| Topic: |
DEVELOP > c-Plus-Plus |
| User: |
"E.T. Grey" |
| Date: |
02 Sep 2006 12:29:31 PM |
| Object: |
pointers anbd references as parameters |
Can I pass a pointer instead of a reference (and vice versa)? - I know a
reference and a pointer are not the same - but i can't help feeling I
can "get away with this".
Bad practise, ok practise, or must be avoided at all times?
If must be avoided, can I hust take the address of a reference and use
that as a pointer? (actually, its mre useful if I can pass a pointer as
reference
PS: Alarm bells ringing though - a ptr can be null, whereas a reference
can't be .. Hmmmm, looks like "terra infestus" (dangerous ground) to me ...
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| User: "Jim Langston" |
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| Title: Re: pointers anbd references as parameters |
01 Sep 2006 05:37:04 PM |
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"E.T. Grey" <nebula@alpha-centauri.com> wrote in message
news:YNGdnRElsP-4ImTZRVnyjw@bt.com...
Can I pass a pointer instead of a reference (and vice versa)? - I know a
reference and a pointer are not the same - but i can't help feeling I can
"get away with this".
It depends on where you want to pass it. If a function or method expects a
reference then you couldn't pass a pointer to it, but you could modify the
function/method to accept a pointer instead.
Bad practise, ok practise, or must be avoided at all times?
Useful practice in some instances.
If must be avoided, can I hust take the address of a reference and use
that as a pointer? (actually, its mre useful if I can pass a pointer as
reference
Hopefully this code shows you what you are asking about.
#include <string>
#include <iostream>
void MyRefF( int& IntR )
{
// How to use IntR as a pointer? This works for me.
int* IntP = &IntR;
*IntP = 20;
std::cout << *IntP << std::endl;
}
int main()
{
int* MyIntP = new int(10);
// So how do we pass MyIntP to MyRefF? Dereference it.
MyRefF( *MyIntP );
std::cout << *MyIntP << std::endl;
delete MyIntP;
MyIntP = NULL;
// Following is undefined behavior, and gives me illegal memory access
// when the variable is used inside of MyRefF
MyRefF( *MyIntP );
std::string wait;
std::getline( std::cin, wait );
}
PS: Alarm bells ringing though - a ptr can be null, whereas a reference
can't be .. Hmmmm, looks like "terra infestus" (dangerous ground) to me
...
And you are right to think that, as the code shows.
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| User: "Alf P. Steinbach" |
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| Title: Re: pointers anbd references as parameters |
02 Sep 2006 12:32:22 PM |
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* E.T. Grey:
PS: Alarm bells ringing though - a ptr can be null, whereas a reference
can't be .. Hmmmm, looks like "terra infestus" (dangerous ground) to me ...
Yes, that's why you preferentially should use references for arguments,
and pointers only where null is a possibility you actively support. One
common exception is to pass pointers for dynamically allocated objects.
A probably much better practice is to use smart pointers for those.
--
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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| User: "Frederick Gotham" |
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| Title: Re: pointers anbd references as parameters |
03 Sep 2006 12:51:28 PM |
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Alf P. Steinbach posted:
* E.T. Grey:
PS: Alarm bells ringing though - a ptr can be null, whereas a reference
can't be .. Hmmmm, looks like "terra infestus" (dangerous ground) to me
...
Yes, that's why you preferentially should use references for arguments,
and pointers only where null is a possibility you actively support.
Another common usage of passing pointers by value is where you are dealing
with an array, e.g.:
void Func(MyClass *p,size_t len)
{
assert(p); assert(len);
do
{
/* Manipulate the array */
} while(--len);
}
--
Frederick Gotham
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| User: "Alf P. Steinbach" |
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| Title: Re: pointers anbd references as parameters |
03 Sep 2006 01:23:28 PM |
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* Frederick Gotham:
Alf P. Steinbach posted:
* E.T. Grey:
PS: Alarm bells ringing though - a ptr can be null, whereas a reference
can't be .. Hmmmm, looks like "terra infestus" (dangerous ground) to me
...
Yes, that's why you preferentially should use references for arguments,
and pointers only where null is a possibility you actively support.
Another common usage of passing pointers by value is where you are dealing
with an array, e.g.:
void Func(MyClass *p,size_t len)
{
assert(p); assert(len);
do
{
/* Manipulate the array */
} while(--len);
}
Use
void func( std::vector<MyClass> const& v )
{
for( ... )
{
...
}
}
--
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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| User: "Frederick Gotham" |
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| Title: Re: pointers anbd references as parameters |
03 Sep 2006 01:32:09 PM |
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Alf P. Steinbach posted:
Use
void func( std::vector<MyClass> const& v )
{
for( ... )
{
...
}
}
I prefer to use arrays for simple tasks. I would find a vector more
appropriate if the code were a little more complicated.
--
Frederick Gotham
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| User: "Thomas J. Gritzan" |
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| Title: Re: pointers anbd references as parameters |
03 Sep 2006 01:49:12 PM |
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Frederick Gotham schrieb:
Alf P. Steinbach posted:
Use
void func( std::vector<MyClass> const& v )
{
for( ... )
{
...
}
}
I prefer to use arrays for simple tasks. I would find a vector more
appropriate if the code were a little more complicated.
I started to use std::vector everytime I would use new[].
Why? Because it's easier and it makes short & clear code. The class holds
the actual size, and deletes the memory in case of exception and normal
cleanup.
The opposite is in my (windows) driver code, where I write more "C-like".
The code looks like this:
<bad code>
Obj* o1 = CreateSomeObject();
if (!o1)
{
// handle error
return;
}
Obj* o2 = CreateSomeOtherObject();
if (!o2)
{
// handle error
free(o1); // don't forget it!
return;
}
Obj* o3 = CreateSomeOtherObject();
if (!o3)
{
// handle error
free(o2);
free(o1); // don't forget one of it!
return;
}
free(o3);
free(o2);
free(o1);
</bad code>
Perhaps I should start using scope guards in kernel code also.
--
Thomas
http://www.netmeister.org/news/learn2quote.html
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| User: "Frederick Gotham" |
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| Title: Re: pointers anbd references as parameters |
03 Sep 2006 12:49:48 PM |
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E.T. Grey posted:
Can I pass a pointer instead of a reference (and vice versa)?
Depends on the context. In general: Yes.
- I know a reference and a pointer are not the same - but i can't help
feeling I can "get away with this".
Pointers are different in that they are an actual object.
A reference is simply a idea, a mystical concept.
Bad practise, ok practise, or must be avoided at all times?
A lot of people like references; they're handier to use.
If must be avoided, can I hust take the address of a reference and use
that as a pointer?
A reference isn't an object (nor a function) -- it doesn't have an address.
Once a reference has been declared, any usage of it is actually usage of
the object which the reference refers to. For example:
int i = 5;
int &r = i;
Func(&r); /* This passes the address of i */
(actually, its mre useful if I can pass a pointer as reference
Depends on context.
--
Frederick Gotham
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| User: "Diego Martins" |
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| Title: Re: pointers anbd references as parameters |
04 Sep 2006 11:45:34 AM |
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Frederick Gotham wrote:
E.T. Grey posted:
Can I pass a pointer instead of a reference (and vice versa)?
Depends on the context. In general: Yes.
when you need a SENTINEL, you can't use references
I use const pointers, in this case
Diego
HP
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