Package reflect implements run-time reflection, allowing a program to manipulate objects with arbitrary types. The typical use is to take a value with static type interface{} and extract its dynamic type information by calling TypeOf, which returns a Type.
A call to ValueOf returns a Value representing the run-time data. Zero takes a Type and returns a Value representing a zero value for that type.
See "The Laws of Reflection" for an introduction to reflection in Go: https://golang.org/doc/articles/laws_of_reflection.html
deepequal.go makefunc.go swapper.go type.go value.go
func Copy(dst, src Value) int
Copy copies the contents of src into dst until either dst has been filled or src has been exhausted. It returns the number of elements copied. Dst and src each must have kind Slice or Array, and dst and src must have the same element type.
As a special case, src can have kind String if the element type of dst is kind Uint8.
func DeepEqual(x, y interface{}) bool DeepEqual reports whether x and y are “deeply equal,” defined as follows. Two values of identical type are deeply equal if one of the following cases applies. Values of distinct types are never deeply equal.
Array values are deeply equal when their corresponding elements are deeply equal.
Struct values are deeply equal if their corresponding fields, both exported and unexported, are deeply equal.
Func values are deeply equal if both are nil; otherwise they are not deeply equal.
Interface values are deeply equal if they hold deeply equal concrete values.
Map values are deeply equal when all of the following are true: they are both nil or both non-nil, they have the same length, and either they are the same map object or their corresponding keys (matched using Go equality) map to deeply equal values.
Pointer values are deeply equal if they are equal using Go's == operator or if they point to deeply equal values.
Slice values are deeply equal when all of the following are true: they are both nil or both non-nil, they have the same length, and either they point to the same initial entry of the same underlying array (that is, &x[0] == &y[0]) or their corresponding elements (up to length) are deeply equal. Note that a non-nil empty slice and a nil slice (for example, []byte{} and []byte(nil)) are not deeply equal.
Other values - numbers, bools, strings, and channels - are deeply equal if they are equal using Go's == operator.
In general DeepEqual is a recursive relaxation of Go's == operator. However, this idea is impossible to implement without some inconsistency. Specifically, it is possible for a value to be unequal to itself, either because it is of func type (uncomparable in general) or because it is a floating-point NaN value (not equal to itself in floating-point comparison), or because it is an array, struct, or interface containing such a value. On the other hand, pointer values are always equal to themselves, even if they point at or contain such problematic values, because they compare equal using Go's == operator, and that is a sufficient condition to be deeply equal, regardless of content. DeepEqual has been defined so that the same short-cut applies to slices and maps: if x and y are the same slice or the same map, they are deeply equal regardless of content.
As DeepEqual traverses the data values it may find a cycle. The second and subsequent times that DeepEqual compares two pointer values that have been compared before, it treats the values as equal rather than examining the values to which they point. This ensures that DeepEqual terminates.
func Swapper(slice interface{}) func(i, j int) Swapper returns a function that swaps the elements in the provided slice.
Swapper panics if the provided interface is not a slice.
ChanDir represents a channel type's direction.
type ChanDir int
const (
RecvDir ChanDir = 1 << iota // <-chan
SendDir // chan<-
BothDir = RecvDir | SendDir // chan
) func (d ChanDir) String() string
A Kind represents the specific kind of type that a Type represents. The zero Kind is not a valid kind.
type Kind uint
const (
Invalid Kind = iota
Bool
Int
Int8
Int16
Int32
Int64
Uint
Uint8
Uint16
Uint32
Uint64
Uintptr
Float32
Float64
Complex64
Complex128
Array
Chan
Func
Interface
Map
Ptr
Slice
String
Struct
UnsafePointer
) func (k Kind) String() string
Method represents a single method.
type Method struct {
// Name is the method name.
// PkgPath is the package path that qualifies a lower case (unexported)
// method name. It is empty for upper case (exported) method names.
// The combination of PkgPath and Name uniquely identifies a method
// in a method set.
// See https://golang.org/ref/spec#Uniqueness_of_identifiers
Name string
PkgPath string
Type Type // method type
Func Value // func with receiver as first argument
Index int // index for Type.Method
}
A SelectCase describes a single case in a select operation. The kind of case depends on Dir, the communication direction.
