Source: marshal.go in package encoding/xml


package xml

import (
	"bufio"
	"bytes"
	"encoding"
	"fmt"
	"io"
	"reflect"
	"strconv"
	"strings"
)

Header is a generic XML header suitable for use with the output of Marshal. This is not automatically added to any output of this package, it is provided as a convenience.

	Header = `<?xml version="1.0" encoding="UTF-8"?>` + "\n"
)

Marshal returns the XML encoding of v. Marshal handles an array or slice by marshaling each of the elements. Marshal handles a pointer by marshaling the value it points at or, if the pointer is nil, by writing nothing. Marshal handles an interface value by marshaling the value it contains or, if the interface value is nil, by writing nothing. Marshal handles all other data by writing one or more XML elements containing the data. The name for the XML elements is taken from, in order of preference: - the tag on the XMLName field, if the data is a struct - the value of the XMLName field of type Name - the tag of the struct field used to obtain the data - the name of the struct field used to obtain the data - the name of the marshaled type The XML element for a struct contains marshaled elements for each of the exported fields of the struct, with these exceptions: - the XMLName field, described above, is omitted. - a field with tag "-" is omitted. - a field with tag "name,attr" becomes an attribute with the given name in the XML element. - a field with tag ",attr" becomes an attribute with the field name in the XML element. - a field with tag ",chardata" is written as character data, not as an XML element. - a field with tag ",cdata" is written as character data wrapped in one or more <![CDATA[ ... ]]> tags, not as an XML element. - a field with tag ",innerxml" is written verbatim, not subject to the usual marshaling procedure. - a field with tag ",comment" is written as an XML comment, not subject to the usual marshaling procedure. It must not contain the "--" string within it. - a field with a tag including the "omitempty" option is omitted if the field value is empty. The empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero. - an anonymous struct field is handled as if the fields of its value were part of the outer struct. - a field implementing Marshaler is written by calling its MarshalXML method. - a field implementing encoding.TextMarshaler is written by encoding the result of its MarshalText method as text. If a field uses a tag "a>b>c", then the element c will be nested inside parent elements a and b. Fields that appear next to each other that name the same parent will be enclosed in one XML element. If the XML name for a struct field is defined by both the field tag and the struct's XMLName field, the names must match. See MarshalIndent for an example. Marshal will return an error if asked to marshal a channel, function, or map.

func Marshal(v interface{}) ([]byte, error) {
	var b bytes.Buffer
	if err := NewEncoder(&b).Encode(v); err != nil {
		return nil, err
	}
	return b.Bytes(), nil
}

Marshaler is the interface implemented by objects that can marshal themselves into valid XML elements. MarshalXML encodes the receiver as zero or more XML elements. By convention, arrays or slices are typically encoded as a sequence of elements, one per entry. Using start as the element tag is not required, but doing so will enable Unmarshal to match the XML elements to the correct struct field. One common implementation strategy is to construct a separate value with a layout corresponding to the desired XML and then to encode it using e.EncodeElement. Another common strategy is to use repeated calls to e.EncodeToken to generate the XML output one token at a time. The sequence of encoded tokens must make up zero or more valid XML elements.

type Marshaler interface {
	MarshalXML(e *Encoder, start StartElement) error
}

MarshalerAttr is the interface implemented by objects that can marshal themselves into valid XML attributes. MarshalXMLAttr returns an XML attribute with the encoded value of the receiver. Using name as the attribute name is not required, but doing so will enable Unmarshal to match the attribute to the correct struct field. If MarshalXMLAttr returns the zero attribute Attr{}, no attribute will be generated in the output. MarshalXMLAttr is used only for struct fields with the "attr" option in the field tag.

type MarshalerAttr interface {
	MarshalXMLAttr(name Name) (Attr, error)
}

MarshalIndent works like Marshal, but each XML element begins on a new indented line that starts with prefix and is followed by one or more copies of indent according to the nesting depth.

func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error) {
	var b bytes.Buffer
	enc := NewEncoder(&b)
	enc.Indent(prefix, indent)
	if err := enc.Encode(v); err != nil {
		return nil, err
	}
	return b.Bytes(), nil
}

An Encoder writes XML data to an output stream.

type Encoder struct {
	p printer
}

NewEncoder returns a new encoder that writes to w.

func NewEncoder(w io.Writer) *Encoder {
	e := &Encoder{printer{Writer: bufio.NewWriter(w)}}
	e.p.encoder = e
	return e
}

