Code Examples
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())
}
}
}
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))
}
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)
}
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"))
}
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)")
}
}
}
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))
}
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)
}
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"))
}
A MapIter is an iterator for ranging over a map.
See [Value.MapRange]. Key returns the key of iter's current map entry. 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. 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. Value returns the value of iter's current map entry.
func Value.MapRange() *MapIter
func Value.SetIterKey(iter *MapIter)
func Value.SetIterValue(iter *MapIter)
Method represents a single method. // func with receiver as first argument // index for Type.Method 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 // method type IsExported reports whether the method is exported.
func Type.Method(int) Method
func Type.MethodByName(string) (Method, bool)
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. // channel to use (for send or receive) // direction of case // value to send (for send)
func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool)
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.CapintDatauintptrLenint
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.DatauintptrLenint
A StructField describes a single field in a struct. // is an embedded field // index sequence for Type.FieldByIndex Name is the field name. // offset within struct, in bytes 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 // field tag string // field type IsExported reports whether the field is exported.
func VisibleFields(t Type) []StructField
func Type.Field(i int) StructField
func Type.FieldByIndex(index []int) StructField
func Type.FieldByName(name string) (StructField, bool)
func Type.FieldByNameFunc(match func(string) bool) (StructField, bool)
func StructOf(fields []StructField) Type
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. 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]. 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.
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. Align returns the alignment in bytes of a value of
this type when allocated in memory. AssignableTo reports whether a value of the type is assignable to type u. 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. CanSeq reports whether a [Value] with this type can be iterated over using [Value.Seq]. CanSeq2 reports whether a [Value] with this type can be iterated over using [Value.Seq2]. ChanDir returns a channel type's direction.
It panics if the type's Kind is not Chan. 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. 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. Elem returns a type's element type.
It panics if the type's Kind is not Array, Chan, Map, Pointer, or Slice. 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()). FieldAlign returns the alignment in bytes of a value of
this type when used as a field in a struct. 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. 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. 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. Implements reports whether the type implements the interface type u. 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()). 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. Key returns a map type's key type.
It panics if the type's Kind is not Map. Kind returns the specific kind of this type. Len returns an array type's length.
It panics if the type's Kind is not Array. 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. 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. Name returns the type's name within its package for a defined type.
For other (non-defined) types it returns the empty string. NumField returns a struct type's field count.
It panics if the type's Kind is not Struct. NumIn returns a function type's input parameter count.
It panics if the type's Kind is not Func. 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. NumOut returns a function type's output parameter count.
It panics if the type's Kind is not Func. 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()). OverflowComplex reports whether the complex128 x cannot be represented by type t.
It panics if t's Kind is not Complex64 or Complex128. OverflowFloat reports whether the float64 x cannot be represented by type t.
It panics if t's Kind is not Float32 or Float64. 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. 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. 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. Size returns the number of bytes needed to store
a value of the given type; it is analogous to unsafe.Sizeof. 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.
Type : expvar.Var
Type : fmt.Stringer
func ArrayOf(length int, elem Type) Type
func ChanOf(dir ChanDir, t Type) Type
func FuncOf(in, out []Type, variadic bool) Type
func MapOf(key, elem Type) Type
func PointerTo(t Type) Type
func PtrTo(t Type) Type
func SliceOf(t Type) Type
func StructOf(fields []StructField) Type
func TypeFor[T]() Type
func TypeOf(i any) Type
func Type.Elem() Type
func Type.In(i int) Type
func Type.Key() Type
func Type.Out(i int) Type
func Value.Type() Type
func database/sql.(*ColumnType).ScanType() Type
func database/sql/driver.RowsColumnTypeScanType.ColumnTypeScanType(index int) Type
func go101.org/nstd.Type[T]() Type
func go101.org/nstd.TypeOf[T](v T) Type
func ArenaNew(a *arena.Arena, typ Type) Value
func ArrayOf(length int, elem Type) Type
func ChanOf(dir ChanDir, t Type) Type
func FuncOf(in, out []Type, variadic bool) Type
func MakeChan(typ Type, buffer int) Value
func MakeFunc(typ Type, fn func(args []Value) (results []Value)) Value
func MakeMap(typ Type) Value
func MakeMapWithSize(typ Type, n int) Value
func MakeSlice(typ Type, len, cap int) Value
func MapOf(key, elem Type) Type
func New(typ Type) Value
func NewAt(typ Type, p unsafe.Pointer) Value
func PointerTo(t Type) Type
func PtrTo(t Type) Type
