Why do you have to pass the address of the pointer instead of the pointer itself when using the Viper library in Go?

Detailed explanation of the UnmarshalKey function of the Go language Viper library and pointer address transfer

This article discusses why the UnmarshalKey function needs to pass the address of the pointer instead of the pointer itself when using the Viper library in Go. We will analyze this problem in combination with code examples and Viper library source code.

The root cause of the problem lies in the reflection mechanism inside the UnmarshalKey function. This function requires an addressable pointer to ungroup the data from the configuration file into the target structure. Although the pointer is a pointer type, it is not an addressable memory address and cannot be modified.

Code examples and problem analysis:

The code examples provided in the article clearly show this problem. Although global.serversetting is of type *setting.serversettings (pointer), it points to an allocated memory address. What the UnmarshalKey function requires is the address of this pointer in order to modify the value in the memory area it points to. Passing global.serversetting directly is equivalent to passing the value of the pointer (i.e., the memory address), rather than the address itself. This makes UnmarshalKey unable to modify the content of the structure pointed to by serversetting .

Viper library source code analysis:

The newdecoder function fragment of the Viper library:

 func newdecoder(config *decoderconfig) (*decoder, error) {
    val := reflect.ValueOf(config.result)
    if val.Kind() != reflect.Ptr {
        return nil, errors.New("result must be a pointer")
    }

    val = val.Elem()
    if !val.CanAddr() {
        return nil, errors.New("result must be addressable (a pointer)")
    }
    // ...
}

This code explains why addressable pointers are needed:

1. val.Kind() != reflect.Ptr : Check whether the passed parameter is a pointer type.
2. val = val.Elem() : Gets the value pointed to by the pointer.
3. !val.CanAddr() : This is the key point. CanAddr() checks whether the value is addressable. If you pass the pointer directly, val.Elem() gets the structure itself, and the structure itself is not addressable because it is not a pointer. Only the address of the pointer is addressable, because the address itself represents a memory location and can be modified.

Verification code and results:

The verification code provided in the article:

 package main

import (
    "fmt"
    "reflect"
)

var a *db

type db struct {
}

func main() {
    val := reflect.ValueOf(a)
    val = val.Elem()
    fmt.Println(val.CanAddr()) // false

    val = reflect.ValueOf(&a)
    val = val.Elem()
    fmt.Println(val.CanAddr()) // true
}

This code verifies that the results returned by CanAddr() method of reflect.ValueOf(a) (pointer itself) and reflect.ValueOf(&a) (pointer's address). Only the address of the pointer can be addressed.

in conclusion:

In order to correctly use the UnmarshalKey function of the Viper library, the address of the pointer to the target structure ( &global.serversetting ) must be passed, rather than the pointer itself ( global.serversetting ). This ensures that the Viper library can correctly ungroup the configuration file data into the target structure. This is not a problem unique to the Viper library, but a reflection of the Go language reflection mechanism and pointer semantics. Understanding Go language pointer and reflection mechanism is crucial to solving such problems.

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