Indent sets the encoder to generate XML in which each element begins on a new indented line that starts with prefix and is followed by one or more copies of indent according to the nesting depth.

func (enc *Encoder) Indent(prefix, indent string) {
	enc.p.prefix = prefix
	enc.p.indent = indent
}

Encode writes the XML encoding of v to the stream. See the documentation for Marshal for details about the conversion of Go values to XML. Encode calls Flush before returning.

func (enc *Encoder) Encode(v interface{}) error {
	err := enc.p.marshalValue(reflect.ValueOf(v), nil, nil)
	if err != nil {
		return err
	}
	return enc.p.Flush()
}

EncodeElement writes the XML encoding of v to the stream, using start as the outermost tag in the encoding. See the documentation for Marshal for details about the conversion of Go values to XML. EncodeElement calls Flush before returning.

func (enc *Encoder) EncodeElement(v interface{}, start StartElement) error {
	err := enc.p.marshalValue(reflect.ValueOf(v), nil, &start)
	if err != nil {
		return err
	}
	return enc.p.Flush()
}

var (
	begComment  = []byte("<!--")
	endComment  = []byte("-->")
	endProcInst = []byte("?>")
)

EncodeToken writes the given XML token to the stream. It returns an error if StartElement and EndElement tokens are not properly matched. EncodeToken does not call Flush, because usually it is part of a larger operation such as Encode or EncodeElement (or a custom Marshaler's MarshalXML invoked during those), and those will call Flush when finished. Callers that create an Encoder and then invoke EncodeToken directly, without using Encode or EncodeElement, need to call Flush when finished to ensure that the XML is written to the underlying writer. EncodeToken allows writing a ProcInst with Target set to "xml" only as the first token in the stream.

func (enc *Encoder) EncodeToken(t Token) error {

	p := &enc.p
	switch t := t.(type) {
	case StartElement:
		if err := p.writeStart(&t); err != nil {
			return err
		}
	case EndElement:
		if err := p.writeEnd(t.Name); err != nil {
			return err
		}
	case CharData:
		escapeText(p, t, false)
	case Comment:
		if bytes.Contains(t, endComment) {
			return fmt.Errorf("xml: EncodeToken of Comment containing --> marker")
		}
		p.WriteString("<!--")
		p.Write(t)
		p.WriteString("-->")
		return p.cachedWriteError()

First token to be encoded which is also a ProcInst with target of xml is the xml declaration. The only ProcInst where target of xml is allowed.

		if t.Target == "xml" && p.Buffered() != 0 {
			return fmt.Errorf("xml: EncodeToken of ProcInst xml target only valid for xml declaration, first token encoded")
		}
		if !isNameString(t.Target) {
			return fmt.Errorf("xml: EncodeToken of ProcInst with invalid Target")
		}
		if bytes.Contains(t.Inst, endProcInst) {
			return fmt.Errorf("xml: EncodeToken of ProcInst containing ?> marker")
		}
		p.WriteString("<?")
		p.WriteString(t.Target)
		if len(t.Inst) > 0 {
			p.WriteByte(' ')
			p.Write(t.Inst)
		}
		p.WriteString("?>")
	case Directive:
		if !isValidDirective(t) {
			return fmt.Errorf("xml: EncodeToken of Directive containing wrong < or > markers")
		}
		p.WriteString("<!")
		p.Write(t)
		p.WriteString(">")
	default:
		return fmt.Errorf("xml: EncodeToken of invalid token type")

	}
	return p.cachedWriteError()
}

isValidDirective reports whether dir is a valid directive text, meaning angle brackets are matched, ignoring comments and strings.

func isValidDirective(dir Directive) bool {
	var (
		depth     int
		inquote   uint8
		incomment bool
	)
	for i, c := range dir {
		switch {
		case incomment:
			if c == '>' {
				if n := 1 + i - len(endComment); n >= 0 && bytes.Equal(dir[n:i+1], endComment) {
					incomment = false
				}

Just ignore anything in comment

		case inquote != 0:
			if c == inquote {
				inquote = 0

Just ignore anything within quotes

		case c == '\'' || c == '"':
			inquote = c
		case c == '<':
			if i+len(begComment) < len(dir) && bytes.Equal(dir[i:i+len(begComment)], begComment) {
				incomment = true
			} else {
				depth++
			}
		case c == '>':
			if depth == 0 {
				return false
			}
			depth--
		}
	}
	return depth == 0 && inquote == 0 && !incomment
}

