## Can You Type Assert Custom Data Structures in Go Plugins?
Oct 29, 2024 pm 02:14 PM
Custom Data Type Sharing Between Go Plugin and Application
In Go, it's possible to share data between a plugin and an application. However, the question arises whether it's feasible to type assert into a custom data structure as opposed to an interface.
Type Assertion with Custom Structures
Consider this scenario:
<code class="go">// plugin.go
package main
type Person struct {
Name string
}
var (
P = Person{
Name: "Emma",
}
)</code>
<code class="go">// app.go
package main
import (
"fmt"
"plugin"
"os"
)
func main() {
plug, err := plugin.Open("./plugin.so")
if err != nil {
fmt.Println(err)
os.Exit(1)
}
sym, err := plug.Lookup("P")
if err != nil {
fmt.Println(err)
os.Exit(1)
}
var p Person
p, ok := sym.(Person)
if !ok {
fmt.Println("Wrong symbol type")
os.Exit(1)
}
fmt.Println(p.Name)
}</code>
When attempting to type assert the symbol P to a Person in the app.go file, you'll encounter an execution time error: "Wrong symbol type."
Solution: Common Type Package
To overcome this limitation, define the custom data type in a separate package and use it in both the plugin and the main application.
<code class="go">// filter/filter.go
package filter
type Filter struct {
Name string
Age int
}</code>
<code class="go">// plugin/main.go
package main
import (
"play/filter"
)
var MyFilter = filter.Filter{
Name: "Bob",
Age: 21,
}
func CreateFilter() filter.Filter {
return filter.Filter{
Name: "Bob",
Age: 21,
}
}</code>
<code class="go">// app/main.go
package main
import (
"fmt"
"log"
"os"
"play/filter"
"plugin"
)
func main() {
p, err := plugin.Open("plugin.so")
if err != nil {
log.Fatal(err)
}
mf, err := p.Lookup("MyFilter")
if err != nil {
log.Fatal(err)
}
f, ok := mf.(*filter.Filter)
if !ok {
log.Fatal("Wrong symbol type")
}
fmt.Printf("%+v\n", f)
}</code>
In this example, the Filter type is defined in a separate package, making it accessible to both the plugin and the main application. As a result, type assertion from the plugin's symbol to a *filter.Filter succeeds.
Note that:
- Variables defined in the main package cannot be referred to from other packages, so avoid using the same type in the plugin and the main application directly.
- Symbols representing variables are returned as pointers, allowing you to modify their values.
- Functions returning values of non-pointer type can be looked up with a function pointer.
- Changing the common type package requires rebuilding the plugin.
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