mapstructure Godoc

mapstructure is a Go library for decoding generic map values to structures and vice versa, while providing helpful error handling.

This library is most useful when decoding values from some data stream (JSON, Gob, etc.) where you don't quite know the structure of the underlying data until you read a part of it. You can therefore read a map[string]interface{} and use this library to decode it into the proper underlying native Go structure.


Standard go get:

$ go get

Usage & Example

For usage and examples see the Godoc.

The Decode function has examples associated with it there.

But Why?!

Go offers fantastic standard libraries for decoding formats such as JSON. The standard method is to have a struct pre-created, and populate that struct from the bytes of the encoded format. This is great, but the problem is if you have configuration or an encoding that changes slightly depending on specific fields. For example, consider this JSON:

  "type": "person",
  "name": "Mitchell"

Perhaps we can't populate a specific structure without first reading the "type" field from the JSON. We could always do two passes over the decoding of the JSON (reading the "type" first, and the rest later). However, it is much simpler to just decode this into a map[string]interface{} structure, read the "type" key, then use something like this library to decode it into the proper structure.



Package mapstructure exposes functionality to convert one arbitrary Go type into another, typically to convert a map[string]interface{} into a native Go structure.

The Go structure can be arbitrarily complex, containing slices, other structs, etc. and the decoder will properly decode nested maps and so on into the proper structures in the native Go struct. See the examples to see what the decoder is capable of.

The simplest function to start with is Decode.

Field Tags

When decoding to a struct, mapstructure will use the field name by default to perform the mapping. For example, if a struct has a field "Username" then mapstructure will look for a key in the source value of "username" (case insensitive).

type User struct {
    Username string

You can change the behavior of mapstructure by using struct tags. The default struct tag that mapstructure looks for is "mapstructure" but you can customize it using DecoderConfig.

Renaming Fields

To rename the key that mapstructure looks for, use the "mapstructure" tag and set a value directly. For example, to change the "username" example above to "user":

type User struct {
    Username string `mapstructure:"user"`

Embedded Structs and Squashing

Embedded structs are treated as if they're another field with that name. By default, the two structs below are equivalent when decoding with mapstructure:

type Person struct {
    Name string

type Friend struct {

type Friend struct {
    Person Person

This would require an input that looks like below:

    "person": map[string]interface{}{"name": "alice"},

If your "person" value is NOT nested, then you can append ",squash" to your tag value and mapstructure will treat it as if the embedded struct were part of the struct directly. Example:

type Friend struct {
    Person `mapstructure:",squash"`

Now the following input would be accepted:

    "name": "alice",

When decoding from a struct to a map, the squash tag squashes the struct fields into a single map. Using the example structs from above:

Friend{Person: Person{Name: "alice"}}

Will be decoded into a map:

    "name": "alice",

DecoderConfig has a field that changes the behavior of mapstructure to always squash embedded structs.

Remainder Values

If there are any unmapped keys in the source value, mapstructure by default will silently ignore them. You can error by setting ErrorUnused in DecoderConfig. If you're using Metadata you can also maintain a slice of the unused keys.

You can also use the ",remain" suffix on your tag to collect all unused values in a map. The field with this tag MUST be a map type and should probably be a "map[string]interface{}" or "map[interface{}]interface{}". See example below:

type Friend struct {
    Name  string
    Other map[string]interface{} `mapstructure:",remain"`

Given the input below, Other would be populated with the other values that weren't used (everything but "name"):

    "name":    "bob",
    "address": "123 Maple St.",

Omit Empty Values

When decoding from a struct to any other value, you may use the ",omitempty" suffix on your tag to omit that value if it equates to the zero value. The zero value of all types is specified in the Go specification.

For example, the zero type of a numeric type is zero ("0"). If the struct field value is zero and a numeric type, the field is empty, and it won't be encoded into the destination type.

type Source {
    Age int `mapstructure:",omitempty"`

Unexported fields

Since unexported (private) struct fields cannot be set outside the package where they are defined, the decoder will simply skip them.

For this output type definition:

type Exported struct {
    private string // this unexported field will be skipped
    Public string

Using this map as input:

    "private": "I will be ignored",
    "Public":  "I made it through!",

The following struct will be decoded:

type Exported struct {
    private: "" // field is left with an empty string (zero value)
    Public: "I made it through!"

Other Configuration

mapstructure is highly configurable. See the DecoderConfig struct for other features and options that are supported.




