A quick one: Passing a reference/pointer to NSString
Hi Guys
What do I need to do in the following code to get theString to take the value
I'm giving it in foo i.e. Hi there?
- (void) aMethod
{
NSString* theString = @;
[self foo:theString];
}
- (NSString*) foo:(NSString*)aString
{
NSString* stringB = @Hi there;
aString = stringB;
}
Thanks guys.
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Re: A quick one: Passing a reference/pointer to NSString
NSString is not mutable, so once created, you can't change its
contents. You can reassign the pointer to another string, as you are
doing - though without knowing your intentions it doesn't really look
right (and could cause a memory leak). Your 'foo' method is prototyped
to return a string but isn't doing that, so maybe that's why you're
not seeing what you expect?
What are you trying to do? If you want to change the content of a
string, you could use NSMutableString, but the code you've posted
isn't a great pattern for string manipulation so the intention of it
isn't clear (to me, anyway).
cheers, Graham
On 23 Jul 2008, at 11:33 am, Jeff Brown wrote:
Hi Guys
What do I need to do in the following code to get theString to take
the value I'm giving it in foo i.e. Hi there?
- (void) aMethod
{
NSString* theString = @;
[self foo:theString];
}
- (NSString*) foo:(NSString*)aString
{
NSString* stringB = @Hi there;
aString = stringB;
}
Thanks guys.
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Re: A quick one: Passing a reference/pointer to NSString
Sorry - it should have been:
- (void) aMethod
{
NSString* string1 = @;
NSString* string2 = @;
string1 = [self foo:string2];
}
- (NSString*) foo:(NSString*)aString
{
NSString* stringA = @Hi there;
NSString* stringB = @Everyone;
aString = stringB;
return stringA;
}
I need to get 2 strings back from foo. I can get foo to return one. I need to
pass the other one in by reference. It seems that since I'm passing a pointer
to an NSString as the argument, it should be fine but it's not.
By the way
How do you reply on this mailing list so that the reply remains part of the
thread?
Start at the new Yahoo!7 for a better online experience. www.yahoo7.com.au
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Re: A quick one: Passing a reference/pointer to NSString
On Jul 22, 2008, at 8:33 PM, Jeff Brown wrote:
What do I need to do in the following code to get theString to take
the value I'm giving it in foo i.e. Hi there?
- (void) aMethod
{
NSString* theString = @;
[self foo:theString];
}
- (NSString*) foo:(NSString*)aString
{
NSString* stringB = @Hi there;
aString = stringB;
}
If you're just inquiring about how to use the language, I think you're
looking for:
- (void) aMethod
{
NSString* theString = nil; // No sense initializing it with a pointer
to a string, when you're about to reassign it.
[self foo:theString]; // Don't pass the pointer theString, pass the
pointer to the pointer theString so that -foo: can write through the
pointer-to-pointer to modify the pointer. Makes sense? ;)
}
- (void) foo:(NSString**)aString // Note the extra asterisk, which
establishes an additional level of indirection
{
*aString = @Hi there; // Use the asterisk to dereference the
pointer-to-pointer to modify the pointed-to pointer
}
However, that's a pretty uncommon technique when using Cocoa. More
common would be for a method to return a string.
Also, keep the memory management guidelines in mind. They are
obscured a bit in trivial examples like this, which only involve
string literals. The convention is that callers of the -foo: method,
since they are not invoking +alloc, +new, -copy..., or -mutableCopy...
need not concern themselves with releasing the returned object. So, -
foo: must be implemented to take care of that itself, often by making
sure the returned object has been autoreleased.
Lastly, the naming conventions for a method like -foo: would be to
prefix it with get. Something like getTitle:. Again, though,
more common would be just a title method which returned the title
rather than outputting it via a by-reference parameter.
Cheers,
Ken
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Re: A quick one: Passing a reference/pointer to NSString
On 23 Jul 2008, at 11:49 am, Jeff Brown wrote:
Sorry - it should have been:
- (void) aMethod
{
NSString* string1 = @;
NSString* string2 = @;
string1 = [self foo:string2];
}
- (NSString*) foo:(NSString*)aString
{
NSString* stringA = @Hi there;
NSString* stringB = @Everyone;
aString = stringB;
return stringA;
}
I need to get 2 strings back from foo. I can get foo to return one.
I need to pass the other one in by reference. It seems that since
I'm passing a pointer to an NSString as the argument, it should be
fine but it's not.
This is my opinion, so take it FWIW: methods that return one value as
a result and another by reference really suck. Sometimes you have no
choice, if what you are returning are scalars, C arrays or simple
structs, but with objects, there's little excuse for it. What's wrong
with returning an array of strings?
