String allocation and instance variables

I am quite new to programming and objective C and I am having a hard time grasping a concept here about allocation, memory management and how they tie in with instance variables. I created a category for NSString called isUrl to test for a prefix of "http:// in a string.

From there, I wanted to test this on a string. However, given that situation I couldn't figure out if I should: 1) Declare a new string as an instance variable with @property to synthesize the accessors 2) Allocate a new string in my implementation 3) Set my variable and allocate it

What would be the difference between these methods and why would I perhaps want to use one over the other? -

As a side note, I am working with a book by O'Reilly and have tried to find answers around the web, but not a lot of luck thus far. I seem to find examples of everything, but I an explanation of how or why they are used is more difficult.

1) NSString* 开发者_JAVA技巧string = [[[NSString alloc]init]autorelease];

string = @"http://www.google.com";

if ( [string isUrl]) {

NSLog(@"Caption is a URL");

NSLog(@"URL %@",string);

string = nil;

2) NSString* string = @"http://www.googl.com";

[string retain];

if ( [string isUrl]) {

NSLog(@"Caption is a URL");

NSLog(@"URL %@",string);

string = nil;

3) Something like this....

@synthesize string;

string.name = "http://www.google.com";

if ( [string.name isUrl]) {

NSLog(@"Caption is a URL");

NSLog(@"URL %@",string);

string = nil;

String literals don't need to be released or retained.

Properties should be used whenever a value associated with an object needs to exist outside of a single function call and you don't need your program to compile under 10.4 or earlier. By your use of nil, it appears you don't need the string to persist, so you should only use a local variable. That's what local variables are for.

NSString* string = @"http://www.google.com";
if ( [string isUrl]) {
    NSLog(@"Caption is a URL");
    NSLog(@"URL %@",string);
}

Apple's Objective-C memory management rules are quite simple. There's really only one rule, with a couple of corollaries.

NSString* string = [[[NSString alloc]init]autorelease];
string = @"http://www.google.com";

You don't need to allocate an object before assigning a different object to the same variable. The first object will simply get deallocated and serves no purpose.

string.name = "http://www.google.com";

"" denotes a C string. For an NSString, you must use the at symbol. The above should be

string.name = @"http://www.google.com";