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func Decode

func Decode(input interface{}, output interface{}) error

    Decode takes an input structure and uses reflection to translate it to the output structure. output must be a pointer to a map or struct.

    mapstructure.Person{Name:"Mitchell", Age:91, Emails:[]string{"one", "two", "three"}, Extra:map[string]string{"twitter":"mitchellh"}}
    Example (EmbeddedStruct)
    Mitchell Hashimoto, San Francisco
    Example (Errors)
    5 error(s) decoding:
    * 'Age' expected type 'int', got unconvertible type 'string', value: 'bad value'
    * 'Emails[0]' expected type 'string', got unconvertible type 'int', value: '1'
    * 'Emails[1]' expected type 'string', got unconvertible type 'int', value: '2'
    * 'Emails[2]' expected type 'string', got unconvertible type 'int', value: '3'
    * 'Name' expected type 'string', got unconvertible type 'int', value: '123'
    Example (Metadata)
    Unused keys: []string{"email"}
    Example (Omitempty)
    &map[Age:0 FirstName:Somebody]
    Example (RemainingData)
    mapstructure.Person{Name:"Mitchell", Age:91, Other:map[string]interface {}{"email":""}}
    Example (Tags)
    mapstructure.Person{Name:"Mitchell", Age:91}
    Example (WeaklyTypedInput)
    mapstructure.Person{Name:"123", Age:42, Emails:[]string{}}

    func DecodeHookExec

    func DecodeHookExec(
    	raw DecodeHookFunc,
    	from reflect.Value, to reflect.Value) (interface{}, error)

      DecodeHookExec executes the given decode hook. This should be used since it'll naturally degrade to the older backwards compatible DecodeHookFunc that took reflect.Kind instead of reflect.Type.

      func DecodeMetadata

      func DecodeMetadata(input interface{}, output interface{}, metadata *Metadata) error

        DecodeMetadata is the same as Decode, but is shorthand to enable metadata collection. See DecoderConfig for more info.

        func WeakDecode

        func WeakDecode(input, output interface{}) error

          WeakDecode is the same as Decode but is shorthand to enable WeaklyTypedInput. See DecoderConfig for more info.

          func WeakDecodeMetadata

          func WeakDecodeMetadata(input interface{}, output interface{}, metadata *Metadata) error

            WeakDecodeMetadata is the same as Decode, but is shorthand to enable both WeaklyTypedInput and metadata collection. See DecoderConfig for more info.

            func WeaklyTypedHook

            func WeaklyTypedHook(
            	f reflect.Kind,
            	t reflect.Kind,
            	data interface{}) (interface{}, error)

              WeaklyTypedHook is a DecodeHookFunc which adds support for weak typing to the decoder.

              Note that this is significantly different from the WeaklyTypedInput option of the DecoderConfig.


              type DecodeHookFunc

              type DecodeHookFunc interface{}

                DecodeHookFunc is the callback function that can be used for data transformations. See "DecodeHook" in the DecoderConfig struct.

                The type must be one of DecodeHookFuncType, DecodeHookFuncKind, or DecodeHookFuncValue. Values are a superset of Types (Values can return types), and Types are a superset of Kinds (Types can return Kinds) and are generally a richer thing to use, but Kinds are simpler if you only need those.

                The reason DecodeHookFunc is multi-typed is for backwards compatibility: we started with Kinds and then realized Types were the better solution, but have a promise to not break backwards compat so we now support both.

                func ComposeDecodeHookFunc

                func ComposeDecodeHookFunc(fs ...DecodeHookFunc) DecodeHookFunc

                  ComposeDecodeHookFunc creates a single DecodeHookFunc that automatically composes multiple DecodeHookFuncs.

                  The composed funcs are called in order, with the result of the previous transformation.

                  func RecursiveStructToMapHookFunc

                  func RecursiveStructToMapHookFunc() DecodeHookFunc

                  func StringToIPHookFunc

                  func StringToIPHookFunc() DecodeHookFunc

                    StringToIPHookFunc returns a DecodeHookFunc that converts strings to net.IP

                    func StringToIPNetHookFunc

                    func StringToIPNetHookFunc() DecodeHookFunc

                      StringToIPNetHookFunc returns a DecodeHookFunc that converts strings to net.IPNet

                      func StringToSliceHookFunc

                      func StringToSliceHookFunc(sep string) DecodeHookFunc

                        StringToSliceHookFunc returns a DecodeHookFunc that converts string to []string by splitting on the given sep.

                        func StringToTimeDurationHookFunc

                        func StringToTimeDurationHookFunc() DecodeHookFunc

                          StringToTimeDurationHookFunc returns a DecodeHookFunc that converts strings to time.Duration.

                          func StringToTimeHookFunc

                          func StringToTimeHookFunc(layout string) DecodeHookFunc

                            StringToTimeHookFunc returns a DecodeHookFunc that converts strings to time.Time.

                            type DecodeHookFuncKind

                            type DecodeHookFuncKind func(reflect.Kind, reflect.Kind, interface{}) (interface{}, error)

                              DecodeHookFuncKind is a DecodeHookFunc which knows only the Kinds of the source and target types.

                              type DecodeHookFuncType

                              type DecodeHookFuncType func(reflect.Type, reflect.Type, interface{}) (interface{}, error)

                                DecodeHookFuncType is a DecodeHookFunc which has complete information about the source and target types.

