Source: convert.go in package database/sql

Type conversions for Scan.


package sql

import (
	"database/sql/driver"
	"errors"
	"fmt"
	"reflect"
	"strconv"
	"time"
	"unicode"
	"unicode/utf8"
)

var errNilPtr = errors.New("destination pointer is nil") // embedded in descriptive error

func describeNamedValue(nv *driver.NamedValue) string {
	if len(nv.Name) == 0 {
		return fmt.Sprintf("$%d", nv.Ordinal)
	}
	return fmt.Sprintf("with name %q", nv.Name)
}

func validateNamedValueName(name string) error {
	if len(name) == 0 {
		return nil
	}
	r, _ := utf8.DecodeRuneInString(name)
	if unicode.IsLetter(r) {
		return nil
	}
	return fmt.Errorf("name %q does not begin with a letter", name)
}

ccChecker wraps the driver.ColumnConverter and allows it to be used as if it were a NamedValueChecker. If the driver ColumnConverter is not present then the NamedValueChecker will return driver.ErrSkip.

type ccChecker struct {
	cci  driver.ColumnConverter
	want int
}

func (c ccChecker) CheckNamedValue(nv *driver.NamedValue) error {
	if c.cci == nil {
		return driver.ErrSkip

The column converter shouldn't be called on any index it isn't expecting. The final error will be thrown in the argument converter loop.

	index := nv.Ordinal - 1
	if c.want <= index {
		return nil
	}

First, see if the value itself knows how to convert itself to a driver type. For example, a NullString struct changing into a string or nil.

	if vr, ok := nv.Value.(driver.Valuer); ok {
		sv, err := callValuerValue(vr)
		if err != nil {
			return err
		}
		if !driver.IsValue(sv) {
			return fmt.Errorf("non-subset type %T returned from Value", sv)
		}
		nv.Value = sv
	}

Second, ask the column to sanity check itself. For example, drivers might use this to make sure that an int64 values being inserted into a 16-bit integer field is in range (before getting truncated), or that a nil can't go into a NOT NULL column before going across the network to get the same error.

	var err error
	arg := nv.Value
	nv.Value, err = c.cci.ColumnConverter(index).ConvertValue(arg)
	if err != nil {
		return err
	}
	if !driver.IsValue(nv.Value) {
		return fmt.Errorf("driver ColumnConverter error converted %T to unsupported type %T", arg, nv.Value)
	}
	return nil
}

defaultCheckNamedValue wraps the default ColumnConverter to have the same function signature as the CheckNamedValue in the driver.NamedValueChecker interface.

func defaultCheckNamedValue(nv *driver.NamedValue) (err error) {
	nv.Value, err = driver.DefaultParameterConverter.ConvertValue(nv.Value)
	return err
}

driverArgsConnLocked converts arguments from callers of Stmt.Exec and Stmt.Query into driver Values. The statement ds may be nil, if no statement is available. ci must be locked.

func driverArgsConnLocked(ci driver.Conn, ds *driverStmt, args []interface{}) ([]driver.NamedValue, error) {
	nvargs := make([]driver.NamedValue, len(args))

-1 means the driver doesn't know how to count the number of placeholders, so we won't sanity check input here and instead let the driver deal with errors.

	want := -1

	var si driver.Stmt
	var cc ccChecker
	if ds != nil {
		si = ds.si
		want = ds.si.NumInput()
		cc.want = want
	}

Check all types of interfaces from the start. Drivers may opt to use the NamedValueChecker for special argument types, then return driver.ErrSkip to pass it along to the column converter.

	nvc, ok := si.(driver.NamedValueChecker)
	if !ok {
		nvc, ok = ci.(driver.NamedValueChecker)
	}
	cci, ok := si.(driver.ColumnConverter)
	if ok {
		cc.cci = cci
	}

Loop through all the arguments, checking each one. If no error is returned simply increment the index and continue. However if driver.ErrRemoveArgument is returned the argument is not included in the query argument list.

	var err error
	var n int
	for _, arg := range args {
		nv := &nvargs[n]
		if np, ok := arg.(NamedArg); ok {
			if err = validateNamedValueName(np.Name); err != nil {
				return nil, err
			}
			arg = np.Value
			nv.Name = np.Name
		}
		nv.Ordinal = n + 1
		nv.Value = arg

Checking sequence has four routes: A: 1. Default B: 1. NamedValueChecker 2. Column Converter 3. Default C: 1. NamedValueChecker 3. Default D: 1. Column Converter 2. Default The only time a Column Converter is called is first or after NamedValueConverter. If first it is handled before the nextCheck label. Thus for repeats tries only when the NamedValueConverter is selected should the Column Converter be used in the retry.

