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mescaline
Guest
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 Posted: Sat Dec 27, 2003 11:16 pm Post subject: The const in the copy ctor |
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Hi,
I can't understand why there is a "const" for a copy constructor
declaration:
X(const X & X1)
When I deine my Copy ctor to be X(X& X1) i.e. avoiding the const still
things remain the same. However every example I see, there is this
extra "const".
Why is adding the const important here? Any explanations welcome
m
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Jeff Schwab
Guest
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| Posted: Sat Dec 27, 2003 11:38 pm Post subject: Re: The const in the copy ctor |
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mescaline wrote:
| Quote: | Hi,
I can't understand why there is a "const" for a copy constructor
declaration:
X(const X & X1)
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So you can copy const objects.
| Quote: | When I deine my Copy ctor to be X(X& X1) i.e. avoiding the const still
things remain the same. However every example I see, there is this
extra "const".
|
In your code, you may not be copying any const objects of class X. Even
so, the const qualifier is a nice way of "promising" that the
copy-constructor will not modify the object being copied.
-Jeff
| Quote: |
Why is adding the const important here? Any explanations welcome
m
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Ralf
Guest
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| Posted: Sun Dec 28, 2003 12:18 am Post subject: Re: The const in the copy ctor |
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The "const" in the copy ctor has two aspects:
1. Normally, it is unusual to change the original while making a copy. With
the "const" you are proposing not to change the original.
2. To have a reference to an constant object makes it possible to copy from
an constant original.
Ralf
www.oop-trainer.de
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Cy Edmunds
Guest
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| Posted: Sun Dec 28, 2003 1:14 am Post subject: Re: The const in the copy ctor |
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"mescaline" <apt2003 (AT) columbia (DOT) edu> wrote
| Quote: | Hi,
I can't understand why there is a "const" for a copy constructor
declaration:
X(const X & X1)
When I deine my Copy ctor to be X(X& X1) i.e. avoiding the const still
things remain the same. However every example I see, there is this
extra "const".
Why is adding the const important here? Any explanations welcome
m
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Consider:
void afunc(const Fred &f)
{
Fred f2(f);
...
}
If Fred's copy constructor is declared
Fred(Fred&);
then afunc won't compile.
Ordinarily copy constructors and assignment operators do not change the
arguments; i.e.
Fred f2(f); // does not change f
f2 = f; // also does not change f
This is indicated in the signatures which should be
Fred (const Fred &);
and
Fred &operator = (const Fred &);
respectively.
--
Cy
http://home.rochester.rr.com/cyhome/
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mescaline
Guest
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| Posted: Sun Dec 28, 2003 3:21 am Post subject: Re: The const in the copy ctor |
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Jeff Schwab <jeffplus (AT) comcast (DOT) net> wrote
| Quote: | mescaline wrote:
Hi,
I can't understand why there is a "const" for a copy constructor
declaration:
X(const X & X1)
So you can copy const objects.
When I deine my Copy ctor to be X(X& X1) i.e. avoiding the const still
things remain the same. However every example I see, there is this
extra "const".
In your code, you may not be copying any const objects of class X. Even
so, the const qualifier is a nice way of "promising" that the
copy-constructor will not modify the object being copied.
-Jeff
Why is adding the const important here? Any explanation
s welcome
m
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Thanks Jeff --
So adding a const is more of a compiler issue than anything else?
i.e.Does adding const improve the way in which the code is compiled?
- m.
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Jeff Schwab
Guest
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| Posted: Sun Dec 28, 2003 3:25 am Post subject: Re: The const in the copy ctor |
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mescaline wrote:
| Quote: | Jeff Schwab <jeffplus (AT) comcast (DOT) net> wrote
mescaline wrote:
Hi,
I can't understand why there is a "const" for a copy constructor
declaration:
X(const X & X1)
So you can copy const objects.
When I deine my Copy ctor to be X(X& X1) i.e. avoiding the const still
things remain the same. However every example I see, there is this
extra "const".
In your code, you may not be copying any const objects of class X. Even
so, the const qualifier is a nice way of "promising" that the
copy-constructor will not modify the object being copied.
-Jeff
Why is adding the const important here? Any explanation
s welcome
m
Thanks Jeff --
So adding a const is more of a compiler issue than anything else?
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Exactly.
| Quote: | i.e.Does adding const improve the way in which the code is compiled?
- m.
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Nope. The optimizer usually doesn't even know anything about "const,"
and must determine on its own whether a variable can actually be changed
at run-time. Be careful to whom you tell this, though; there's a lot of
people out there using "const" with religious fervor, believing with all
their soul that this practice makes them efficient programmers. :)
-Jeff
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Jeff Schwab
Guest
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| Posted: Sun Dec 28, 2003 3:35 am Post subject: Re: The const in the copy ctor |
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| Quote: | So adding a const is more of a compiler issue than
anything else?
Exactly.
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A pang of guilt compells me to add that by using const, you are implying
certain guarantees to the clients of your code. It is true that the
only time you *need* const is to make stuff compile, and that the
compiled code is unlikely to be any better for const having been used.
However, const is also a way to document your code's behavior. It's a
lot like adding a comment that says a variable will not be altered
within a certain section of code. Adding comments doesn't make your
code any more efficient, but it might make the programmers who use it
more efficient.
Ok, guilt assuaged. :)
-Jef
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