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src/generics/trait-objects.md

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+# Trait Objects
+
+We've seen how a function can take arguments which implement a trait:
+
+```rust,editable
+use std::fmt::Display;
+
+fn print<T: Display>(x: T) {
+    println!("Your value: {}", x);
+}
+
+fn main() {
+    print(123);
+    print("Hello");
+}
+```
+
+However, how can we store a collection of mixed types which implement `Display`?
+
+```rust,editable,compile_fail
+fn main() {
+    let xs = vec![123, "Hello"];
+}
+```
+
+For this, we need _trait objects_:
+
+```rust,editable
+use std::fmt::Display;
+
+fn main() {
+    let xs: Vec<Box<dyn Display>> = vec![Box::new(123), Box::new("Hello")];
+    for x in xs {
+        println!("x: {x}");
+    }
+}
+```
+
+Memory layout after allocating `xs`:
+
+```bob
+ Stack                             Heap
+.- - - - - - - - - - - - - -.     .- - - - - - - - - - - - - - - - - - - - - - - -.
+:                           :     :                                               :
+:    xs                     :     :                                               :
+:   +-----------+-------+   :     :   +-----+-----+                               :
+:   | ptr       |   o---+---+-----+-->| o o | o o |                               :
+:   | len       |     2 |   :     :   +-|-|-+-|-|-+                               :
+:   | capacity  |     2 |   :     :     | |   | |   +----+----+----+----+----+    :
+:   +-----------+-------+   :     :     | |   | '-->| H  | e  | l  | l  | o  |    :
+:                           :     :     | |   |     +----+----+----+----+----+    :
+`- - - - - - - - - - - - - -'     :     | |   |                                   :
+                                  :     | |   |     +-------------------------+   :
+                                  :     | |   '---->| "<str as Display>::fmt" |   :
+                                  :     | |         +-------------------------+   :
+                                  :     | |                                       :
+                                  :     | |   +-------------------------+         :
+                                  :     | '-->| "<i32 as Display>::fmt" |         :
+                                  :     |     +-------------------------+         :
+                                  :     |                                         :
+                                  :     |     +----+----+----+----+               :
+                                  :     '---->| 7b | 00 | 00 | 00 |               :
+                                  :           +----+----+----+----+               :
+                                  :                                               :
+                                  :                                               :
+                                  '- - - - - - - - - - - - - - - - - - - - - - - -'
+```
+
+Similarly, you need a trait object if you want to return different values
+implementing a trait:
+
+```rust,editable
+fn numbers(n: i32) -> Box<dyn Iterator<Item=i32>> {
+    if n > 0 {
+        Box::new(0..n)
+    } else {
+        Box::new((n..0).rev())
+    }
+}
+
+fn main() {
+    println!("{:?}", numbers(-5).collect::<Vec<_>>());
+    println!("{:?}", numbers(5).collect::<Vec<_>>());
+}
+
+```