Flush flushes any buffered XML to the underlying writer. See the EncodeToken documentation for details about when it is necessary.

func (enc *Encoder) Flush() error {
	return enc.p.Flush()
}

type printer struct {
	*bufio.Writer
	encoder    *Encoder
	seq        int
	indent     string
	prefix     string
	depth      int
	indentedIn bool
	putNewline bool
	attrNS     map[string]string // map prefix -> name space
	attrPrefix map[string]string // map name space -> prefix
	prefixes   []string
	tags       []Name
}

createAttrPrefix finds the name space prefix attribute to use for the given name space, defining a new prefix if necessary. It returns the prefix.

func (p *printer) createAttrPrefix(url string) string {
	if prefix := p.attrPrefix[url]; prefix != "" {
		return prefix
	}

The "http://www.w3.org/XML/1998/namespace" name space is predefined as "xml" and must be referred to that way. (The "http://www.w3.org/2000/xmlns/" name space is also predefined as "xmlns", but users should not be trying to use that one directly - that's our job.)

	if url == xmlURL {
		return xmlPrefix
	}

Need to define a new name space.

	if p.attrPrefix == nil {
		p.attrPrefix = make(map[string]string)
		p.attrNS = make(map[string]string)
	}

Pick a name. We try to use the final element of the path but fall back to _.

	prefix := strings.TrimRight(url, "/")
	if i := strings.LastIndex(prefix, "/"); i >= 0 {
		prefix = prefix[i+1:]
	}
	if prefix == "" || !isName([]byte(prefix)) || strings.Contains(prefix, ":") {
		prefix = "_"

xmlanything is reserved and any variant of it regardless of case should be matched, so: (('X'|'x') ('M'|'m') ('L'|'l')) See Section 2.3 of https://www.w3.org/TR/REC-xml/

	if len(prefix) >= 3 && strings.EqualFold(prefix[:3], "xml") {
		prefix = "_" + prefix
	}

Name is taken. Find a better one.

		for p.seq++; ; p.seq++ {
			if id := prefix + "_" + strconv.Itoa(p.seq); p.attrNS[id] == "" {
				prefix = id
				break
			}
		}
	}

	p.attrPrefix[url] = prefix
	p.attrNS[prefix] = url

	p.WriteString(`xmlns:`)
	p.WriteString(prefix)
	p.WriteString(`="`)
	EscapeText(p, []byte(url))
	p.WriteString(`" `)

	p.prefixes = append(p.prefixes, prefix)

	return prefix
}

deleteAttrPrefix removes an attribute name space prefix.

func (p *printer) deleteAttrPrefix(prefix string) {
	delete(p.attrPrefix, p.attrNS[prefix])
	delete(p.attrNS, prefix)
}

func (p *printer) markPrefix() {
	p.prefixes = append(p.prefixes, "")
}

func (p *printer) popPrefix() {
	for len(p.prefixes) > 0 {
		prefix := p.prefixes[len(p.prefixes)-1]
		p.prefixes = p.prefixes[:len(p.prefixes)-1]
		if prefix == "" {
			break
		}
		p.deleteAttrPrefix(prefix)
	}
}

var (
	marshalerType     = reflect.TypeOf((*Marshaler)(nil)).Elem()
	marshalerAttrType = reflect.TypeOf((*MarshalerAttr)(nil)).Elem()
	textMarshalerType = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
)

marshalValue writes one or more XML elements representing val. If val was obtained from a struct field, finfo must have its details.

func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplate *StartElement) error {
	if startTemplate != nil && startTemplate.Name.Local == "" {
		return fmt.Errorf("xml: EncodeElement of StartElement with missing name")
	}

	if !val.IsValid() {
		return nil
	}
	if finfo != nil && finfo.flags&fOmitEmpty != 0 && isEmptyValue(val) {
		return nil
	}

Drill into interfaces and pointers. This can turn into an infinite loop given a cyclic chain, but it matches the Go 1 behavior.

	for val.Kind() == reflect.Interface || val.Kind() == reflect.Ptr {
		if val.IsNil() {
			return nil
		}
		val = val.Elem()
	}

	kind := val.Kind()
	typ := val.Type()

Check for marshaler.

