Documentation ¶
Overview ¶
Package mapstruct 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, mapstruct will use the field name by default to perform the mapping. For example, if a struct has a field "Username" then mapstruct will look for a key in the source value of "username" (case insensitive).
type User struct { Username string }
You can change the behavior of mapstruct by using struct tags. The default struct tag that mapstruct looks for is "mapstruct" but you can customize it using Config.
Renaming Fields ¶
To rename the key that mapstruct looks for, use the "mapstruct" tag and set a value directly. For example, to change the "username" example above to "user":
type User struct { Username string `mapstruct:"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 mapstruct:
type Person struct { Name string } type Friend struct { Person } type Friend struct { Person Person }
This would require an input that looks like below:
map[string]interface{}{ "person": map[string]interface{}{"name": "alice"}, }
If your "person" value is NOT nested, then you can append ",squash" to your tag value and mapstruct will treat it as if the embedded struct were part of the struct directly. Example:
type Friend struct { Person `mapstruct:",squash"` }
Now the following input would be accepted:
map[string]interface{}{ "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:
map[string]interface{}{ "name": "alice", }
Config has a field that changes the behavior of mapstruct to always squash embedded structs.
Remainder Values ¶
If there are any unmapped keys in the source value, mapstruct by default will silently ignore them. You can error by setting ErrorUnused in Config. 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{} `mapstruct:",remain"` }
Given the input below, Other would be populated with the other values that weren't used (everything but "name"):
map[string]interface{}{ "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 `mapstruct:",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:
map[string]interface{}{ "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 ¶
mapstruct is highly configurable. See the Config struct for other features and options that are supported.
Index ¶
- Variables
- func Decode(input interface{}, output interface{}) error
- func DecodeHookExec(raw HookFunc, from, to reflect.Value) (interface{}, error)
- func DecodeMetadata(input interface{}, output interface{}, metadata *Metadata) error
- func WeakDecode(input, output interface{}) error
- func WeakDecodeMetadata(input interface{}, output interface{}, metadata *Metadata) error
- func WeaklyTypedHook(f reflect.Kind, t reflect.Kind, data interface{}) (interface{}, error)
- type Config
- type Decoder
- type Error
- type HookFunc
- func ComposeDecodeHookFunc(fs ...HookFunc) HookFunc
- func RecursiveStructToMapHookFunc() HookFunc
- func StringToIPHookFunc() HookFunc
- func StringToIPNetHookFunc() HookFunc
- func StringToSliceHookFunc(sep string) HookFunc
- func StringToTimeDurationHookFunc() HookFunc
- func StringToTimeHookFunc(layout string) HookFunc
- type HookFuncKind
- type HookFuncType
- type HookFuncValue
- type Metadata
Examples ¶
Constants ¶
This section is empty.
Variables ¶
nolint:gochecknoglobals
Functions ¶
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.
Example ¶
type Person struct { Name string Age int Emails []string Extra map[string]string } // This input can come from anywhere, but typically comes from // something like decoding JSON where we're not quite sure of the // struct initially. input := map[string]interface{}{ "name": "Mitchell", "age": 91, "emails": []string{"one", "two", "three"}, "extra": map[string]string{ "twitter": "mitchellh", }, } var result Person err := Decode(input, &result) if err != nil { panic(err) } fmt.Printf("%#v", result)
Output: mapstruct.Person{Name:"Mitchell", Age:91, Emails:[]string{"one", "two", "three"}, Extra:map[string]string{"twitter":"mitchellh"}}
Example (EmbeddedStruct) ¶
// Squashing multiple embedded structs is allowed using the squash tag. // This is demonstrated by creating a composite struct of multiple types // and decoding into it. In this case, a person can carry with it both // a Family and a Location, as well as their own FirstName. type Family struct { LastName string } type Location struct { City string } type Person struct { Family `mapstruct:",squash"` Location `mapstruct:",squash"` FirstName string } input := map[string]interface{}{ "FirstName": "Mitchell", "LastName": "Hashimoto", "City": "San Francisco", } var result Person err := Decode(input, &result) if err != nil { panic(err) } fmt.Printf("%s %s, %s", result.FirstName, result.LastName, result.City)
Output: Mitchell Hashimoto, San Francisco
Example (Errors) ¶
type Person struct { Name string Age int Emails []string Extra map[string]string } // This input can come from anywhere, but typically comes from // something like decoding JSON where we're not quite sure of the // struct initially. input := map[string]interface{}{ "name": 123, "age": "bad value", "emails": []int{1, 2, 3}, } var result Person err := Decode(input, &result) if err == nil { panic("should have an error") } fmt.Println(err.Error())
Output: 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) ¶
type Person struct { Name string Age int } // This input can come from anywhere, but typically comes from // something like decoding JSON where we're not quite sure of the // struct initially. input := map[string]interface{}{ "name": "Mitchell", "age": 91, "email": "foo@bar.com", } // For metadata, we make a more advanced Config so we can // more finely configure the decoder that is used. In this case, we // just tell the decoder we want to track metadata. var md Metadata var result Person config := &Config{ Metadata: &md, Result: &result, } decoder, err := NewDecoder(config) if err != nil { panic(err) } if err := decoder.Decode(input); err != nil { panic(err) } fmt.Printf("Unused keys: %#v", md.Unused)
