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#288 in Memory management
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Map Box
Map the value in a Box, re-using the allocation when possible.
For example, this code will not re-allocate.
use map_box::Map;
let b = Box::new(42u64);
let b = b.map_box(|v| v as i64);
The signature of map
is:
impl<T1> Box<T1> {
fn map_box<T2>(self, f: impl FnMut(T1) -> T2) -> Box<T2>;
}
Limitations
If the alignment requirements of the type changes, even if the alignment becomes lower, the Box needs to be reallocated. This is because:
- alloc::dealloc requires the layout to be identical to the layout that the allocation was made with
- alloc::realloc only takes a new size, it cannot change the layout of the allocation downwards