There is an easier way to avoid the memory leak with objc::rc::autoreleasepool. Here is the original idea SSheldon/rust-objc-foundation#15 (comment)
Added it to the test and benchmark.
xcode-select --install
brew install cargo-instruments
cargo instruments -t Allocationspub fn leak(str: &str) {
let nsstrig = NSString::from_str(str);
nsstrig.as_str(); // if you replace just this one line leak is gone
}INSString.as_str() internally uses UTF8String property of NSString. Apple doc says that the memory behind this pointer has a lifetime shorter than a lifetime of an NSString itself. But apparently, this is not entirely true. At least, this statement is not valid for strings that contain characters outside the ASCI range. And sometimes for strings that do not.
Sadly, I did not find any reason for that.
So in the end, the actual leak occurs not in INSString.as_str() but, I guess, in objc runtime.
Yes. NSString::getCString (Apple doc) and we can use it like this:
pub fn convert_with_vec(nsstring: Id<NSString>) -> String {
let string_size: usize = unsafe { msg_send![nsstring, lengthOfBytesUsingEncoding: 4] };
let mut buffer: Vec<u8> = vec![0_u8; string_size + 1];
let is_success: bool = unsafe {
msg_send![nsstring, getCString:buffer.as_mut_ptr() maxLength:string_size+1 encoding:4]
};
if is_success {
// before from_vec_with_nul can be used https://github.com/rust-lang/rust/pull/89292
// nul termination from the buffer should be removed by hands
buffer.pop();
unsafe {
CString::from_vec_unchecked(buffer)
.to_str()
.unwrap()
.to_string()
}
} else {
// In case getCString failed there is no point in creating CString
// Original NSString::as_str() swallows all the errors.
// Not sure if that is the correct approach, but we also don`t have errors here.
"".to_string()
}
}If you change in main.rs leak to no_leak_vec and will run again
cargo instruments -t AllocationsYou will see no leak at all. And if you run benchmark
cargo becnhYou will see that performance even better.
to string/old time: [12.551 us 12.644 us 12.733 us]
to string/autorelease time: [12.775 us 12.998 us 13.207 us]
to string/vec time: [10.107 us 10.208 us 10.307 us] The only problem I see with this solution is that it has a different return type (String instead of &str). If you know how to fix that, or any other idea on how to do things better - please let me know.
If you want to know how it all started - here is initial blog post, pull request to a copypasta. Autorelease idea