How to Efficiently Merge Go Structs While Prioritizing Specific Values?

Merging Struct Fields Efficiently

Problem

Consider a Config struct with multiple fields. You have two structs of this type, DefaultConfig with default values and FileConfig loaded from a file. The goal is to merge these structs, giving priority to values in FileConfig while preserving unset fields.

Reflection-Based Solution

One approach involves using reflection to compare field values and selectively update those in DefaultConfig:

func merge(default *Config, file *Config) (*Config) {
  b := reflect.ValueOf(default).Elem()
  o := reflect.ValueOf(file).Elem()

  for i := 0; i <p>However, this method requires careful handling of zero values, as it could lead to unintended overwrites.</p><h3 id="Encoding-JSON-Based-Solution">Encoding/JSON-Based Solution</h3><p>A more elegant and zero-effort solution utilizes the encoding/json package:</p><pre class="brush:php;toolbar:false">import (
  "encoding/json"
  "io/ioutil"
)

var defConfig = &Config{
  S1: "def1",
  S2: "def2",
  S3: "def3",
}

func main() {
  fileContent, err := ioutil.ReadFile("config.json")
  if err != nil {
    panic(err)
  }

  err = json.NewDecoder(bytes.NewReader(fileContent)).Decode(&defConfig)
  if err != nil {
    panic(err)
  }

  fmt.Printf("%+v", defConfig)
}

With this approach:

• Missing values in config.json will retain the default values.
• Values in config.json will override default values, even if they are set to zero.
• Explicit zero values in config.json will overwrite non-zero default values.

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