reflect

package standard library
go1.23.3 Latest Latest
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 Go to latest Published: Nov 6, 2024 License: BSD-3-Clause Imports: 14 Imported by: 680,452

Documentation

Overview

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

Index

Examples

Constants

View Source 
const Ptr = Pointer

Ptr is the old name for the Pointer kind.

Variables

This section is empty.

Functions

func Copy 

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 

func DeepEqual(x, y any) 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 added in go1.8 

func Swapper(slice any) 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.

Types

type ChanDir 

type ChanDir int

ChanDir represents a channel type's direction. 

const (
	RecvDir ChanDir             = 1 << iota // <-chan
	SendDir                                 // chan<-
	BothDir = RecvDir | SendDir             // chan
)

func (ChanDir) String 

func (d ChanDir) String() string

type Kind 

type Kind uint

A Kind represents the specific kind of type that a Type represents. The zero Kind is not a valid kind.

Example 
package main

import (
	"fmt"
	"reflect"
)

func main() {
	for _, v := range []any{"hi", 42, func() {}} {
		switch v := reflect.ValueOf(v); v.Kind() {
		case reflect.String:
			fmt.Println(v.String())
		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
			fmt.Println(v.Int())
		default:
			fmt.Printf("unhandled kind %s", v.Kind())
		}
	}

}

Output:

hi
42
unhandled kind func

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
	Pointer
	Slice
	String
	Struct
	UnsafePointer
)

func (Kind) String 

func (k Kind) String() string

String returns the name of k.

type MapIter added in go1.12 

type MapIter struct {
	// contains filtered or unexported fields
}

A MapIter is an iterator for ranging over a map. See Value.MapRange.

func (*MapIter) Key added in go1.12 

func (iter *MapIter) Key() Value

Key returns the key of iter's current map entry.

func (*MapIter) Next added in go1.12 

func (iter *MapIter) Next() bool

Next advances the map iterator and reports whether there is another entry. It returns false when iter is exhausted; subsequent calls to MapIter.Key, MapIter.Value, or MapIter.Next will panic.

func (*MapIter) Reset added in go1.18 

func (iter *MapIter) Reset(v Value)

Reset modifies iter to iterate over v. It panics if v's Kind is not Map and v is not the zero Value. Reset(Value{}) causes iter to not to refer to any map, which may allow the previously iterated-over map to be garbage collected.

func (*MapIter) Value added in go1.12 

func (iter *MapIter) Value() Value

Value returns the value of iter's current map entry.

type Method 

type Method struct {
	// Name is the method name.
	Name string

	// 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
	PkgPath string

	Type  Type  // method type
	Func  Value // func with receiver as first argument
	Index int   // index for Type.Method
}

Method represents a single method.

func (Method) IsExported added in go1.17 

func (m Method) IsExported() bool

IsExported reports whether the method is exported.

type SelectCase added in go1.1 

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 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 SelectDir added in go1.1 

type SelectDir int

A SelectDir describes the communication direction of a select case. 

const (
	SelectSend    SelectDir // case Chan <- Send
	SelectRecv              // case <-Chan:
	SelectDefault           // default
)

type SliceHeader deprecated 

type SliceHeader struct {
	Data uintptr
	Len  int
	Cap  int
}

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.

Deprecated: Use unsafe.Slice or unsafe.SliceData instead.

type StringHeader deprecated 

type StringHeader struct {
	Data uintptr
	Len  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.

Deprecated: Use unsafe.String or unsafe.StringData instead.

type StructField 

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 StructField describes a single field in a struct.

func VisibleFields added in go1.17 

func VisibleFields(t Type) []StructField

VisibleFields returns all the visible fields in t, which must be a struct type. A field is defined as visible if it's accessible directly with a FieldByName call. The returned fields include fields inside anonymous struct members and unexported fields. They follow the same order found in the struct, with anonymous fields followed immediately by their promoted fields.

For each element e of the returned slice, the corresponding field can be retrieved from a value v of type t by calling v.FieldByIndex(e.Index).

func (StructField) IsExported added in go1.17 

func (f StructField) IsExported() bool

IsExported reports whether the field is exported.

type StructTag 

type StructTag string

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.