	if val.CanInterface() && typ.Implements(marshalerType) {
		return p.marshalInterface(val.Interface().(Marshaler), defaultStart(typ, finfo, startTemplate))
	}
	if val.CanAddr() {
		pv := val.Addr()
		if pv.CanInterface() && pv.Type().Implements(marshalerType) {
			return p.marshalInterface(pv.Interface().(Marshaler), defaultStart(pv.Type(), finfo, startTemplate))
		}
	}

Check for text marshaler.

	if val.CanInterface() && typ.Implements(textMarshalerType) {
		return p.marshalTextInterface(val.Interface().(encoding.TextMarshaler), defaultStart(typ, finfo, startTemplate))
	}
	if val.CanAddr() {
		pv := val.Addr()
		if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
			return p.marshalTextInterface(pv.Interface().(encoding.TextMarshaler), defaultStart(pv.Type(), finfo, startTemplate))
		}
	}

Slices and arrays iterate over the elements. They do not have an enclosing tag.

	if (kind == reflect.Slice || kind == reflect.Array) && typ.Elem().Kind() != reflect.Uint8 {
		for i, n := 0, val.Len(); i < n; i++ {
			if err := p.marshalValue(val.Index(i), finfo, startTemplate); err != nil {
				return err
			}
		}
		return nil
	}

	tinfo, err := getTypeInfo(typ)
	if err != nil {
		return err
	}

Create start element. Precedence for the XML element name is: 0. startTemplate 1. XMLName field in underlying struct; 2. field name/tag in the struct field; and 3. type name

	var start StartElement

	if startTemplate != nil {
		start.Name = startTemplate.Name
		start.Attr = append(start.Attr, startTemplate.Attr...)
	} else if tinfo.xmlname != nil {
		xmlname := tinfo.xmlname
		if xmlname.name != "" {
			start.Name.Space, start.Name.Local = xmlname.xmlns, xmlname.name
		} else {
			fv := xmlname.value(val, dontInitNilPointers)
			if v, ok := fv.Interface().(Name); ok && v.Local != "" {
				start.Name = v
			}
		}
	}
	if start.Name.Local == "" && finfo != nil {
		start.Name.Space, start.Name.Local = finfo.xmlns, finfo.name
	}
	if start.Name.Local == "" {
		name := typ.Name()
		if name == "" {
			return &UnsupportedTypeError{typ}
		}
		start.Name.Local = name
	}

Attributes

	for i := range tinfo.fields {
		finfo := &tinfo.fields[i]
		if finfo.flags&fAttr == 0 {
			continue
		}
		fv := finfo.value(val, dontInitNilPointers)

		if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
			continue
		}

		if fv.Kind() == reflect.Interface && fv.IsNil() {
			continue
		}

		name := Name{Space: finfo.xmlns, Local: finfo.name}
		if err := p.marshalAttr(&start, name, fv); err != nil {
			return err
		}
	}

	if err := p.writeStart(&start); err != nil {
		return err
	}

	if val.Kind() == reflect.Struct {
		err = p.marshalStruct(tinfo, val)
	} else {
		s, b, err1 := p.marshalSimple(typ, val)
		if err1 != nil {
			err = err1
		} else if b != nil {
			EscapeText(p, b)
		} else {
			p.EscapeString(s)
		}
	}
	if err != nil {
		return err
	}

	if err := p.writeEnd(start.Name); err != nil {
		return err
	}

	return p.cachedWriteError()
}

marshalAttr marshals an attribute with the given name and value, adding to start.Attr.

func (p *printer) marshalAttr(start *StartElement, name Name, val reflect.Value) error {
	if val.CanInterface() && val.Type().Implements(marshalerAttrType) {
		attr, err := val.Interface().(MarshalerAttr).MarshalXMLAttr(name)
		if err != nil {
			return err
		}
		if attr.Name.Local != "" {
			start.Attr = append(start.Attr, attr)
		}
		return nil
	}

	if val.CanAddr() {
		pv := val.Addr()
		if pv.CanInterface() && pv.Type().Implements(marshalerAttrType) {
			attr, err := pv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
			if err != nil {
				return err
			}
			if attr.Name.Local != "" {
				start.Attr = append(start.Attr, attr)
			}
			return nil
		}
	}

	if val.CanInterface() && val.Type().Implements(textMarshalerType) {
		text, err := val.Interface().(encoding.TextMarshaler).MarshalText()
		if err != nil {
			return err
		}
		start.Attr = append(start.Attr, Attr{name, string(text)})
		return nil
	}

	if val.CanAddr() {
		pv := val.Addr()
		if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
			text, err := pv.Interface().(encoding.TextMarshaler).MarshalText()
			if err != nil {
				return err
			}
			start.Attr = append(start.Attr, Attr{name, string(text)})
			return nil
		}
	}

