NSString和Objective-C内存管理ARC

Question

I'm a little confused when coding an Objective-C project. The ARC is on. Here is a sample code:

NSString *foo = [[NSString alloc] initWithUTF8String:"This is a C string."];

// Use foo here...

foo = @"This is an ObjC string."

Here are my questions:

1. Do I need to explicitly terminate C string with '\\0' in initWithUTF8String: method, or it is okay to omit NULL terminator?

2. Is there any memory leakage when I reuse foo as a pointer and assign new Objective-C string to it? Why?

3. If I change NSString to other class, like NSObject or my own class, is there any difference for question 2? (Initialize an object and then reassign other value directly to it.)

Thank you!

Answer 1

1. You must have the null terminator. From the documentation: "bytes - A NULL-terminated C array of bytes in UTF-8 encoding. This value must not be NULL."
2. No. The compiler will insert implicit release of the previous value and retain the new one since you declared foo with (implicit) strong semantics. From the documentation: "__strong is the default. An object remains “alive” as long as there is a strong pointer to it."
3. In general, no.

Answer 2

1. An explicit \\0 is not required because in C (and hence Objective C), quoted string literals are null-terminated implicitly by the compiler. Here's a similar question.

Do string literals that end with a null-terminator contain an extra null-terminator?

2. No memory leakage. The ARC-configured compiler will generate code to release the first string that was being referenced before assigning the new string.

3. No change. You may get a compile-time warning if the types aren't compatible.

Answer 3

For the first question, what Apple's official documentation tells me is:

Returns a string created by copying the data from a given C array of UTF8-encoded bytes.

• (id)stringWithUTF8String:(const char *)bytes Parameters bytes A NULL-terminated C array of bytes in UTF8 encoding.

But since string literal is NULL terminated by default (as @TomSwift points out), it's okay to omit it.