                                func TextUnmarshallerHookFunc

                                func TextUnmarshallerHookFunc() DecodeHookFuncType

                                  TextUnmarshallerHookFunc returns a DecodeHookFunc that applies strings to the UnmarshalText function, when the target type implements the encoding.TextUnmarshaler interface

                                  type DecodeHookFuncValue

                                  type DecodeHookFuncValue func(from reflect.Value, to reflect.Value) (interface{}, error)

                                    DecodeHookFuncRaw is a DecodeHookFunc which has complete access to both the source and target values.

                                    type Decoder

                                    type Decoder struct {
                                    	// contains filtered or unexported fields

                                      A Decoder takes a raw interface value and turns it into structured data, keeping track of rich error information along the way in case anything goes wrong. Unlike the basic top-level Decode method, you can more finely control how the Decoder behaves using the DecoderConfig structure. The top-level Decode method is just a convenience that sets up the most basic Decoder.

                                      func NewDecoder

                                      func NewDecoder(config *DecoderConfig) (*Decoder, error)

                                        NewDecoder returns a new decoder for the given configuration. Once a decoder has been returned, the same configuration must not be used again.

                                        func (*Decoder) Decode

                                        func (d *Decoder) Decode(input interface{}) error

                                          Decode decodes the given raw interface to the target pointer specified by the configuration.

                                          type DecoderConfig

                                          type DecoderConfig struct {
                                          	// DecodeHook, if set, will be called before any decoding and any
                                          	// type conversion (if WeaklyTypedInput is on). This lets you modify
                                          	// the values before they're set down onto the resulting struct. The
                                          	// DecodeHook is called for every map and value in the input. This means
                                          	// that if a struct has embedded fields with squash tags the decode hook
                                          	// is called only once with all of the input data, not once for each
                                          	// embedded struct.
                                          	// If an error is returned, the entire decode will fail with that error.
                                          	DecodeHook DecodeHookFunc
                                          	// If ErrorUnused is true, then it is an error for there to exist
                                          	// keys in the original map that were unused in the decoding process
                                          	// (extra keys).
                                          	ErrorUnused bool
                                          	// ZeroFields, if set to true, will zero fields before writing them.
                                          	// For example, a map will be emptied before decoded values are put in
                                          	// it. If this is false, a map will be merged.
                                          	ZeroFields bool
                                          	// If WeaklyTypedInput is true, the decoder will make the following
                                          	// "weak" conversions:
                                          	//   - bools to string (true = "1", false = "0")
                                          	//   - numbers to string (base 10)
                                          	//   - bools to int/uint (true = 1, false = 0)
                                          	//   - strings to int/uint (base implied by prefix)
                                          	//   - int to bool (true if value != 0)
                                          	//   - string to bool (accepts: 1, t, T, TRUE, true, True, 0, f, F,
                                          	//     FALSE, false, False. Anything else is an error)
                                          	//   - empty array = empty map and vice versa
                                          	//   - negative numbers to overflowed uint values (base 10)
                                          	//   - slice of maps to a merged map
                                          	//   - single values are converted to slices if required. Each
                                          	//     element is weakly decoded. For example: "4" can become []int{4}
                                          	//     if the target type is an int slice.
                                          	WeaklyTypedInput bool
                                          	// Squash will squash embedded structs.  A squash tag may also be
                                          	// added to an individual struct field using a tag.  For example:
                                          	//  type Parent struct {
                                          	//      Child `mapstructure:",squash"`
                                          	//  }
                                          	Squash bool
                                          	// Metadata is the struct that will contain extra metadata about
                                          	// the decoding. If this is nil, then no metadata will be tracked.
                                          	Metadata *Metadata
                                          	// Result is a pointer to the struct that will contain the decoded
                                          	// value.
                                          	Result interface{}
                                          	// The tag name that mapstructure reads for field names. This
                                          	// defaults to "mapstructure"
                                          	TagName string

                                            DecoderConfig is the configuration that is used to create a new decoder and allows customization of various aspects of decoding.

                                            type Error

                                            type Error struct {
                                            	Errors []string

                                              Error implements the error interface and can represents multiple errors that occur in the course of a single decode.

                                              func (*Error) Error

                                              func (e *Error) Error() string

                                              func (*Error) WrappedErrors

                                              func (e *Error) WrappedErrors() []error

                                                WrappedErrors implements the errwrap.Wrapper interface to make this return value more useful with the errwrap and go-multierror libraries.

                                                type Metadata

                                                type Metadata struct {
                                                	// Keys are the keys of the structure which were successfully decoded
                                                	Keys []string
                                                	// Unused is a slice of keys that were found in the raw value but
                                                	// weren't decoded since there was no matching field in the result interface
                                                	Unused []string

                                                  Metadata contains information about decoding a structure that is tedious or difficult to get otherwise.