If Dir is SelectDefault, the case represents a default case. Chan and Send must be zero Values.
If Dir is SelectSend, the case represents a send operation. Normally Chan's underlying value must be a channel, and Send's underlying value must be assignable to the channel's element type. As a special case, if Chan is a zero Value, then the case is ignored, and the field Send will also be ignored and may be either zero or non-zero.
If Dir is SelectRecv, the case represents a receive operation. Normally Chan's underlying value must be a channel and Send must be a zero Value. If Chan is a zero Value, then the case is ignored, but Send must still be a zero Value. When a receive operation is selected, the received Value is returned by Select.
type SelectCase struct {
Dir SelectDir // direction of case
Chan Value // channel to use (for send or receive)
Send Value // value to send (for send)
}
A SelectDir describes the communication direction of a select case.
type SelectDir int
const (
SelectSend SelectDir // case Chan <- Send
SelectRecv // case <-Chan:
SelectDefault // default
) SliceHeader is the runtime representation of a slice. It cannot be used safely or portably and its representation may change in a later release. Moreover, the Data field is not sufficient to guarantee the data it references will not be garbage collected, so programs must keep a separate, correctly typed pointer to the underlying data.
type SliceHeader struct {
Data uintptr
Len int
Cap int
}
StringHeader is the runtime representation of a string. It cannot be used safely or portably and its representation may change in a later release. Moreover, the Data field is not sufficient to guarantee the data it references will not be garbage collected, so programs must keep a separate, correctly typed pointer to the underlying data.
type StringHeader struct {
Data uintptr
Len int
}
A StructField describes a single field in a struct.
type StructField struct {
// Name is the field name.
Name string
// PkgPath is the package path that qualifies a lower case (unexported)
// field name. It is empty for upper case (exported) field names.
// See https://golang.org/ref/spec#Uniqueness_of_identifiers
PkgPath string
Type Type // field type
Tag StructTag // field tag string
Offset uintptr // offset within struct, in bytes
Index []int // index sequence for Type.FieldByIndex
Anonymous bool // is an embedded field
}
A StructTag is the tag string in a struct field.
By convention, tag strings are a concatenation of optionally space-separated key:"value" pairs. Each key is a non-empty string consisting of non-control characters other than space (U+0020 ' '), quote (U+0022 '"'), and colon (U+003A ':'). Each value is quoted using U+0022 '"' characters and Go string literal syntax.
type StructTag string
package main
import (
"fmt"
"reflect"
)
func main() {
type S struct {
F string `species:"gopher" color:"blue"`
}
s := S{}
st := reflect.TypeOf(s)
field := st.Field(0)
fmt.Println(field.Tag.Get("color"), field.Tag.Get("species"))
}
func (tag StructTag) Get(key string) string
Get returns the value associated with key in the tag string. If there is no such key in the tag, Get returns the empty string. If the tag does not have the conventional format, the value returned by Get is unspecified. To determine whether a tag is explicitly set to the empty string, use Lookup.
func (tag StructTag) Lookup(key string) (value string, ok bool)
Lookup returns the value associated with key in the tag string. If the key is present in the tag the value (which may be empty) is returned. Otherwise the returned value will be the empty string. The ok return value reports whether the value was explicitly set in the tag string. If the tag does not have the conventional format, the value returned by Lookup is unspecified.
package main
import (
"fmt"
"reflect"
)
func main() {
type S struct {
F0 string `alias:"field_0"`
F1 string `alias:""`
F2 string
}
s := S{}
st := reflect.TypeOf(s)
for i := 0; i < st.NumField(); i++ {
field := st.Field(i)
if alias, ok := field.Tag.Lookup("alias"); ok {
if alias == "" {
fmt.Println("(blank)")
} else {
fmt.Println(alias)
}
} else {
fmt.Println("(not specified)")
}
}
}
Type is the representation of a Go type.
Not all methods apply to all kinds of types. Restrictions, if any, are noted in the documentation for each method. Use the Kind method to find out the kind of type before calling kind-specific methods. Calling a method inappropriate to the kind of type causes a run-time panic.