Example 
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"))

}

Output:

blue gopher

func (StructTag) Get 

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 StructTag.Lookup.

func (StructTag) Lookup added in go1.7 

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.

Example 
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)")
		}
	}

}

Output:

field_0
(blank)
(not specified)

type Type 

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,
	// and only exported methods are accessible.
	//
	// For an interface type, the returned Method's Type field gives the
	// method signature, without a receiver, and the Func field is nil.
	//
	// Methods are sorted in lexicographic order.
	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 methods accessible using Method.
	//
	// For a non-interface type, it returns the number of exported methods.
	//
	// For an interface type, it returns the number of exported and unexported methods.
	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.
	// Even if ConvertibleTo returns true, the conversion may still panic.
	// For example, a slice of type []T is convertible to *[N]T,
	// but the conversion will panic if its length is less than N.
	ConvertibleTo(u Type) bool

	// Comparable reports whether values of this type are comparable.
	// Even if Comparable returns true, the comparison may still panic.
	// For example, values of interface type are comparable,
	// but the comparison will panic if their dynamic type is not 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, Pointer, 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.
	// If the returned field is promoted from an embedded struct,
	// then Offset in the returned StructField is the offset in
	// the embedded struct.
	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.
	//
	// If the returned field is promoted from an embedded struct,
	// then Offset in the returned StructField is the offset in
	// the embedded struct.
	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

	// OverflowComplex reports whether the complex128 x cannot be represented by type t.
	// It panics if t's Kind is not Complex64 or Complex128.
	OverflowComplex(x complex128) bool

	// OverflowFloat reports whether the float64 x cannot be represented by type t.
	// It panics if t's Kind is not Float32 or Float64.
	OverflowFloat(x float64) bool

	// OverflowInt reports whether the int64 x cannot be represented by type t.
	// It panics if t's Kind is not Int, Int8, Int16, Int32, or Int64.
	OverflowInt(x int64) bool

	// OverflowUint reports whether the uint64 x cannot be represented by type t.
	// It panics if t's Kind is not Uint, Uintptr, Uint8, Uint16, Uint32, or Uint64.
	OverflowUint(x uint64) bool

	// CanSeq reports whether a [Value] with this type can be iterated over using [Value.Seq].
	CanSeq() bool

	// CanSeq2 reports whether a [Value] with this type can be iterated over using [Value.Seq2].
	CanSeq2() bool
	// contains filtered or unexported methods
}

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.

func ArrayOf added in go1.5 

func ArrayOf(length int, elem Type) Type

ArrayOf returns the array type with the given length 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 added in go1.1 

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 added in go1.5 

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 added in go1.1 

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 PointerTo added in go1.18 

func PointerTo(t Type) Type

PointerTo returns the pointer type with element t. For example, if t represents type Foo, PointerTo(t) represents *Foo.

func PtrTo deprecated 

func PtrTo(t Type) Type

PtrTo returns the pointer type with element t. For example, if t represents type Foo, PtrTo(t) represents *Foo.

PtrTo is the old spelling of PointerTo. The two functions behave identically.

Deprecated: Superseded by PointerTo.

func SliceOf added in go1.1 

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 added in go1.7 

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 support promoted methods of embedded fields and panics if passed unexported StructFields.

Example 
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)

}

Output:

value: &{Height:0.4 Age:2}
json:  {"height":0.4,"age":2}
value: &{Height:1.5 Age:10}

func TypeFor added in go1.22.0 

func TypeFor[T any]() Type

TypeFor returns the Type that represents the type argument T.

func TypeOf 

func TypeOf(i any) Type

TypeOf returns the reflection Type that represents the dynamic type of i. If i is a nil interface value, TypeOf returns nil.

Example 
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))

}

Output:

true

type Value 

type Value struct {
	// contains filtered or unexported fields
}

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 Value.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.

func Append 

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 

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 

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 

func MakeChan(typ Type, buffer int) Value

MakeChan creates a new channel with the specified type and buffer size.

func MakeFunc added in go1.1 

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.

