Question : Best way to pass default struct into viper

[matthieugouel]

Hello everyone,

I hope there is no duplicate for that.
I'm new to Golang therefore to viper. My use case is to have a structure as a default value, that can be overridden via an other way (env variable, ...). I want to avoid using SetDefault since it could be a lot of config variable in nested configuration.

I managed to to that by marshaling my structure with JSON and than passing the byte slice into ReadConfig .

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"strings"

	"github.com/spf13/viper"
)

// Config : Application configuration
type Config struct {
	Jwt JwtConfig
}

// JwtConfig : JWT configuration
type JwtConfig struct {
	Secret string
}

func main() {

	cfg := Config{
		Jwt: JwtConfig{
			Secret: "helicopter",
		},
	}

	viper.SetConfigType("json")
	cfgJSON, err := json.Marshal(cfg)
	if err != nil {
		panic(err)
	}

	viper.ReadConfig((bytes.NewBuffer(cfgJSON)))

	viper.SetEnvPrefix("TEST")
	viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
	viper.AutomaticEnv()

	fmt.Println(viper.Get("jwt.secret"))
}

My question is : is it the prefered way to do it ? Or maybe is there a more efficient way to do that without JSON/YAML marshalling ?

Best,
Matthieu.

[kylechadha]

Would like to know about this as well. Seems like it would be a pretty common use case?

[hbagdi]

Same here. Is there anyway to do this?

[josuebrunel]

Right there in the documentation

[RahilRehan]

Having a default struct config which has lowest preference and can be set easily would be super awesome to have.

[shacharovercast]

Right there in the documentation

this works! thanks!

[robivictor]

This would be really helpful and a pretty common use case — a possible addition to V2.

[github-actions]

Issues with no activity for 30 days are marked stale and subject to being closed.

[github-actions]

This issue has been closed for inactivity. Please re-open if this was a mistake.

[semihbkgr]

I solved this by taking my typed DefaultConfig() struct, converting it to a map[string]any via mapstructure.Decode, and then registering those values with v.SetDefault(...) before loading the YAML config and env vars. It works as expected in my case (env > config > defaults), but I’m not sure whether relying on mapstructure for struct→map conversion is guaranteed to be safe across all cases and future versions.

func LoadConfig(yamlConfig string) (Config, error) {
	v := viper.New()

	v.SetEnvPrefix("APP")
	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
	v.AutomaticEnv()

	// 1) defaults from struct -> map -> viper defaults
	defaultMap := map[string]any{}
	if err := mapstructure.Decode(DefaultConfig(), &defaultMap); err != nil {
		return Config{}, err
	}
	for k, val := range defaultMap {
		v.SetDefault(k, val)
	}

	// 2) config file content from string
	v.SetConfigType("yaml")
	if err := v.ReadConfig(strings.NewReader(yamlConfig)); err != nil {
		return Config{}, err
	}

	// 3) unmarshal final resolved config
	var cfg Config
	if err := v.Unmarshal(&cfg); err != nil {
		return Config{}, err
	}
	return cfg, nil
}