Dereference or skip nil pointer, interface values.

	switch val.Kind() {
	case reflect.Ptr, reflect.Interface:
		if val.IsNil() {
			return nil
		}
		val = val.Elem()
	}

Walk slices.

	if val.Kind() == reflect.Slice && val.Type().Elem().Kind() != reflect.Uint8 {
		n := val.Len()
		for i := 0; i < n; i++ {
			if err := p.marshalAttr(start, name, val.Index(i)); err != nil {
				return err
			}
		}
		return nil
	}

	if val.Type() == attrType {
		start.Attr = append(start.Attr, val.Interface().(Attr))
		return nil
	}

	s, b, err := p.marshalSimple(val.Type(), val)
	if err != nil {
		return err
	}
	if b != nil {
		s = string(b)
	}
	start.Attr = append(start.Attr, Attr{name, s})
	return nil
}

defaultStart returns the default start element to use, given the reflect type, field info, and start template.

func defaultStart(typ reflect.Type, finfo *fieldInfo, startTemplate *StartElement) StartElement {

Precedence for the XML element name is as above, except that we do not look inside structs for the first field.

	if startTemplate != nil {
		start.Name = startTemplate.Name
		start.Attr = append(start.Attr, startTemplate.Attr...)
	} else if finfo != nil && finfo.name != "" {
		start.Name.Local = finfo.name
		start.Name.Space = finfo.xmlns
	} else if typ.Name() != "" {
		start.Name.Local = typ.Name()

Must be a pointer to a named type, since it has the Marshaler methods.

		start.Name.Local = typ.Elem().Name()
	}
	return start
}

marshalInterface marshals a Marshaler interface value.

Push a marker onto the tag stack so that MarshalXML cannot close the XML tags that it did not open.

	p.tags = append(p.tags, Name{})
	n := len(p.tags)

	err := val.MarshalXML(p.encoder, start)
	if err != nil {
		return err
	}

Make sure MarshalXML closed all its tags. p.tags[n-1] is the mark.

	if len(p.tags) > n {
		return fmt.Errorf("xml: %s.MarshalXML wrote invalid XML: <%s> not closed", receiverType(val), p.tags[len(p.tags)-1].Local)
	}
	p.tags = p.tags[:n-1]
	return nil
}

marshalTextInterface marshals a TextMarshaler interface value.

func (p *printer) marshalTextInterface(val encoding.TextMarshaler, start StartElement) error {
	if err := p.writeStart(&start); err != nil {
		return err
	}
	text, err := val.MarshalText()
	if err != nil {
		return err
	}
	EscapeText(p, text)
	return p.writeEnd(start.Name)
}

writeStart writes the given start element.

func (p *printer) writeStart(start *StartElement) error {
	if start.Name.Local == "" {
		return fmt.Errorf("xml: start tag with no name")
	}

	p.tags = append(p.tags, start.Name)
	p.markPrefix()

	p.writeIndent(1)
	p.WriteByte('<')
	p.WriteString(start.Name.Local)

	if start.Name.Space != "" {
		p.WriteString(` xmlns="`)
		p.EscapeString(start.Name.Space)
		p.WriteByte('"')
	}

Attributes

	for _, attr := range start.Attr {
		name := attr.Name
		if name.Local == "" {
			continue
		}
		p.WriteByte(' ')
		if name.Space != "" {
			p.WriteString(p.createAttrPrefix(name.Space))
			p.WriteByte(':')
		}
		p.WriteString(name.Local)
		p.WriteString(`="`)
		p.EscapeString(attr.Value)
		p.WriteByte('"')
	}
	p.WriteByte('>')
	return nil
}

func (p *printer) writeEnd(name Name) error {
	if name.Local == "" {
		return fmt.Errorf("xml: end tag with no name")
	}
	if len(p.tags) == 0 || p.tags[len(p.tags)-1].Local == "" {
		return fmt.Errorf("xml: end tag </%s> without start tag", name.Local)
	}
	if top := p.tags[len(p.tags)-1]; top != name {
		if top.Local != name.Local {
			return fmt.Errorf("xml: end tag </%s> does not match start tag <%s>", name.Local, top.Local)
		}
		return fmt.Errorf("xml: end tag </%s> in namespace %s does not match start tag <%s> in namespace %s", name.Local, name.Space, top.Local, top.Space)
	}
	p.tags = p.tags[:len(p.tags)-1]