		checker := defaultCheckNamedValue
		nextCC := false
		switch {
		case nvc != nil:
			nextCC = cci != nil
			checker = nvc.CheckNamedValue
		case cci != nil:
			checker = cc.CheckNamedValue
		}

	nextCheck:
		err = checker(nv)
		switch err {
		case nil:
			n++
			continue
		case driver.ErrRemoveArgument:
			nvargs = nvargs[:len(nvargs)-1]
			continue
		case driver.ErrSkip:
			if nextCC {
				nextCC = false
				checker = cc.CheckNamedValue
			} else {
				checker = defaultCheckNamedValue
			}
			goto nextCheck
		default:
			return nil, fmt.Errorf("sql: converting argument %s type: %v", describeNamedValue(nv), err)
		}
	}

Check the length of arguments after conversion to allow for omitted arguments.

	if want != -1 && len(nvargs) != want {
		return nil, fmt.Errorf("sql: expected %d arguments, got %d", want, len(nvargs))
	}

	return nvargs, nil

}

convertAssign is the same as convertAssignRows, but without the optional rows argument.

func convertAssign(dest, src interface{}) error {
	return convertAssignRows(dest, src, nil)
}

convertAssignRows copies to dest the value in src, converting it if possible. An error is returned if the copy would result in loss of information. dest should be a pointer type. If rows is passed in, the rows will be used as the parent for any cursor values converted from a driver.Rows to a *Rows.

Common cases, without reflect.

	switch s := src.(type) {
	case string:
		switch d := dest.(type) {
		case *string:
			if d == nil {
				return errNilPtr
			}
			*d = s
			return nil
		case *[]byte:
			if d == nil {
				return errNilPtr
			}
			*d = []byte(s)
			return nil
		case *RawBytes:
			if d == nil {
				return errNilPtr
			}
			*d = append((*d)[:0], s...)
			return nil
		}
	case []byte:
		switch d := dest.(type) {
		case *string:
			if d == nil {
				return errNilPtr
			}
			*d = string(s)
			return nil
		case *interface{}:
			if d == nil {
				return errNilPtr
			}
			*d = cloneBytes(s)
			return nil
		case *[]byte:
			if d == nil {
				return errNilPtr
			}
			*d = cloneBytes(s)
			return nil
		case *RawBytes:
			if d == nil {
				return errNilPtr
			}
			*d = s
			return nil
		}
	case time.Time:
		switch d := dest.(type) {
		case *time.Time:
			*d = s
			return nil
		case *string:
			*d = s.Format(time.RFC3339Nano)
			return nil
		case *[]byte:
			if d == nil {
				return errNilPtr
			}
			*d = []byte(s.Format(time.RFC3339Nano))
			return nil
		case *RawBytes:
			if d == nil {
				return errNilPtr
			}
			*d = s.AppendFormat((*d)[:0], time.RFC3339Nano)
			return nil
		}
	case decimalDecompose:
		switch d := dest.(type) {
		case decimalCompose:
			return d.Compose(s.Decompose(nil))
		}
	case nil:
		switch d := dest.(type) {
		case *interface{}:
			if d == nil {
				return errNilPtr
			}
			*d = nil
			return nil
		case *[]byte:
			if d == nil {
				return errNilPtr
			}
			*d = nil
			return nil
		case *RawBytes:
			if d == nil {
				return errNilPtr
			}
			*d = nil
			return nil

The driver is returning a cursor the client may iterate over.

	case driver.Rows:
		switch d := dest.(type) {
		case *Rows:
			if d == nil {
				return errNilPtr
			}
			if rows == nil {
				return errors.New("invalid context to convert cursor rows, missing parent *Rows")
			}
			rows.closemu.Lock()
			*d = Rows{
				dc:          rows.dc,
				releaseConn: func(error) {},
				rowsi:       s,

Chain the cancel function.

			parentCancel := rows.cancel

When Rows.cancel is called, the closemu will be locked as well. So we can access rs.lasterr.