Output: Unused keys: []string{"email"}
Example (Omitempty) ¶
// Add omitempty annotation to avoid map keys for empty values type Family struct { LastName string } type Location struct { City string } type Person struct { *Family `mapstruct:",omitempty"` *Location `mapstruct:",omitempty"` Age int FirstName string } result := &map[string]interface{}{} input := Person{FirstName: "Somebody"} err := Decode(input, &result) if err != nil { panic(err) } fmt.Printf("%+v", result)
Output: &map[Age:0 FirstName:Somebody]
Example (RemainingData) ¶
// Note that the mapstruct tags defined in the struct type // can indicate which fields the values are mapped to. type Person struct { Name string Age int Other map[string]interface{} `mapstruct:",remain"` } input := map[string]interface{}{ "name": "Mitchell", "age": 91, "email": "mitchell@example.com", } var result Person err := Decode(input, &result) if err != nil { panic(err) } fmt.Printf("%#v", result)
Output: mapstruct.Person{Name:"Mitchell", Age:91, Other:map[string]interface {}{"email":"mitchell@example.com"}}
Example (Tags) ¶
// Note that the mapstruct tags defined in the struct type // can indicate which fields the values are mapped to. type Person struct { Name string `mapstruct:"person_name"` Age int `mapstruct:"person_age"` } input := map[string]interface{}{ "person_name": "Mitchell", "person_age": 91, } var result Person err := Decode(input, &result) if err != nil { panic(err) } fmt.Printf("%#v", result)
Output: mapstruct.Person{Name:"Mitchell", Age:91}
Example (WeaklyTypedInput) ¶
type Person struct { Name string Age int Emails []string } // This input can come from anywhere, but typically comes from // something like decoding JSON, generated by a weakly typed language // such as PHP. input := map[string]interface{}{ "name": 123, // number => string "age": "42", // string => number "emails": map[string]interface{}{}, // empty map => empty array } var result Person config := &Config{ WeaklyTypedInput: true, Result: &result, } decoder, err := NewDecoder(config) if err != nil { panic(err) } err = decoder.Decode(input) if err != nil { panic(err) } fmt.Printf("%#v", result)
Output: mapstruct.Person{Name:"123", Age:42, Emails:[]string{}}
func DecodeHookExec ¶
DecodeHookExec executes the given decode hook. This should be used since it'll naturally degrade to the older backwards compatible HookFunc that took reflect.Kind instead of reflect.Type.
func DecodeMetadata ¶
DecodeMetadata is the same as Decode, but is shorthand to enable metadata collection. See Config for more info.
func WeakDecode ¶
func WeakDecode(input, output interface{}) error
WeakDecode is the same as Decode but is shorthand to enable WeaklyTypedInput. See Config for more info.
func WeakDecodeMetadata ¶
WeakDecodeMetadata is the same as Decode, but is shorthand to enable both WeaklyTypedInput and metadata collection. See Config for more info.
Types ¶
type Config ¶
type Config struct { // Hook, 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 // Hook 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. Hook HookFunc // 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 `mapstruct:",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 mapstruct reads for field names. This // defaults to ["mapstruct", "field", "json", "yaml"] TagNames []string }
Config is the configuration that is used to create a new decoder and allows customization of various aspects of decoding.
type Decoder ¶
type Decoder struct {
*Config
}
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 Config structure. The top-level Decode method is just a convenience that sets up the most basic Decoder.
func NewDecoder ¶
NewDecoder returns a new decoder for the given configuration. Once a decoder has been returned, the same configuration must not be used again.
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) WrappedErrors ¶
WrappedErrors implements the errwrap.Wrapper interface to make this return value more useful with the errwrap and go-multierror libraries.
type HookFunc ¶
type HookFunc interface{}
HookFunc is the callback function that can be used for data transformations. See "Hook" in the Config struct.
The type must be one of HookFuncType, HookFuncKind, or HookFuncValue. 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 HookFunc 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 ¶
ComposeDecodeHookFunc creates a single HookFunc that automatically composes multiple DecodeHookFuncs.
The composed funcs are called in order, with the result of the previous transformation.
func RecursiveStructToMapHookFunc ¶
func RecursiveStructToMapHookFunc() HookFunc
func StringToIPHookFunc ¶
func StringToIPHookFunc() HookFunc
StringToIPHookFunc returns a HookFunc that converts strings to net.IP
func StringToIPNetHookFunc ¶
func StringToIPNetHookFunc() HookFunc
StringToIPNetHookFunc returns a HookFunc that converts strings to net.IPNet
func StringToSliceHookFunc ¶
StringToSliceHookFunc returns a HookFunc that converts string to []string by splitting on the given sep.
func StringToTimeDurationHookFunc ¶
func StringToTimeDurationHookFunc() HookFunc
StringToTimeDurationHookFunc returns a HookFunc that converts strings to time.Duration.
func StringToTimeHookFunc ¶
StringToTimeHookFunc returns a HookFunc that converts strings to time.Time.
type HookFuncKind ¶
HookFuncKind is a HookFunc which knows only the Kinds of the source and target types.
type HookFuncType ¶
HookFuncType is a HookFunc which has complete information about the source and target types.
func TextUnmarshallerHookFunc ¶
func TextUnmarshallerHookFunc() HookFuncType
TextUnmarshallerHookFunc returns a HookFunc that applies strings to the UnmarshalText function, when the target type implements the encoding.TextUnmarshaler interface
type HookFuncValue ¶
DecodeHookFuncRaw is a HookFunc which has complete access to both the source and target values.
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.