Update the ways to get a pointer from a box
Show how to get a pointer without destroying the box. Use `boxed::into_raw` instead of `mem::transmute`.
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@ -33,31 +33,34 @@
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//! let my_speed_ptr: *mut i32 = &mut my_speed;
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//! ```
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//!
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//! To get a pointer to a boxed value, dereference the box:
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//!
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//! ```
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//! let my_num: Box<i32> = Box::new(10);
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//! let my_num_ptr: *const i32 = &*my_num;
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//! let mut my_speed: Box<i32> = Box::new(88);
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//! let my_speed_ptr: *mut i32 = &mut *my_speed;
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//! ```
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//!
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//! This does not take ownership of the original allocation
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//! and requires no resource management later,
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//! but you must not use the pointer after its lifetime.
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//!
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//! ## 2. Transmute an owned box (`Box<T>`).
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//! ## 2. Consume a box (`Box<T>`).
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//!
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//! The `transmute` function takes, by value, whatever it's given
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//! and returns it as whatever type is requested, as long as the
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//! types are the same size. Because `Box<T>` and `*mut T` have the same
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//! representation they can be trivially,
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//! though unsafely, transformed from one type to the other.
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//! The `into_raw` function consumes a box and returns
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//! the raw pointer. It doesn't destroy `T` or deallocate any memory.
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//!
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//! ```
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//! use std::mem;
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//! use std::boxed;
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//!
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//! unsafe {
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//! let my_num: Box<i32> = Box::new(10);
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//! let my_num: *const i32 = mem::transmute(my_num);
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//! let my_speed: Box<i32> = Box::new(88);
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//! let my_speed: *mut i32 = mem::transmute(my_speed);
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//! let my_speed: *mut i32 = boxed::into_raw(my_speed);
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//!
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//! // By taking ownership of the original `Box<T>` though
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//! // we are obligated to transmute it back later to be destroyed.
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//! drop(mem::transmute::<_, Box<i32>>(my_speed));
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//! drop(mem::transmute::<_, Box<i32>>(my_num));
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//! // we are obligated to put it together later to be destroyed.
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//! drop(Box::from_raw(my_speed));
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//! }
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//! ```
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//!
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