25 Jan 2016

A little refactoring

I checked the API of the LogMsg struct in my syslog-ng-common crate. Basically, a LogMsg contains key-value pairs so it behaves like a map. It’s API wasn’t so clear as it mirrored the name of the C original C functions.

Problem

There were three problematic methods:

• get_value_by_name(): returns the value assigned to a key
• get_value(): returns the value assigned to a handle
• set_value(): inserts a new key-value pair

A handle represents an entry in the memory where the value can be directly accessed. There were some code duplication between the function. For example, get_value_by_name() looked up the handle corresponding to the given key, then used the handle to get the value, while get_value_by_name() took a handle as its parameter and used that to get the value.

I had only one setter method (set_value()) but it’s useful to set a value via a handle.

Solution

I imagined two functions: * get(): return the value assigned to a key * insert(): insert a new key-value pair

But it’d be nice to have these functions working with handles and string references as well…

But Rust has generics! So what if we make get() generic over types which implement Into<NVHandle>?

pub fn get<K: Into<NVHandle>>(&self, key: K) -> &str {
    let handle = key.into();
    //...
    let value = logmsg::__log_msg_get_value(self.0, handle.0, &mut size);
    // ...
    value
}

We just need to implement it for &str:

impl<'a> Into<NVHandle> for &'a str {
    fn into(self) -> NVHandle {
        let name = CString::new(self).unwrap();
        let handle = unsafe { logmsg::log_msg_get_value_handle(name.as_ptr()) };
        NVHandle(handle)
    }
}

This trick can be used for the insert() function as well. The API looks much cleaner and there is no duplicated code.

Till next time,
Tibor Benke at 00:00