Anyway, to your problem:
NSStrings are immutable.
If you *really* want to return a string object by reference, your
method would look like this:
- (void) leakLikeABastard:(NSString**) aString
{
*aString = @new string;
}
a much better way to do what you are apparently trying to do is:
- (NSArray*) foo
{
return [NSArray arrayWithObjects:@Hi there, @Everyone, nil];
}
But since it's not really too clear what your ultimate aim is, this
may not be the best way either.
By the way
How do you reply on this mailing list so that the reply remains part
of the thread?
Just hit reply (all) - it worked, your message is in the same thread.
cheers, Graham
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Re: A quick one: Passing a reference/pointer to NSString
On Jul 22, 2008, at 8:49 PM, Jeff Brown wrote:
Sorry - it should have been:
- (void) aMethod
{
NSString* string1 = @;
NSString* string2 = @;
string1 = [self foo:string2];
}
- (NSString*) foo:(NSString*)aString
{
NSString* stringA = @Hi there;
NSString* stringB = @Everyone;
aString = stringB;
return stringA;
}
I need to get 2 strings back from foo. I can get foo to return one.
I need to pass the other one in by reference.
Alternatives to consider:
*) Have the method return an NSArray* containing the strings
*) Have the method return a struct which has two NSString* fields
*) Have a method such as:
- (void) getGreeting:(NSString**)greeting andAdressee:
(NSString**)addressee;
That is, if you're going to write a get-style method, probably better
to have it supply both outputs through by-reference parameters than to
supply one via by-reference parameter and another via return value.
It seems that since I'm passing a pointer to an NSString as the
argument, it should be fine but it's not.
It was doing exactly what you told it to. In your original code, you
had a variable aString which was a pointer to an NSString object.
Your code then assigned a different value to the pointer, making it
point to a different object. However, aString is local to the -foo:
method. Changing what it pointed to did not affect the pointer
theString in aMethod. (At the point where -foo: was called, the
contents of the theString variable was copied to the aString variable
in the new context. After that copy the two variables are entirely
independent. They both initially pointed to the same object, but that
changed when you assigned a new pointer to aString.)
By the way
How do you reply on this mailing list so that the reply remains part
of the thread?
I generally just do a Reply All.
Cheers,
Ken
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Re: A quick one: Passing a reference/pointer to NSString
On Jul 22, 2008, at 9:07 PM, Graham Cox wrote:
- (void) leakLikeABastard:(NSString**) aString
{
*aString = @new string;
}
There's nothing about returning an object pointer by reference that's
inherently prone to leaking. The more likely problem is in the caller
of such a method. If the caller supplies the address of a variable
which already holds a strong reference to an object, then passing that
variable by reference to this method would cause the caller to lose
track of its pointer. It would thus be unable to later release its
strong reference, causing a leak.
Cheers,
Ken
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Re: A quick one: Passing a reference/pointer to NSString
On Jul 22, 2008, at 9:14 PM, Ken Thomases wrote: Alternatives to consider: *) Have the method return an NSArray* containing the strings *) Have the method return a struct which has two NSString* fields *) Have a method such as: - (void) getGreeting:(NSString**)greeting andAdressee: (NSString**)addressee; That is, if you're going to write a get-style method, probably better to have it supply both outputs through by-reference parameters than to supply one via by-reference parameter and another via return value. More alternatives that I forgot to list: *) Have the method return an NSDictionary* with keys for accessing the two strings *) Have separate methods: -(NSString*) greeting; -(NSString*) addressee; *) The usual justification for returning two values from one method is that the two are closely interrelated. It maybe doesn't make sense for them to exist in isolation from one another. (The trivial examples discussed don't illustrate this.) In such a case, that might be a design clue that you need a separate class to represent whatever single conceptual idea spans the two tidbits of information. So, you might want a custom class which represents the single concept which has two (or more) properties. Then, -foo (no colon) would return an instance of that class. Cheers, Ken ___ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to [EMAIL PROTECTED]
Re: A quick one: Passing a reference/pointer to NSString
I know, the problem lies with the caller. But this approach is bad form in my view, and does require that you take more care to avoid leaks. Graham On 23 Jul 2008, at 12:19 pm, Ken Thomases wrote: There's nothing about returning an object pointer by reference that's inherently prone to leaking. ___ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to [EMAIL PROTECTED]
Re: A quick one: Passing a reference/pointer to NSString
Thanks guys for all the advice. Much appreciated. Jeff Start at the new Yahoo!7 for a better online experience. www.yahoo7.com.au ___ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to [EMAIL PROTECTED]