func SliceAt(typ Type, p unsafe.Pointer, n int) Value
func SliceOf(t Type) Type
func VisibleFields(t Type) []StructField
func Zero(typ Type) Value
func Type.AssignableTo(u Type) bool
func Type.ConvertibleTo(u Type) bool
func Type.Implements(u Type) bool
func Value.CanConvert(t Type) bool
func Value.Convert(t Type) Value
func internal/fuzz.CheckCorpus(vals []any, types []Type) error
func internal/fuzz.ReadCorpus(dir string, types []Type) ([]fuzz.CorpusEntry, error)
func testing/internal/testdeps.TestDeps.CheckCorpus(vals []any, types []Type) error
func testing/internal/testdeps.TestDeps.CoordinateFuzzing(timeout time.Duration, limit int64, minimizeTimeout time.Duration, minimizeLimit int64, parallel int, seed []fuzz.CorpusEntry, types []Type, corpusDir, cacheDir string) (err error)
func testing/internal/testdeps.TestDeps.ReadCorpus(dir string, types []Type) ([]fuzz.CorpusEntry, error)
func testing/quick.Value(t Type, rand *rand.Rand) (value Value, ok bool)
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. 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. Bool returns v's underlying value.
It panics if v's kind is not [Bool]. 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. 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. 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. 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. CanComplex reports whether [Value.Complex] can be used without panicking. 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. CanFloat reports whether [Value.Float] can be used without panicking. CanInt reports whether Int can be used without panicking. CanInterface reports whether [Value.Interface] can be used without panicking. 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. CanUint reports whether [Value.Uint] can be used without panicking. Cap returns v's capacity.
It panics if v's Kind is not [Array], [Chan], [Slice] or pointer to [Array]. 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]. Close closes the channel v.
It panics if v's Kind is not [Chan] or
v is a receive-only channel. 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. Complex returns v's underlying value, as a complex128.
It panics if v's Kind is not [Complex64] or [Complex128] 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. 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. 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. 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. 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. 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. 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]. 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. Float returns v's underlying value, as a float64.
It panics if v's Kind is not [Float32] or [Float64] 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. 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. Int returns v's underlying value, as an int64.
It panics if v's Kind is not [Int], [Int8], [Int16], [Int32], or [Int64]. 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. 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. 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. 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. IsZero reports whether v is the zero value for its type.
It panics if the argument is invalid. Kind returns v's Kind.
If v is the zero Value ([Value.IsValid] returns false), Kind returns Invalid. Len returns v's length.
It panics if v's Kind is not [Array], [Chan], [Map], [Slice], [String], or pointer to [Array]. 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. 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. 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()
...
} 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. 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. NumField returns the number of fields in the struct v.
It panics if v's Kind is not [Struct]. 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. OverflowComplex reports whether the complex128 x cannot be represented by v's type.
It panics if v's Kind is not [Complex64] or [Complex128]. OverflowFloat reports whether the float64 x cannot be represented by v's type.
It panics if v's Kind is not [Float32] or [Float64]. 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]. 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]. 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. 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. 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. 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. 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. 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. SetBool sets v's underlying value.
It panics if v's Kind is not [Bool] or if [Value.CanSet] returns false. SetBytes sets v's underlying value.
It panics if v's underlying value is not a slice of bytes. 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. 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. 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. 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. 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. 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. 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. 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. SetPointer sets the [unsafe.Pointer] value v to x.
It panics if v's Kind is not [UnsafePointer]. SetString sets v's underlying value to x.
It panics if v's Kind is not [String] or if [Value.CanSet] returns false. 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. SetZero sets v to be the zero value of v's type.
It panics if [Value.CanSet] returns false. 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. 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. 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. 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. 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. Type returns v's type. Uint returns v's underlying value, as a uint64.
It panics if v's Kind is not [Uint], [Uintptr], [Uint8], [Uint16], [Uint32], or [Uint64]. 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. 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.
