Map Entries are Unaddressable
Evan Hicks
The Problem
The error:
./prog.go:15:10: cannot call pointer method on m[“bar”]
./prog.go:15:10: cannot take the address of m[“bar”]
package main
import (
"fmt"
)
type Bar struct {}
func (b *Bar) Fizz() {
fmt.Println("buzz")
}
func main() {
m := map[string]Bar{"bar": Bar{}}
m["bar"].Fizz()
}
The Solution
There are actually two different bugs here, hence the two different compile errors. The first bug is that the function Fizz is defined on a pointer receiver *Bar and m is a map of non-pointer Bar. Normally this isn’t an issue, because Go will automatically create a pointer to the object and then call Fizz on that. However, since map entries are not addressable, Go cannot create a pointer and this error occurs.
The fix:
There are two ways to fix this. The first is to change m to type map[string]*Bar and then have &Bar{} as the value. The other solution shown below is to change the Fizz method to not be a pointer receiver.
package main
import (
"fmt"
)
type Bar struct {}
func (b Bar) Fizz() {
fmt.Println("buzz")
}
func main() {
m := map[string]Bar{"bar": Bar{}}
m["bar"].Fizz()
}
Further Reading
If you’re looking to get a deeper understanding of how Go application monitoring works, take a look at the following articles: