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course/4-structs/exercises/4-embedded_structs/readme.md

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+# Embedded Structs
+
+Go is not an [object-oriented](https://en.wikipedia.org/wiki/Object-oriented_programming) language. However, embedded structs provide a kind of *data-only* inheritance that can be useful at times. Keep in mind, Go doesn't support classes or inheritance in the complete sense, embedded structs are just a way to elevate and share fields between struct definitions.
+
+```go
+type car struct {
+  make string
+  model string
+}
+
+type truck struct {
+  // "car" is embedded, so the definition of a
+  // "truck" now also additionally contains all
+  // of the fields of the car struct
+  car
+  bedSize int
+}
+```
+
+## Embedded vs nested
+
+* An embedded struct's fields are accessed at the top level, unlike nested structs.
+* Promoted fields can be accessed like normal fields except that they can't be used in [composite literals](https://golang.org/ref/spec#Composite_literals)
+
+```go
+lanesTruck := truck{
+  bedSize: 10,
+  car: car{
+    make: "toyota",
+    model: "camry",
+  },
+}
+
+fmt.Println(lanesTruck.bedSize)
+
+// embedded fields promoted to the top-level
+// instead of lanesTruck.car.make
+fmt.Println(lanesTruck.make)
+fmt.Println(lanesTruck.model)
+```
+
+## Assignment
+
+At Textio, a "user" struct represents an account holder, and a "sender" is just a "user" with some "sender" specific data. A "sender" is a user that has a `rateLimit` field that tells us how many messages they are allowed to send.
+
+Fix the system by using an embedded struct as expected by the test code.