	p.writeIndent(-1)
	p.WriteByte('<')
	p.WriteByte('/')
	p.WriteString(name.Local)
	p.WriteByte('>')
	p.popPrefix()
	return nil
}

func (p *printer) marshalSimple(typ reflect.Type, val reflect.Value) (string, []byte, error) {
	switch val.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return strconv.FormatInt(val.Int(), 10), nil, nil
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		return strconv.FormatUint(val.Uint(), 10), nil, nil
	case reflect.Float32, reflect.Float64:
		return strconv.FormatFloat(val.Float(), 'g', -1, val.Type().Bits()), nil, nil
	case reflect.String:
		return val.String(), nil, nil
	case reflect.Bool:
		return strconv.FormatBool(val.Bool()), nil, nil
	case reflect.Array:
		if typ.Elem().Kind() != reflect.Uint8 {
			break

[...]byte

		var bytes []byte
		if val.CanAddr() {
			bytes = val.Slice(0, val.Len()).Bytes()
		} else {
			bytes = make([]byte, val.Len())
			reflect.Copy(reflect.ValueOf(bytes), val)
		}
		return "", bytes, nil
	case reflect.Slice:
		if typ.Elem().Kind() != reflect.Uint8 {
			break

[]byte

		return "", val.Bytes(), nil
	}
	return "", nil, &UnsupportedTypeError{typ}
}

var ddBytes = []byte("--")

indirect drills into interfaces and pointers, returning the pointed-at value. If it encounters a nil interface or pointer, indirect returns that nil value. This can turn into an infinite loop given a cyclic chain, but it matches the Go 1 behavior.

func indirect(vf reflect.Value) reflect.Value {
	for vf.Kind() == reflect.Interface || vf.Kind() == reflect.Ptr {
		if vf.IsNil() {
			return vf
		}
		vf = vf.Elem()
	}
	return vf
}

func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
	s := parentStack{p: p}
	for i := range tinfo.fields {
		finfo := &tinfo.fields[i]
		if finfo.flags&fAttr != 0 {
			continue
		}
		vf := finfo.value(val, dontInitNilPointers)

The field is behind an anonymous struct field that's nil. Skip it.

			continue
		}

		switch finfo.flags & fMode {
		case fCDATA, fCharData:
			emit := EscapeText
			if finfo.flags&fMode == fCDATA {
				emit = emitCDATA
			}
			if err := s.trim(finfo.parents); err != nil {
				return err
			}
			if vf.CanInterface() && vf.Type().Implements(textMarshalerType) {
				data, err := vf.Interface().(encoding.TextMarshaler).MarshalText()
				if err != nil {
					return err
				}
				if err := emit(p, data); err != nil {
					return err
				}
				continue
			}
			if vf.CanAddr() {
				pv := vf.Addr()
				if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
					data, err := pv.Interface().(encoding.TextMarshaler).MarshalText()
					if err != nil {
						return err
					}
					if err := emit(p, data); err != nil {
						return err
					}
					continue
				}
			}

			var scratch [64]byte
			vf = indirect(vf)
			switch vf.Kind() {
			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
				if err := emit(p, strconv.AppendInt(scratch[:0], vf.Int(), 10)); err != nil {
					return err
				}
			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
				if err := emit(p, strconv.AppendUint(scratch[:0], vf.Uint(), 10)); err != nil {
					return err
				}
			case reflect.Float32, reflect.Float64:
				if err := emit(p, strconv.AppendFloat(scratch[:0], vf.Float(), 'g', -1, vf.Type().Bits())); err != nil {
					return err
				}
			case reflect.Bool:
				if err := emit(p, strconv.AppendBool(scratch[:0], vf.Bool())); err != nil {
					return err
				}
			case reflect.String:
				if err := emit(p, []byte(vf.String())); err != nil {
					return err
				}
			case reflect.Slice:
				if elem, ok := vf.Interface().([]byte); ok {
					if err := emit(p, elem); err != nil {
						return err
					}
				}
			}
			continue

