How Can I Efficiently Merge Two Structs with Overlapping Fields, Prioritizing One Struct\'s Values?

Merging Fields of Identical Structs

Problem: Given two structs with possibly overlapping fields, how can one merge them, prioritizing the fields of the second struct over the first?

In the example provided, the Config struct has several fields. The goal is to combine two instances of this struct (DefaultConfig and FileConfig), with FileConfig taking precedence. However, FileConfig may have missing fields.

Reflection Approach:

The provided code snippet uses reflection to check if a field's value in FileConfig is not the default for its type. If so, it sets the field in DefaultConfig to the FileConfig value.

Simplified JSON-Based Approach:

An alternative and efficient approach is to use the encoding/json package to decode the contents of FileConfig into a copy of DefaultConfig. This method offers several benefits:

• Handles Missing Values Automatically: Fields that are missing from FileConfig will be filled in with the default values from DefaultConfig.
• Overrides with File Values: Fields that are present in FileConfig will override the values in DefaultConfig.
• Preserves Zero Values: Even zero values explicitly set in FileConfig will overwrite the non-zero default values in DefaultConfig.

Implementation:

import (
    "encoding/json"
)

type Config struct {
    S1 string
    S2 string
    S3 string
    S4 string
    S5 string
}

func MergeConfig(defaultConfig, fileConfig *Config) *Config {
    // Make a copy of the default configuration
    mergedConfig := &Config{*defaultConfig}

    // Unmarshal the file configuration into the merged configuration
    if err := json.Unmarshal([]byte(fileConfig), mergedConfig); err != nil {
        // Handle error
    }

    return mergedConfig
}

Usage:

// Load the configuration from a file
fileContent := `{"S2":"file-s2","S3":"","S5":"file-s5"}`
fileConfig := &Config{}
if err := json.NewDecoder(strings.NewReader(fileContent)).Decode(fileConfig); err != nil {
    // Handle error
}

// Initialize the default configuration
defConfig := &Config{
    S1: "",
    S2: "",
    S3: "abc",
    S4: "def",
    S5: "ghi",
}

// Merge the configurations
mergedConfig := MergeConfig(defConfig, fileConfig)

fmt.Println(mergedConfig)

Output:

&{S1: S2:file-s2 S3: S4:def S5:file-s5}

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