because all the members are private.
Does the default constructor initialize the values of i, c & iPtr?
In this case, the default constructor will be generated by the compiler. It
does nothing, so no, the values won't get initialized. |
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NewToCPP
*nix forums beginner
Joined: 08 Dec 2005
Posts: 31
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| Posted: Thu Jul 20, 2006 7:25 pm Post subject:
Re: does the default constructor initialize values?
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Frederick:
What are the default values for each of those members?
I guess for "i ==> 0", "iPtr ==> NULL" what is default for c?
Frederick Gotham wrote:
| Quote: | NewToCPP posted:
class A
{
int i;
char c;
int * iPtr;
};
That is equivalent to:
struct A {
private:
int i;
char c;
int *p;
};
Let's remove the "private" as it just complicates the example. So we have:
struct A {
int i;
char c;
int *p;
};
int main()
{
A obj1; /* Contains garbage */
A obj2 = {}; /* Each member gets its default value */
A obj3 = A(); /* Each member gets its default value */
A static obj4; /* Each member gets its default value */
}
--
Frederick Gotham |
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Frederick Gotham
*nix forums Guru
Joined: 09 Jun 2006
Posts: 502
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| Posted: Thu Jul 20, 2006 7:46 pm Post subject:
Re: does the default constructor initialize values?
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NewToCPP posted:
| Quote: | Frederick:
What are the default values for each of those members?
I guess for "i ==> 0", "iPtr ==> NULL" what is default for c?
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Numeric types, e.g. integer types and floating point types, get 0.
("char" is a numeric type.)
"bool" becomes false.
Pointers get the null pointer value.
In all cases, it's as if you'd written:
IntrinsicType object = 0;
--
Frederick Gotham |
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NewToCPP
*nix forums beginner
Joined: 08 Dec 2005
Posts: 31
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| Posted: Thu Jul 20, 2006 10:21 pm Post subject:
Re: does the default constructor initialize values?
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What does it do in case of non-public members?
Frederick Gotham wrote:
| Quote: | NewToCPP posted:
Frederick:
What are the default values for each of those members?
I guess for "i ==> 0", "iPtr ==> NULL" what is default for c?
Numeric types, e.g. integer types and floating point types, get 0.
("char" is a numeric type.)
"bool" becomes false.
Pointers get the null pointer value.
In all cases, it's as if you'd written:
IntrinsicType object = 0;
--
Frederick Gotham |
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Alan Johnson
*nix forums Guru Wannabe
Joined: 14 May 2005
Posts: 206
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| Posted: Fri Jul 21, 2006 2:51 am Post subject:
Re: does the default constructor initialize values?
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NewToCPP wrote:
| Quote: | Rolf Magnus wrote:
NewToCPP wrote:
does the default constructor initialize values?
That depends.
I have a class as defined below:
class A
{
int i;
char c;
int * iPtr;
};
Note that this class is quite useless, because all the members are private.
Does the default constructor initialize the values of i, c & iPtr?
In this case, the default constructor will be generated by the compiler. It
does nothing, so no, the values won't get initialized.
Does it initialize if the members are public?
|
No.
Also, look at the fourth bullet point in this FAQ:
http://www.parashift.com/c++-faq-lite/how-to-post.html#faq-5.4
--
Alan Johnson |
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Frederick Gotham
*nix forums Guru
Joined: 09 Jun 2006
Posts: 502
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| Posted: Fri Jul 21, 2006 3:30 am Post subject:
Re: does the default constructor initialize values?
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NewToCPP posted:
| Quote: | What does it do in case of non-public members?
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Everything's the same, regardless of whether a member is public, private or
protected.
--
Frederick Gotham |
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