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https://github.com/jbranchaud/til
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66 lines
1.7 KiB
Markdown
66 lines
1.7 KiB
Markdown
# Pass A Struct To A Function
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Go operates as _pass-by-value_ which means that when we pass a struct to a
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function, the receiving function gets a copy of the struct. Two things worth
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noticing about that are 1) an extra memory allocation happens when calling the
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function and 2) altering the struct does not affect the original in the calling
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context.
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On the other hand, we can have a function that takes a pointer to a struct.
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When we call that function, we have a reference to the memory location of the
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struct instead of a copy of the struct. That means no additional allocation and
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modifications to the dereferenced struct are modifications to the original in
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the calling context.
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Here is an example that demonstrates both of these. Notice the printed output
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that is included in comments at the end which shows memory locations and
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contents of the struct at various points.
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```go
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package main
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import "fmt"
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type Order struct {
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Item string
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Quantity int
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DineIn bool
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}
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func main() {
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order := Order{Item: "taco", Quantity: 3, DineIn: true}
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fmt.Println("Order:", order)
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fmt.Printf("main - Loc: %p\n", &order)
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doubledOrder := doubleOrder(order)
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fmt.Println("Double Order:", doubledOrder)
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fmt.Println("Original Order:", order)
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doubleOrderPtr(&order)
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fmt.Println("Double Order Ptr:", order)
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}
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func doubleOrder(order Order) Order {
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fmt.Printf("doubleOrder - Loc: %p\n", &order)
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order.Quantity *= 2
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return order
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}
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func doubleOrderPtr(order *Order) {
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fmt.Printf("doubleOrderPtr - Loc: %p\n", order)
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(*order).Quantity *= 2
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}
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// Order: {taco 3 true}
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// main - Loc: 0xc0000b4000
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// doubleOrder - Loc: 0xc0000b4040
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// Double Order: {taco 6 true}
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// Original Order: {taco 3 true}
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// doubleOrderPtr - Loc: 0xc0000b4000
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// Double Order Ptr: {taco 6 true}
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```
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