Type values are comparable, such as with the == operator, so they can be used as map keys. Two Type values are equal if they represent identical types.
type Type interface {
// Align returns the alignment in bytes of a value of
// this type when allocated in memory.
Align() int
// FieldAlign returns the alignment in bytes of a value of
// this type when used as a field in a struct.
FieldAlign() int
// Method returns the i'th method in the type's method set.
// It panics if i is not in the range [0, NumMethod()).
//
// For a non-interface type T or *T, the returned Method's Type and Func
// fields describe a function whose first argument is the receiver.
//
// For an interface type, the returned Method's Type field gives the
// method signature, without a receiver, and the Func field is nil.
Method(int) Method
// MethodByName returns the method with that name in the type's
// method set and a boolean indicating if the method was found.
//
// For a non-interface type T or *T, the returned Method's Type and Func
// fields describe a function whose first argument is the receiver.
//
// For an interface type, the returned Method's Type field gives the
// method signature, without a receiver, and the Func field is nil.
MethodByName(string) (Method, bool)
// NumMethod returns the number of exported methods in the type's method set.
NumMethod() int
// Name returns the type's name within its package for a defined type.
// For other (non-defined) types it returns the empty string.
Name() string
// PkgPath returns a defined type's package path, that is, the import path
// that uniquely identifies the package, such as "encoding/base64".
// If the type was predeclared (string, error) or not defined (*T, struct{},
// []int, or A where A is an alias for a non-defined type), the package path
// will be the empty string.
PkgPath() string
// Size returns the number of bytes needed to store
// a value of the given type; it is analogous to unsafe.Sizeof.
Size() uintptr
// String returns a string representation of the type.
// The string representation may use shortened package names
// (e.g., base64 instead of "encoding/base64") and is not
// guaranteed to be unique among types. To test for type identity,
// compare the Types directly.
String() string
// Kind returns the specific kind of this type.
Kind() Kind
// Implements reports whether the type implements the interface type u.
Implements(u Type) bool
// AssignableTo reports whether a value of the type is assignable to type u.
AssignableTo(u Type) bool
// ConvertibleTo reports whether a value of the type is convertible to type u.
ConvertibleTo(u Type) bool
// Comparable reports whether values of this type are comparable.
Comparable() bool
// Bits returns the size of the type in bits.
// It panics if the type's Kind is not one of the
// sized or unsized Int, Uint, Float, or Complex kinds.
Bits() int
// ChanDir returns a channel type's direction.
// It panics if the type's Kind is not Chan.
ChanDir() ChanDir
// IsVariadic reports whether a function type's final input parameter
// is a "..." parameter. If so, t.In(t.NumIn() - 1) returns the parameter's
// implicit actual type []T.
//
// For concreteness, if t represents func(x int, y ... float64), then
//
// t.NumIn() == 2
// t.In(0) is the reflect.Type for "int"
// t.In(1) is the reflect.Type for "[]float64"
// t.IsVariadic() == true
//
// IsVariadic panics if the type's Kind is not Func.
IsVariadic() bool
// Elem returns a type's element type.
// It panics if the type's Kind is not Array, Chan, Map, Ptr, or Slice.
Elem() Type
// Field returns a struct type's i'th field.
// It panics if the type's Kind is not Struct.
// It panics if i is not in the range [0, NumField()).
Field(i int) StructField
// FieldByIndex returns the nested field corresponding
// to the index sequence. It is equivalent to calling Field
// successively for each index i.
// It panics if the type's Kind is not Struct.
FieldByIndex(index []int) StructField
// FieldByName returns the struct field with the given name
// and a boolean indicating if the field was found.
FieldByName(name string) (StructField, bool)
// FieldByNameFunc returns the struct field with a name
// that satisfies the match function and a boolean indicating if
// the field was found.
//
// FieldByNameFunc considers the fields in the struct itself
// and then the fields in any embedded structs, in breadth first order,
// stopping at the shallowest nesting depth containing one or more
// fields satisfying the match function. If multiple fields at that depth
// satisfy the match function, they cancel each other
// and FieldByNameFunc returns no match.
// This behavior mirrors Go's handling of name lookup in
// structs containing embedded fields.
FieldByNameFunc(match func(string) bool) (StructField, bool)
// In returns the type of a function type's i'th input parameter.
