How Can I Efficiently Merge Two Go Structs of the Same Type?

Merging Structs of the Same Type with Reflection or JSON Unmarshalling

In Go, you may encounter a scenario where you need to merge two structs of the same type, with values from one struct potentially overriding those in another. This article explores two approaches to achieve this: using Go's reflection capabilities or unmarshalling JSON data into a prepared default configuration.

Reflection Approach (Not Recommended)

The reflection approach, as presented below, requires checking for zero values to determine which fields to override. However, this solution is not ideal because:

<br>func merge(default <em>Config, file </em>Config) (*Config) {<br>  b := reflect.ValueOf(default).Elem()<br>  o := reflect.ValueOf(file).Elem()</p>
<p>for i := 0; i < b.NumField(); i   {</p><div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">defaultField := b.Field(i)
fileField := o.Field(i)
if defaultField.Interface() != reflect.Zero(fileField.Type()).Interface() {
 defaultField.Set(reflect.ValueOf(fileField.Interface()))
}

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}

return default
}

• It relies on reflection, which can be inefficient and error-prone.
• It assumes that zero values are always set in the default configuration, which may not be true in all cases.
• It does not account for explicit overrides to zero values in the file configuration.

JSON Unmarshalling Approach (Recommended)

A more elegant and flexible approach is to use Go's encoding/json package and unmarshal JSON data into a prepared default configuration. This method provides several advantages:

• Handles missing values gracefully: Values missing in the file configuration will be filled in with the default values.
• Overrides default values: Values provided in the file configuration will override the default values, regardless of their type.
• Supports explicit overrides to zero values: Values explicitly set to zero in the file configuration will override non-zero default values.

To demonstrate this approach:

1. Create a default configuration struct defConfig with your default values.
2. Load the JSON data from the file into a string variable fileContent.
3. Use json.NewDecoder() to unmarshal the JSON data into defConfig.

Example code:

var defConfig = &Config{
    S1: "",
    S2: "",
    S3: "abc",
    S4: "def",
    S5: "ghi",
}

const fileContent = `{"S2":"file-s2","S3":"","S5":"file-s5"}`

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

This solution ensures that values in the default configuration are overwritten by the values in the file configuration, providing a reliable and flexible way to merge structs of the same type.

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