Why is undefined: error thrown while passing custom struct as pointers? - go

Question

package main
import (
"encoding/json"
"fmt"
)
func main() {
type CustomInfo struct {
Name string
Size int
}
type Error struct {
ErrorCode int
ErrorMsg string
}
type Product struct {
Fruit string
CInfo CustomInfo
Err Error
}
var pr1 = Product{
Fruit: "Orange",
CInfo: CustomInfo{
Name: "orango botanica",
Size: 3,
},
Err: Error{
ErrorMsg: "",
},
}
var pr2 = Product{
Fruit: "Apple",
CInfo: CustomInfo{
Name: "appleo botanica",
Size: 4,
},
Err: Error{
ErrorMsg: "",
},
}
var products []Product
products = append(products, pr1, pr2)
mrshl, _ := json.Marshal(products)
var productsRes []Product
err := json.Unmarshal([]byte(mrshl), &productsRes)
if err != nil {
fmt.Println(err)
}
//fmt.Println(productsRes[0].Fruit)
//fmt.Println(productsRes[1])
//fmt.Println(unmrshl)
validate(&productsRes)
}
func validate(bRes *Product){
fmt.Println(bRes[0].Fruit)
fmt.Println(bRes[1])
}
Why do I get ./prog.go:61:22: undefined: Product ?

Answer 1

I modified your updated playground example a bit here.
You don't want a pointer to the slice, you just want to pass the slice itself. It's not inherently wrong to pass a pointer, it's just unnecessary here. A slice means: "I (main) give you (validate) access to an array I have made." The slice header provides the user-of-the-slice:
access to the array (via indexing: bRes[i] is the i-th element of the array);
the length of the array: len(bRes)—the for loops use this implicitly; and
the capacity of the array (not used in this example).
By writing to bRes[i] we can update any or all of the fields of one of the Products in the underlying array. This is what the second loop I added to validate does.
Note: lines 47-48, which read:
var products []Product
products = append(products, pr1, pr2)
uses append a little oddly: since we just have the two products, we could build the slice directly with:
products := []Product{pr1, pr2}
The value of products will be nil initially. The nil slice header says, in effect, that the length and capacity are both zero, and there is no underlying array after all. Appending to a nil slice always causes append to allocate a new underlying array. The append function returns the new slice, which uses the new array.1 So there's a tiny bit of wasted effort in setting up this nil slice, only to throw it out. Again, it's not wrong, it's just unnecessary.
(Meanwhile, you get +1 point for checking for an error from json.Unmarshal, but -1 point, or maybe minus half a point, for not checking for an error from json.Marshal. 😀)
1append always constructs a new slice header. The new header may re-use the old array, in some cases. or it may use a new array. The append operation will re-use the old, already-existing array if and only if the appended elements fit into the existing array based on the capacity indicated by the original slice header. Since a nil header has a capacity of zero, its existing array cannot be used here.

Answer 2

Your struct definition is in main and thus out of scope for validate, it can only be used inside of your main function. It should work when you move your struct definitions out of main
Also, your validate function should probably accept a []Product (slice of Product), not a *Product (pointer to single Product)