// It panics if the type's Kind is not Func.
// It panics if i is not in the range [0, NumIn()).
In(i int) Type
// Key returns a map type's key type.
// It panics if the type's Kind is not Map.
Key() Type
// Len returns an array type's length.
// It panics if the type's Kind is not Array.
Len() int
// NumField returns a struct type's field count.
// It panics if the type's Kind is not Struct.
NumField() int
// NumIn returns a function type's input parameter count.
// It panics if the type's Kind is not Func.
NumIn() int
// NumOut returns a function type's output parameter count.
// It panics if the type's Kind is not Func.
NumOut() int
// Out returns the type of a function type's i'th output parameter.
// It panics if the type's Kind is not Func.
// It panics if i is not in the range [0, NumOut()).
Out(i int) Type
// contains filtered or unexported methods
} func ArrayOf(count int, elem Type) Type
ArrayOf returns the array type with the given count and element type. For example, if t represents int, ArrayOf(5, t) represents [5]int.
If the resulting type would be larger than the available address space, ArrayOf panics.
func ChanOf(dir ChanDir, t Type) Type
ChanOf returns the channel type with the given direction and element type. For example, if t represents int, ChanOf(RecvDir, t) represents <-chan int.
The gc runtime imposes a limit of 64 kB on channel element types. If t's size is equal to or exceeds this limit, ChanOf panics.
func FuncOf(in, out []Type, variadic bool) Type
FuncOf returns the function type with the given argument and result types. For example if k represents int and e represents string, FuncOf([]Type{k}, []Type{e}, false) represents func(int) string.
The variadic argument controls whether the function is variadic. FuncOf panics if the in[len(in)-1] does not represent a slice and variadic is true.
func MapOf(key, elem Type) Type
MapOf returns the map type with the given key and element types. For example, if k represents int and e represents string, MapOf(k, e) represents map[int]string.
If the key type is not a valid map key type (that is, if it does not implement Go's == operator), MapOf panics.
func PtrTo(t Type) Type
PtrTo returns the pointer type with element t. For example, if t represents type Foo, PtrTo(t) represents *Foo.
func SliceOf(t Type) Type
SliceOf returns the slice type with element type t. For example, if t represents int, SliceOf(t) represents []int.
func StructOf(fields []StructField) Type
StructOf returns the struct type containing fields. The Offset and Index fields are ignored and computed as they would be by the compiler.
StructOf currently does not generate wrapper methods for embedded fields and panics if passed unexported StructFields. These limitations may be lifted in a future version.
package main
import (
"bytes"
"encoding/json"
"fmt"
"reflect"
)
func main() {
typ := reflect.StructOf([]reflect.StructField{
{
Name: "Height",
Type: reflect.TypeOf(float64(0)),
Tag: `json:"height"`,
},
{
Name: "Age",
Type: reflect.TypeOf(int(0)),
Tag: `json:"age"`,
},
})
v := reflect.New(typ).Elem()
v.Field(0).SetFloat(0.4)
v.Field(1).SetInt(2)
s := v.Addr().Interface()
w := new(bytes.Buffer)
if err := json.NewEncoder(w).Encode(s); err != nil {
panic(err)
}
fmt.Printf("value: %+v\n", s)
fmt.Printf("json: %s", w.Bytes())
r := bytes.NewReader([]byte(`{"height":1.5,"age":10}`))
if err := json.NewDecoder(r).Decode(s); err != nil {
panic(err)
}
fmt.Printf("value: %+v\n", s)
}
func TypeOf(i interface{}) Type TypeOf returns the reflection Type that represents the dynamic type of i. If i is a nil interface value, TypeOf returns nil.
package main
import (
"fmt"
"io"
"os"
"reflect"
)
func main() {
// As interface types are only used for static typing, a
// common idiom to find the reflection Type for an interface
// type Foo is to use a *Foo value.
writerType := reflect.TypeOf((*io.Writer)(nil)).Elem()
fileType := reflect.TypeOf((*os.File)(nil))
fmt.Println(fileType.Implements(writerType))
}
Value is the reflection interface to a Go value.