Value : database/sql/driver.Validator
Value : expvar.Var
Value : fmt.Stringer
func Append(s Value, x ...Value) Value
func AppendSlice(s, t Value) Value
func ArenaNew(a *arena.Arena, typ Type) Value
func Indirect(v Value) Value
func MakeChan(typ Type, buffer int) Value
func MakeFunc(typ Type, fn func(args []Value) (results []Value)) Value
func MakeMap(typ Type) Value
func MakeMapWithSize(typ Type, n int) Value
func MakeSlice(typ Type, len, cap int) Value
func New(typ Type) Value
func NewAt(typ Type, p unsafe.Pointer) Value
func Select(cases []SelectCase) (chosen int, recv Value, recvOK bool)
func SliceAt(typ Type, p unsafe.Pointer, n int) Value
func ValueOf(i any) Value
func Zero(typ Type) Value
func (*MapIter).Key() Value
func (*MapIter).Value() Value
func Value.Addr() Value
func Value.Call(in []Value) []Value
func Value.CallSlice(in []Value) []Value
func Value.Convert(t Type) Value
func Value.Elem() Value
func Value.Field(i int) Value
func Value.FieldByIndex(index []int) Value
func Value.FieldByIndexErr(index []int) (Value, error)
func Value.FieldByName(name string) Value
func Value.FieldByNameFunc(match func(string) bool) Value
func Value.Index(i int) Value
func Value.MapIndex(key Value) Value
func Value.MapKeys() []Value
func Value.Method(i int) Value
func Value.MethodByName(name string) Value
func Value.Recv() (x Value, ok bool)
func Value.Slice(i, j int) Value
func Value.Slice3(i, j, k int) Value
func Value.TryRecv() (x Value, ok bool)
func go101.org/nstd.ValueOf[T](v T) Value
func testing/quick.Value(t Type, rand *rand.Rand) (value Value, ok bool)
func testing/quick.Generator.Generate(rand *rand.Rand, size int) Value
func Append(s Value, x ...Value) Value
func Append(s Value, x ...Value) Value
func AppendSlice(s, t Value) Value
func Copy(dst, src Value) int
func Indirect(v Value) Value
func (*MapIter).Reset(v Value)
func Value.Call(in []Value) []Value
func Value.CallSlice(in []Value) []Value
func Value.Equal(u Value) bool
func Value.MapIndex(key Value) Value
func Value.Send(x Value)
func Value.Set(x Value)
func Value.SetMapIndex(key, elem Value)
func Value.TrySend(x Value) bool
func encoding/gob.(*Decoder).DecodeValue(v Value) error
func encoding/gob.(*Encoder).EncodeValue(value Value) error
func go/ast.NotNilFilter(_ string, v Value) bool
func internal/fmtsort.Sort(mapValue Value) fmtsort.SortedMap
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.KindKindMethodstring(*ValueError) Error() string
*ValueError : error
Package-Level Functions (total 29)
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.
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.
ArenaNew returns a [Value] representing a pointer to a new zero value for the
specified type, allocating storage for it in the provided arena. That is,
the returned Value's Type is [PointerTo](typ).
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.
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.
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].
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.
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.
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.
MakeChan creates a new channel with the specified type and buffer size.
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.
MakeMap creates a new map with the specified type.
MakeMapWithSize creates a new map with the specified type
and initial space for approximately n elements.
MakeSlice creates a new zero-initialized slice value
for the specified slice type, length, and capacity.
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.
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).
NewAt returns a Value representing a pointer to a value of the
specified type, using p as that pointer.
PointerTo returns the pointer type with element t.
For example, if t represents type Foo, PointerTo(t) represents *Foo.
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].
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.
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].
SliceOf returns the slice type with element type t.
For example, if t represents int, SliceOf(t) represents []int.
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.
Swapper returns a function that swaps the elements in the provided
slice.
Swapper panics if the provided interface is not a slice.
Type Parameters:
T: any TypeFor returns the [Type] that represents the type argument T.
TypeOf returns the reflection [Type] that represents the dynamic type of i.
If i is a nil interface value, TypeOf returns nil.
ValueOf returns a new Value initialized to the concrete value
stored in the interface i. ValueOf(nil) returns the zero Value.
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).
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.
The pages are generated with Goldsv0.7.0-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
Please follow @zigo_101 (reachable from the left QR code) to get the latest news of Golds.
