reflect

package standard library
go1.27.1 Latest Latest
Warning

This package is not in the latest version of its module.

Go to latest
Published: Sep 1, 2026 License: BSD-3-Clause Imports: 18 Imported by: 831,409

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. It dst is an Array, it panics if Value.CanSet returns false.

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.

func TypeAssert added in go1.25.0

func TypeAssert[T any](v Value) (T, bool)

TypeAssert is semantically equivalent to:

v2, ok := v.Interface().(T)

Note that this function, just as the type assertion above, might return:

  • ok == false when v.Type() == reflect.TypeFor[T]() For example, when both T and v are interface types and v.IsNil() == true. In that case v.Interface() returns a nil interface value, and the assertion .(T) fails with ok == false.

  • ok == true when v.Type() != reflect.TypeFor[T](). For example, when T is an interface type and v holds a value whose concrete type implements T.

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 field := range st.Fields() {
		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.
	//
	// Calling this method will force the linker to retain all exported methods in all packages.
	// This may make the executable binary larger but will not affect execution time.
	Method(int) Method

	// Methods returns an iterator over each method in the type's method set. The sequence is
	// equivalent to calling Method successively for each index i in the range [0, NumMethod()).
	Methods() iter.Seq[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.
	//
	// Calling this method will cause the linker to retain all methods with this name in all packages.
	// If the linker can't determine the name, it will retain all exported methods.
	// This may make the executable binary larger but will not affect execution time.
	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

	// Fields returns an iterator over each struct field for struct type t. The sequence is
	// equivalent to calling Field successively for each index i in the range [0, NumField()).
	// It panics if the type's Kind is not Struct.
	Fields() iter.Seq[StructField]