Not all methods apply to all kinds of values. Restrictions, if any, are noted in the documentation for each method. Use the Kind method to find out the kind of value before calling kind-specific methods. Calling a method inappropriate to the kind of type causes a run time panic.
The zero Value represents no value. Its IsValid method returns false, its Kind method returns Invalid, its String method returns "<invalid Value>", and all other methods panic. Most functions and methods never return an invalid value. If one does, its documentation states the conditions explicitly.
A Value can be used concurrently by multiple goroutines provided that the underlying Go value can be used concurrently for the equivalent direct operations.
To compare two Values, compare the results of the Interface method. Using == on two Values does not compare the underlying values they represent.
type Value struct {
// contains filtered or unexported fields
}
func Append(s Value, x ...Value) Value
Append appends the values x to a slice s and returns the resulting slice. As in Go, each x's value must be assignable to the slice's element type.
func AppendSlice(s, t Value) Value
AppendSlice appends a slice t to a slice s and returns the resulting slice. The slices s and t must have the same element type.
func Indirect(v Value) Value
Indirect returns the value that v points to. If v is a nil pointer, Indirect returns a zero Value. If v is not a pointer, Indirect returns v.
func MakeChan(typ Type, buffer int) Value
MakeChan creates a new channel with the specified type and buffer size.
func MakeFunc(typ Type, fn func(args []Value) (results []Value)) Value
MakeFunc returns a new function of the given Type that wraps the function fn. When called, that new function does the following:
- converts its arguments to a slice of Values. - runs results := fn(args). - returns the results as a slice of Values, one per formal result.
The implementation fn can assume that the argument Value slice has the number and type of arguments given by typ. If typ describes a variadic function, the final Value is itself a slice representing the variadic arguments, as in the body of a variadic function. The result Value slice returned by fn must have the number and type of results given by typ.
The Value.Call method allows the caller to invoke a typed function in terms of Values; in contrast, MakeFunc allows the caller to implement a typed function in terms of Values.
The Examples section of the documentation includes an illustration of how to use MakeFunc to build a swap function for different types.
package main
import (
"fmt"
"reflect"
)
func main() {
// swap is the implementation passed to MakeFunc.
// It must work in terms of reflect.Values so that it is possible
// to write code without knowing beforehand what the types
// will be.
swap := func(in []reflect.Value) []reflect.Value {
return []reflect.Value{in[1], in[0]}
}
// makeSwap expects fptr to be a pointer to a nil function.
// It sets that pointer to a new function created with MakeFunc.
// When the function is invoked, reflect turns the arguments
// into Values, calls swap, and then turns swap's result slice
// into the values returned by the new function.
makeSwap := func(fptr interface{}) {
// fptr is a pointer to a function.
// Obtain the function value itself (likely nil) as a reflect.Value
// so that we can query its type and then set the value.
fn := reflect.ValueOf(fptr).Elem()
// Make a function of the right type.
v := reflect.MakeFunc(fn.Type(), swap)
// Assign it to the value fn represents.
fn.Set(v)
}
// Make and call a swap function for ints.
var intSwap func(int, int) (int, int)
makeSwap(&intSwap)
fmt.Println(intSwap(0, 1))
// Make and call a swap function for float64s.
var floatSwap func(float64, float64) (float64, float64)
makeSwap(&floatSwap)
fmt.Println(floatSwap(2.72, 3.14))
}
func MakeMap(typ Type) Value
MakeMap creates a new map with the specified type.
func MakeMapWithSize(typ Type, n int) Value
MakeMapWithSize creates a new map with the specified type and initial space for approximately n elements.
func MakeSlice(typ Type, len, cap int) Value
MakeSlice creates a new zero-initialized slice value for the specified slice type, length, and capacity.
func New(typ Type) Value
New returns a Value representing a pointer to a new zero value for the specified type. That is, the returned Value's Type is PtrTo(typ).
func NewAt(typ Type, p unsafe.Pointer) Value
NewAt returns a Value representing a pointer to a value of the specified type, using p as that pointer.
func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool)
Select executes a select operation described by the list of cases. Like the Go select statement, it blocks until at least one of the cases can proceed, makes a uniform pseudo-random choice, and then executes that case. It returns the index of the chosen case and, if that case was a receive operation, the value received and a boolean indicating whether the value corresponds to a send on the channel (as opposed to a zero value received because the channel is closed).