Example 
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 any) {
		// 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))

}

Output:

1 0
3.14 2.72

func MakeMap 

func MakeMap(typ Type) Value

MakeMap creates a new map with the specified type.

func MakeMapWithSize added in go1.9 

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 

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 

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 PointerTo(typ).

func NewAt 

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 added in go1.1 

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). Select supports a maximum of 65536 cases.

func SliceAt added in go1.23.0 

func SliceAt(typ Type, p unsafe.Pointer, n int) Value

SliceAt returns a Value representing a slice whose underlying data starts at p, with length and capacity equal to n.

This is like unsafe.Slice.

func ValueOf 

func ValueOf(i any) Value

ValueOf returns a new Value initialized to the concrete value stored in the interface i. ValueOf(nil) returns the zero Value.

func Zero 

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 (Value) Addr 

func (v Value) Addr() Value

Addr returns a pointer value representing the address of v. It panics if Value.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 (Value) Bool 

func (v Value) Bool() bool

Bool returns v's underlying value. It panics if v's kind is not Bool.

func (Value) Bytes 

func (v Value) Bytes() []byte

Bytes returns v's underlying value. It panics if v's underlying value is not a slice of bytes or an addressable array of bytes.

func (Value) Call 

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 (Value) CallSlice 

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 (Value) CanAddr 

func (v Value) CanAddr() bool

CanAddr reports whether the value's address can be obtained with Value.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 Value.Addr will panic.

func (Value) CanComplex added in go1.18 

func (v Value) CanComplex() bool

CanComplex reports whether Value.Complex can be used without panicking.

func (Value) CanConvert added in go1.17 

func (v Value) CanConvert(t Type) bool

CanConvert reports whether the value v can be converted to type t. If v.CanConvert(t) returns true then v.Convert(t) will not panic.

func (Value) CanFloat added in go1.18 

func (v Value) CanFloat() bool

CanFloat reports whether Value.Float can be used without panicking.

func (Value) CanInt added in go1.18 

func (v Value) CanInt() bool

CanInt reports whether Int can be used without panicking.

func (Value) CanInterface 

func (v Value) CanInterface() bool

CanInterface reports whether Value.Interface can be used without panicking.

func (Value) CanSet 

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 Value.Set or any type-specific setter (e.g., Value.SetBool, Value.SetInt) will panic.

func (Value) CanUint added in go1.18 

func (v Value) CanUint() bool

CanUint reports whether Value.Uint can be used without panicking.

func (Value) Cap 

func (v Value) Cap() int

Cap returns v's capacity. It panics if v's Kind is not Array, Chan, Slice or pointer to Array.

func (Value) Clear added in go1.21.0 

func (v Value) Clear()

Clear clears the contents of a map or zeros the contents of a slice.

It panics if v's Kind is not Map or Slice.

func (Value) Close 

func (v Value) Close()

Close closes the channel v. It panics if v's Kind is not Chan or v is a receive-only channel.

func (Value) Comparable added in go1.20 

func (v Value) Comparable() bool

Comparable reports whether the value v is comparable. If the type of v is an interface, this checks the dynamic type. If this reports true then v.Interface() == x will not panic for any x, nor will v.Equal(u) for any Value u.

func (Value) Complex 

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 (Value) Convert added in go1.1 

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, or if converting v to type t panics, Convert panics.

func (Value) Elem 

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 Pointer. It returns the zero Value if v is nil.

func (Value) Equal added in go1.20 

func (v Value) Equal(u Value) bool

Equal reports true if v is equal to u. For two invalid values, Equal will report true. For an interface value, Equal will compare the value within the interface. Otherwise, If the values have different types, Equal will report false. Otherwise, for arrays and structs Equal will compare each element in order, and report false if it finds non-equal elements. During all comparisons, if values of the same type are compared, and the type is not comparable, Equal will panic.

func (Value) Field 

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 (Value) FieldByIndex 

func (v Value) FieldByIndex(index []int) Value

FieldByIndex returns the nested field corresponding to index. It panics if evaluation requires stepping through a nil pointer or a field that is not a struct.

