为类型引用实现特征时返回对临时值的引用时出错

Question

I am trying to implement a simple trait which returns a reference to some data ( GetData ). For different reasons, I need to implement this trait for reference to my type ( &Base<T> ). I also would like to have a wrapper type that owns the base type and implements the trait.

The simplified example looks like this:

    // Trait
    trait GetData<T> {
      fn get_data(&self) -> &T;
    }
    
    // Base type
    struct Base<T> {
      pub data: T,
    }
    
    impl<T> GetData<T> for &Base<T> {
      fn get_data(&self) -> &T {
        &self.data
      }
    }
    
    // Wrapper type owning a Base type
    struct Wrapper<T> {
      base: Base<T>,
      //...
    }
    
    impl<T> GetData<T> for &Wrapper<T> {
      fn get_data(&self) -> &T {
        (&self.base).get_data()  // E0515: returns a value referencing data owned by the current function 
        //&self.base.data        // This works fine!
      }
    }

When implementing get_data for a &Wrapper<T> I get an error of returning a temporary value from my function. As I understand &self.base is borrowed by GetData::get_data . I am not sure how to resolve it though. All my attempts to add explicit lifetimes didn't work so far.

Answer 1

The issue is with this:

    impl<T> GetData<T> for &Base<T> {
      fn get_data(&self) -> &T {
        &self.data
      }
    }

Now in all honesty I've no idea why you'd bother implementing only for &Base instead of just implementing for Base . But that's the cause of the issue because the lifetime of the Base and the lifetime of the self are not the same and your self is an &&Base<T> .

This means in:

      fn get_data(&self) -> &T {
        (&self.base).get_data()  // E0515: returns a value referencing data owned by the current function 
        //&self.base.data        // This works fine!
      }

the lifetime of the result is that of &self.base which is purely local to the function, instead of being the expected lifetime of self .

If you

    impl<T> GetData<T> for Base<T> {
      fn get_data(&self) -> &T {
        &self.data
      }
    }

then everything works out of the box.

Can't help but think the entire thing smells of OO-thinking, a GetData trait stinks of Java/C# interface.