func ValueOf(i interface{}) Value ValueOf returns a new Value initialized to the concrete value stored in the interface i. ValueOf(nil) returns the zero Value.
func Zero(typ Type) Value
Zero returns a Value representing the zero value for the specified type. The result is different from the zero value of the Value struct, which represents no value at all. For example, Zero(TypeOf(42)) returns a Value with Kind Int and value 0. The returned value is neither addressable nor settable.
func (v Value) Addr() Value
Addr returns a pointer value representing the address of v. It panics if CanAddr() returns false. Addr is typically used to obtain a pointer to a struct field or slice element in order to call a method that requires a pointer receiver.
func (v Value) Bool() bool
Bool returns v's underlying value. It panics if v's kind is not Bool.
func (v Value) Bytes() []byte
Bytes returns v's underlying value. It panics if v's underlying value is not a slice of bytes.
func (v Value) Call(in []Value) []Value
Call calls the function v with the input arguments in. For example, if len(in) == 3, v.Call(in) represents the Go call v(in[0], in[1], in[2]). Call panics if v's Kind is not Func. It returns the output results as Values. As in Go, each input argument must be assignable to the type of the function's corresponding input parameter. If v is a variadic function, Call creates the variadic slice parameter itself, copying in the corresponding values.
func (v Value) CallSlice(in []Value) []Value
CallSlice calls the variadic function v with the input arguments in, assigning the slice in[len(in)-1] to v's final variadic argument. For example, if len(in) == 3, v.CallSlice(in) represents the Go call v(in[0], in[1], in[2]...). CallSlice panics if v's Kind is not Func or if v is not variadic. It returns the output results as Values. As in Go, each input argument must be assignable to the type of the function's corresponding input parameter.
func (v Value) CanAddr() bool
CanAddr reports whether the value's address can be obtained with Addr. Such values are called addressable. A value is addressable if it is an element of a slice, an element of an addressable array, a field of an addressable struct, or the result of dereferencing a pointer. If CanAddr returns false, calling Addr will panic.
func (v Value) CanInterface() bool
CanInterface reports whether Interface can be used without panicking.
func (v Value) CanSet() bool
CanSet reports whether the value of v can be changed. A Value can be changed only if it is addressable and was not obtained by the use of unexported struct fields. If CanSet returns false, calling Set or any type-specific setter (e.g., SetBool, SetInt) will panic.
func (v Value) Cap() int
Cap returns v's capacity. It panics if v's Kind is not Array, Chan, or Slice.
func (v Value) Close()
Close closes the channel v. It panics if v's Kind is not Chan.
func (v Value) Complex() complex128
Complex returns v's underlying value, as a complex128. It panics if v's Kind is not Complex64 or Complex128
func (v Value) Convert(t Type) Value
Convert returns the value v converted to type t. If the usual Go conversion rules do not allow conversion of the value v to type t, Convert panics.
func (v Value) Elem() Value
Elem returns the value that the interface v contains or that the pointer v points to. It panics if v's Kind is not Interface or Ptr. It returns the zero Value if v is nil.
func (v Value) Field(i int) Value
Field returns the i'th field of the struct v. It panics if v's Kind is not Struct or i is out of range.
func (v Value) FieldByIndex(index []int) Value
FieldByIndex returns the nested field corresponding to index. It panics if v's Kind is not struct.
func (v Value) FieldByName(name string) Value
FieldByName returns the struct field with the given name. It returns the zero Value if no field was found. It panics if v's Kind is not struct.
func (v Value) FieldByNameFunc(match func(string) bool) Value
FieldByNameFunc returns the struct field with a name that satisfies the match function. It panics if v's Kind is not struct. It returns the zero Value if no field was found.
func (v Value) Float() float64
Float returns v's underlying value, as a float64. It panics if v's Kind is not Float32 or Float64
func (v Value) Index(i int) Value
Index returns v's i'th element. It panics if v's Kind is not Array, Slice, or String or i is out of range.