Example 
package main

import (
	"fmt"
	"reflect"
)

func main() {
	// This example shows a case in which the name of a promoted field
	// is hidden by another field: FieldByName will not work, so
	// FieldByIndex must be used instead.
	type user struct {
		firstName string
		lastName  string
	}

	type data struct {
		user
		firstName string
		lastName  string
	}

	u := data{
		user:      user{"Embedded John", "Embedded Doe"},
		firstName: "John",
		lastName:  "Doe",
	}

	s := reflect.ValueOf(u).FieldByIndex([]int{0, 1})
	fmt.Println("embedded last name:", s)

}

Output:

embedded last name: Embedded Doe

func (Value) FieldByIndexErr added in go1.18 

func (v Value) FieldByIndexErr(index []int) (Value, error)

FieldByIndexErr returns the nested field corresponding to index. It returns an error if evaluation requires stepping through a nil pointer, but panics if it must step through a field that is not a struct.

func (Value) FieldByName 

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.

Example 
package main

import (
	"fmt"
	"reflect"
)

func main() {
	type user struct {
		firstName string
		lastName  string
	}
	u := user{firstName: "John", lastName: "Doe"}
	s := reflect.ValueOf(u)

	fmt.Println("Name:", s.FieldByName("firstName"))
}

Output:

Name: John

func (Value) FieldByNameFunc 

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 (Value) Float 

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 (Value) Grow added in go1.20 

func (v Value) Grow(n int)

Grow increases the slice's capacity, if necessary, to guarantee space for another n elements. After Grow(n), at least n elements can be appended to the slice without another allocation.

It panics if v's Kind is not a Slice or if n is negative or too large to allocate the memory.

func (Value) Index 

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 (Value) Int 

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 (Value) Interface 

func (v Value) Interface() (i any)

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 (Value) InterfaceData deprecated 

func (v Value) InterfaceData() [2]uintptr

InterfaceData returns a pair of unspecified uintptr values. It panics if v's Kind is not Interface.

In earlier versions of Go, this function returned the interface's value as a uintptr pair. As of Go 1.4, the implementation of interface values precludes any defined use of InterfaceData.

Deprecated: The memory representation of interface values is not compatible with InterfaceData.

func (Value) IsNil 

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 (Value) IsValid 

func (v Value) IsValid() bool

IsValid reports whether v represents a value. It returns false if v is the zero Value. If Value.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 (Value) IsZero added in go1.13 

func (v Value) IsZero() bool

IsZero reports whether v is the zero value for its type. It panics if the argument is invalid.

func (Value) Kind 

func (v Value) Kind() Kind

Kind returns v's Kind. If v is the zero Value (Value.IsValid returns false), Kind returns Invalid.

func (Value) Len 

func (v Value) Len() int

Len returns v's length. It panics if v's Kind is not Array, Chan, Map, Slice, String, or pointer to Array.

func (Value) MapIndex 

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 (Value) MapKeys 

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 (Value) MapRange added in go1.12 

func (v Value) MapRange() *MapIter

MapRange returns a range iterator for a map. It panics if v's Kind is not Map.

Call MapIter.Next to advance the iterator, and MapIter.Key/MapIter.Value to access each entry. MapIter.Next returns false when the iterator is exhausted. MapRange follows the same iteration semantics as a range statement.

Example: 

iter := reflect.ValueOf(m).MapRange()
for iter.Next() {
	k := iter.Key()
	v := iter.Value()
	...
}

func (Value) Method 

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 (Value) MethodByName 

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 (Value) NumField 

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 (Value) NumMethod 

func (v Value) NumMethod() int

NumMethod returns the number of methods in the value's method set.

For a non-interface type, it returns the number of exported methods.

For an interface type, it returns the number of exported and unexported methods.

func (Value) OverflowComplex 

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 (Value) OverflowFloat 

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 (Value) OverflowInt 

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 (Value) OverflowUint 

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 (Value) Pointer 

func (v Value) Pointer() uintptr

Pointer returns v's value as a uintptr. It panics if v's Kind is not Chan, Func, Map, Pointer, Slice, String, 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.

If v's Kind is String, the returned pointer is to the first element of the underlying bytes of string.