		case fComment:
			if err := s.trim(finfo.parents); err != nil {
				return err
			}
			vf = indirect(vf)
			k := vf.Kind()
			if !(k == reflect.String || k == reflect.Slice && vf.Type().Elem().Kind() == reflect.Uint8) {
				return fmt.Errorf("xml: bad type for comment field of %s", val.Type())
			}
			if vf.Len() == 0 {
				continue
			}
			p.writeIndent(0)
			p.WriteString("<!--")
			dashDash := false
			dashLast := false
			switch k {
			case reflect.String:
				s := vf.String()
				dashDash = strings.Contains(s, "--")
				dashLast = s[len(s)-1] == '-'
				if !dashDash {
					p.WriteString(s)
				}
			case reflect.Slice:
				b := vf.Bytes()
				dashDash = bytes.Contains(b, ddBytes)
				dashLast = b[len(b)-1] == '-'
				if !dashDash {
					p.Write(b)
				}
			default:
				panic("can't happen")
			}
			if dashDash {
				return fmt.Errorf(`xml: comments must not contain "--"`)
			}

"--->" is invalid grammar. Make it "- -->"

				p.WriteByte(' ')
			}
			p.WriteString("-->")
			continue

		case fInnerXML:
			vf = indirect(vf)
			iface := vf.Interface()
			switch raw := iface.(type) {
			case []byte:
				p.Write(raw)
				continue
			case string:
				p.WriteString(raw)
				continue
			}

		case fElement, fElement | fAny:
			if err := s.trim(finfo.parents); err != nil {
				return err
			}
			if len(finfo.parents) > len(s.stack) {
				if vf.Kind() != reflect.Ptr && vf.Kind() != reflect.Interface || !vf.IsNil() {
					if err := s.push(finfo.parents[len(s.stack):]); err != nil {
						return err
					}
				}
			}
		}
		if err := p.marshalValue(vf, finfo, nil); err != nil {
			return err
		}
	}
	s.trim(nil)
	return p.cachedWriteError()
}

return the bufio Writer's cached write error

func (p *printer) cachedWriteError() error {
	_, err := p.Write(nil)
	return err
}

func (p *printer) writeIndent(depthDelta int) {
	if len(p.prefix) == 0 && len(p.indent) == 0 {
		return
	}
	if depthDelta < 0 {
		p.depth--
		if p.indentedIn {
			p.indentedIn = false
			return
		}
		p.indentedIn = false
	}
	if p.putNewline {
		p.WriteByte('\n')
	} else {
		p.putNewline = true
	}
	if len(p.prefix) > 0 {
		p.WriteString(p.prefix)
	}
	if len(p.indent) > 0 {
		for i := 0; i < p.depth; i++ {
			p.WriteString(p.indent)
		}
	}
	if depthDelta > 0 {
		p.depth++
		p.indentedIn = true
	}
}

type parentStack struct {
	p     *printer
	stack []string
}

trim updates the XML context to match the longest common prefix of the stack and the given parents. A closing tag will be written for every parent popped. Passing a zero slice or nil will close all the elements.

func (s *parentStack) trim(parents []string) error {
	split := 0
	for ; split < len(parents) && split < len(s.stack); split++ {
		if parents[split] != s.stack[split] {
			break
		}
	}
	for i := len(s.stack) - 1; i >= split; i-- {
		if err := s.p.writeEnd(Name{Local: s.stack[i]}); err != nil {
			return err
		}
	}
	s.stack = s.stack[:split]
	return nil
}

push adds parent elements to the stack and writes open tags.

func (s *parentStack) push(parents []string) error {
	for i := 0; i < len(parents); i++ {
		if err := s.p.writeStart(&StartElement{Name: Name{Local: parents[i]}}); err != nil {
			return err
		}
	}
	s.stack = append(s.stack, parents...)
	return nil
}

UnsupportedTypeError is returned when Marshal encounters a type that cannot be converted into XML.

type UnsupportedTypeError struct {
	Type reflect.Type
}

func (e *UnsupportedTypeError) Error() string {
	return "xml: unsupported type: " + e.Type.String()
}

func isEmptyValue(v reflect.Value) bool {
	switch v.Kind() {
	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
		return v.Len() == 0
	case reflect.Bool:
		return !v.Bool()
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return v.Int() == 0
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		return v.Uint() == 0
	case reflect.Float32, reflect.Float64:
		return v.Float() == 0
	case reflect.Interface, reflect.Ptr:
		return v.IsNil()
	}
	return false