func (v Value) Int() int64
Int returns v's underlying value, as an int64. It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
func (v Value) Interface() (i interface{}) Interface returns v's current value as an interface{}. It is equivalent to:
var i interface{} = (v's underlying value)
It panics if the Value was obtained by accessing unexported struct fields.
func (v Value) InterfaceData() [2]uintptr
InterfaceData returns the interface v's value as a uintptr pair. It panics if v's Kind is not Interface.
func (v Value) IsNil() bool
IsNil reports whether its argument v is nil. The argument must be a chan, func, interface, map, pointer, or slice value; if it is not, IsNil panics. Note that IsNil is not always equivalent to a regular comparison with nil in Go. For example, if v was created by calling ValueOf with an uninitialized interface variable i, i==nil will be true but v.IsNil will panic as v will be the zero Value.
func (v Value) IsValid() bool
IsValid reports whether v represents a value. It returns false if v is the zero Value. If IsValid returns false, all other methods except String panic. Most functions and methods never return an invalid value. If one does, its documentation states the conditions explicitly.
func (v Value) Kind() Kind
Kind returns v's Kind. If v is the zero Value (IsValid returns false), Kind returns Invalid.
func (v Value) Len() int
Len returns v's length. It panics if v's Kind is not Array, Chan, Map, Slice, or String.
func (v Value) MapIndex(key Value) Value
MapIndex returns the value associated with key in the map v. It panics if v's Kind is not Map. It returns the zero Value if key is not found in the map or if v represents a nil map. As in Go, the key's value must be assignable to the map's key type.
func (v Value) MapKeys() []Value
MapKeys returns a slice containing all the keys present in the map, in unspecified order. It panics if v's Kind is not Map. It returns an empty slice if v represents a nil map.
func (v Value) Method(i int) Value
Method returns a function value corresponding to v's i'th method. The arguments to a Call on the returned function should not include a receiver; the returned function will always use v as the receiver. Method panics if i is out of range or if v is a nil interface value.
func (v Value) MethodByName(name string) Value
MethodByName returns a function value corresponding to the method of v with the given name. The arguments to a Call on the returned function should not include a receiver; the returned function will always use v as the receiver. It returns the zero Value if no method was found.
func (v Value) NumField() int
NumField returns the number of fields in the struct v. It panics if v's Kind is not Struct.
func (v Value) NumMethod() int
NumMethod returns the number of exported methods in the value's method set.
func (v Value) OverflowComplex(x complex128) bool
OverflowComplex reports whether the complex128 x cannot be represented by v's type. It panics if v's Kind is not Complex64 or Complex128.
func (v Value) OverflowFloat(x float64) bool
OverflowFloat reports whether the float64 x cannot be represented by v's type. It panics if v's Kind is not Float32 or Float64.
func (v Value) OverflowInt(x int64) bool
OverflowInt reports whether the int64 x cannot be represented by v's type. It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64.
func (v Value) OverflowUint(x uint64) bool
OverflowUint reports whether the uint64 x cannot be represented by v's type. It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) Pointer() uintptr
Pointer returns v's value as a uintptr. It returns uintptr instead of unsafe.Pointer so that code using reflect cannot obtain unsafe.Pointers without importing the unsafe package explicitly. It panics if v's Kind is not Chan, Func, Map, Ptr, Slice, or UnsafePointer.
If v's Kind is Func, the returned pointer is an underlying code pointer, but not necessarily enough to identify a single function uniquely. The only guarantee is that the result is zero if and only if v is a nil func Value.
If v's Kind is Slice, the returned pointer is to the first element of the slice. If the slice is nil the returned value is 0. If the slice is empty but non-nil the return value is non-zero.
func (v Value) Recv() (x Value, ok bool)
Recv receives and returns a value from the channel v. It panics if v's Kind is not Chan. The receive blocks until a value is ready. The boolean value ok is true if the value x corresponds to a send on the channel, false if it is a zero value received because the channel is closed.
func (v Value) Send(x Value)
Send sends x on the channel v. It panics if v's kind is not Chan or if x's type is not the same type as v's element type. As in Go, x's value must be assignable to the channel's element type.
func (v Value) Set(x Value)
Set assigns x to the value v. It panics if CanSet returns false. As in Go, x's value must be assignable to v's type.