It's preferred to use uintptr(Value.UnsafePointer()) to get the equivalent result.

func (Value) Recv 

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 (Value) Send 

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 (Value) Seq added in go1.23.0 

func (v Value) Seq() iter.Seq[Value]

Seq returns an iter.Seq[Value] that loops over the elements of v. If v's kind is Func, it must be a function that has no results and that takes a single argument of type func(T) bool for some type T. If v's kind is Pointer, the pointer element type must have kind Array. Otherwise v's kind must be Int, Int8, Int16, Int32, Int64, Uint, Uint8, Uint16, Uint32, Uint64, Uintptr, Array, Chan, Map, Slice, or String.

func (Value) Seq2 added in go1.23.0 

func (v Value) Seq2() iter.Seq2[Value, Value]

Seq2 returns an iter.Seq2[Value, Value] that loops over the elements of v. If v's kind is Func, it must be a function that has no results and that takes a single argument of type func(K, V) bool for some type K, V. If v's kind is Pointer, the pointer element type must have kind Array. Otherwise v's kind must be Array, Map, Slice, or String.

func (Value) Set 

func (v Value) Set(x Value)

Set assigns x to the value v. It panics if Value.CanSet returns false. As in Go, x's value must be assignable to v's type and must not be derived from an unexported field.

func (Value) SetBool 

func (v Value) SetBool(x bool)

SetBool sets v's underlying value. It panics if v's Kind is not Bool or if Value.CanSet returns false.

func (Value) SetBytes 

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 (Value) SetCap added in go1.2 

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 (Value) SetComplex 

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 Value.CanSet returns false.

func (Value) SetFloat 

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 Value.CanSet returns false.

func (Value) SetInt 

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 Value.CanSet returns false.

func (Value) SetIterKey added in go1.18 

func (v Value) SetIterKey(iter *MapIter)

SetIterKey assigns to v the key of iter's current map entry. It is equivalent to v.Set(iter.Key()), but it avoids allocating a new Value. As in Go, the key must be assignable to v's type and must not be derived from an unexported field.

func (Value) SetIterValue added in go1.18 

func (v Value) SetIterValue(iter *MapIter)

SetIterValue assigns to v the value of iter's current map entry. It is equivalent to v.Set(iter.Value()), but it avoids allocating a new Value. As in Go, the value must be assignable to v's type and must not be derived from an unexported field.

func (Value) SetLen 

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 (Value) SetMapIndex 

func (v Value) SetMapIndex(key, elem Value)

SetMapIndex sets the element associated with key in the map v to elem. It panics if v's Kind is not Map. If elem 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 elem must be assignable to the map's key type, and elem's value must be assignable to the map's elem type.

func (Value) SetPointer 

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 (Value) SetString 

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 Value.CanSet returns false.

func (Value) SetUint 

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 Value.CanSet returns false.

func (Value) SetZero added in go1.20 

func (v Value) SetZero()

SetZero sets v to be the zero value of v's type. It panics if Value.CanSet returns false.

func (Value) Slice 

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 (Value) Slice3 added in go1.2 

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 (Value) String 

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 (Value) TryRecv 

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 (Value) TrySend 

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 (Value) Type 

func (v Value) Type() Type

Type returns v's type.

func (Value) Uint 

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 (Value) UnsafeAddr 

func (v Value) UnsafeAddr() uintptr

UnsafeAddr returns a pointer to v's data, as a uintptr. It panics if v is not addressable.

It's preferred to use uintptr(Value.Addr().UnsafePointer()) to get the equivalent result.

func (Value) UnsafePointer added in go1.18 

func (v Value) UnsafePointer() unsafe.Pointer

UnsafePointer returns v's value as a unsafe.Pointer. It panics if v's Kind is not Chan, Func, Map, Pointer, Slice, String 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 nil. If the slice is empty but non-nil the return value is non-nil.

If v's Kind is String, the returned pointer is to the first element of the underlying bytes of string.

type ValueError 

type ValueError struct {
	Method string
	Kind   Kind
}

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.

func (*ValueError) Error 

func (e *ValueError) Error() string

Notes

Bugs

  • 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.

Source Files

View all Source files

Directories

Path Synopsis
internal
example1
example2

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