func (v Value) SetBool(x bool)
SetBool sets v's underlying value. It panics if v's Kind is not Bool or if CanSet() is false.
func (v Value) SetBytes(x []byte)
SetBytes sets v's underlying value. It panics if v's underlying value is not a slice of bytes.
func (v Value) SetCap(n int)
SetCap sets v's capacity to n. It panics if v's Kind is not Slice or if n is smaller than the length or greater than the capacity of the slice.
func (v Value) SetComplex(x complex128)
SetComplex sets v's underlying value to x. It panics if v's Kind is not Complex64 or Complex128, or if CanSet() is false.
func (v Value) SetFloat(x float64)
SetFloat sets v's underlying value to x. It panics if v's Kind is not Float32 or Float64, or if CanSet() is false.
func (v Value) SetInt(x int64)
SetInt sets v's underlying value to x. It panics if v's Kind is not Int, Int8, Int16, Int32, or Int64, or if CanSet() is false.
func (v Value) SetLen(n int)
SetLen sets v's length to n. It panics if v's Kind is not Slice or if n is negative or greater than the capacity of the slice.
func (v Value) SetMapIndex(key, val Value)
SetMapIndex sets the value associated with key in the map v to val. It panics if v's Kind is not Map. If val is the zero Value, SetMapIndex deletes the key from the map. Otherwise if v holds a nil map, SetMapIndex will panic. As in Go, key's value must be assignable to the map's key type, and val's value must be assignable to the map's value type.
func (v Value) SetPointer(x unsafe.Pointer)
SetPointer sets the unsafe.Pointer value v to x. It panics if v's Kind is not UnsafePointer.
func (v Value) SetString(x string)
SetString sets v's underlying value to x. It panics if v's Kind is not String or if CanSet() is false.
func (v Value) SetUint(x uint64)
SetUint sets v's underlying value to x. It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64, or if CanSet() is false.
func (v Value) Slice(i, j int) Value
Slice returns v[i:j]. It panics if v's Kind is not Array, Slice or String, or if v is an unaddressable array, or if the indexes are out of bounds.
func (v Value) Slice3(i, j, k int) Value
Slice3 is the 3-index form of the slice operation: it returns v[i:j:k]. It panics if v's Kind is not Array or Slice, or if v is an unaddressable array, or if the indexes are out of bounds.
func (v Value) String() string
String returns the string v's underlying value, as a string. String is a special case because of Go's String method convention. Unlike the other getters, it does not panic if v's Kind is not String. Instead, it returns a string of the form "<T value>" where T is v's type. The fmt package treats Values specially. It does not call their String method implicitly but instead prints the concrete values they hold.
func (v Value) TryRecv() (x Value, ok bool)
TryRecv attempts to receive a value from the channel v but will not block. It panics if v's Kind is not Chan. If the receive delivers a value, x is the transferred value and ok is true. If the receive cannot finish without blocking, x is the zero Value and ok is false. If the channel is closed, x is the zero value for the channel's element type and ok is false.
func (v Value) TrySend(x Value) bool
TrySend attempts to send x on the channel v but will not block. It panics if v's Kind is not Chan. It reports whether the value was sent. As in Go, x's value must be assignable to the channel's element type.
func (v Value) Type() Type
Type returns v's type.
func (v Value) Uint() uint64
Uint returns v's underlying value, as a uint64. It panics if v's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
func (v Value) UnsafeAddr() uintptr
UnsafeAddr returns a pointer to v's data. It is for advanced clients that also import the "unsafe" package. It panics if v is not addressable.
A ValueError occurs when a Value method is invoked on a Value that does not support it. Such cases are documented in the description of each method.
type ValueError struct {
Method string
Kind Kind
}
func (e *ValueError) Error() string
FieldByName and related functions consider struct field names to be equal if the names are equal, even if they are unexported names originating in different packages. The practical effect of this is that the result of t.FieldByName("x") is not well defined if the struct type t contains multiple fields named x (embedded from different packages). FieldByName may return one of the fields named x or may report that there are none. See https://golang.org/issue/4876 for more details.
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Licensed under the Creative Commons Attribution License 3.0.
https://golang.org/pkg/reflect/