				d.close(rows.lasterr)
				if parentCancel != nil {
					parentCancel()
				}
			}
			rows.closemu.Unlock()
			return nil
		}
	}

	var sv reflect.Value

	switch d := dest.(type) {
	case *string:
		sv = reflect.ValueOf(src)
		switch sv.Kind() {
		case reflect.Bool,
			reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
			reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64,
			reflect.Float32, reflect.Float64:
			*d = asString(src)
			return nil
		}
	case *[]byte:
		sv = reflect.ValueOf(src)
		if b, ok := asBytes(nil, sv); ok {
			*d = b
			return nil
		}
	case *RawBytes:
		sv = reflect.ValueOf(src)
		if b, ok := asBytes([]byte(*d)[:0], sv); ok {
			*d = RawBytes(b)
			return nil
		}
	case *bool:
		bv, err := driver.Bool.ConvertValue(src)
		if err == nil {
			*d = bv.(bool)
		}
		return err
	case *interface{}:
		*d = src
		return nil
	}

	if scanner, ok := dest.(Scanner); ok {
		return scanner.Scan(src)
	}

	dpv := reflect.ValueOf(dest)
	if dpv.Kind() != reflect.Ptr {
		return errors.New("destination not a pointer")
	}
	if dpv.IsNil() {
		return errNilPtr
	}

	if !sv.IsValid() {
		sv = reflect.ValueOf(src)
	}

	dv := reflect.Indirect(dpv)
	if sv.IsValid() && sv.Type().AssignableTo(dv.Type()) {
		switch b := src.(type) {
		case []byte:
			dv.Set(reflect.ValueOf(cloneBytes(b)))
		default:
			dv.Set(sv)
		}
		return nil
	}

	if dv.Kind() == sv.Kind() && sv.Type().ConvertibleTo(dv.Type()) {
		dv.Set(sv.Convert(dv.Type()))
		return nil
	}

The following conversions use a string value as an intermediate representation to convert between various numeric types. This also allows scanning into user defined types such as "type Int int64". For symmetry, also check for string destination types.

	switch dv.Kind() {
	case reflect.Ptr:
		if src == nil {
			dv.Set(reflect.Zero(dv.Type()))
			return nil
		}
		dv.Set(reflect.New(dv.Type().Elem()))
		return convertAssignRows(dv.Interface(), src, rows)
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		if src == nil {
			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
		}
		s := asString(src)
		i64, err := strconv.ParseInt(s, 10, dv.Type().Bits())
		if err != nil {
			err = strconvErr(err)
			return fmt.Errorf("converting driver.Value type %T (%q) to a %s: %v", src, s, dv.Kind(), err)
		}
		dv.SetInt(i64)
		return nil
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		if src == nil {
			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
		}
		s := asString(src)
		u64, err := strconv.ParseUint(s, 10, dv.Type().Bits())
		if err != nil {
			err = strconvErr(err)
			return fmt.Errorf("converting driver.Value type %T (%q) to a %s: %v", src, s, dv.Kind(), err)
		}
		dv.SetUint(u64)
		return nil
	case reflect.Float32, reflect.Float64:
		if src == nil {
			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
		}
		s := asString(src)
		f64, err := strconv.ParseFloat(s, dv.Type().Bits())
		if err != nil {
			err = strconvErr(err)
			return fmt.Errorf("converting driver.Value type %T (%q) to a %s: %v", src, s, dv.Kind(), err)
		}
		dv.SetFloat(f64)
		return nil
	case reflect.String:
		if src == nil {
			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
		}
		switch v := src.(type) {
		case string:
			dv.SetString(v)
			return nil
		case []byte:
			dv.SetString(string(v))
			return nil
		}
	}

	return fmt.Errorf("unsupported Scan, storing driver.Value type %T into type %T", src, dest)
}

func strconvErr(err error) error {
	if ne, ok := err.(*strconv.NumError); ok {
		return ne.Err
	}
	return err
}

func cloneBytes(b []byte) []byte {
	if b == nil {
		return nil
	}
	c := make([]byte, len(b))
	copy(c, b)
	return c
}

func asString(src interface{}) string {
	switch v := src.(type) {
	case string:
		return v
	case []byte:
		return string(v)
	}
	rv := reflect.ValueOf(src)
	switch rv.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return strconv.FormatInt(rv.Int(), 10)
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		return strconv.FormatUint(rv.Uint(), 10)
	case reflect.Float64:
		return strconv.FormatFloat(rv.Float(), 'g', -1, 64)
	case reflect.Float32:
		return strconv.FormatFloat(rv.Float(), 'g', -1, 32)
	case reflect.Bool:
		return strconv.FormatBool(rv.Bool())
	}
	return fmt.Sprintf("%v", src)
}

func asBytes(buf []byte, rv reflect.Value) (b []byte, ok bool) {
	switch rv.Kind() {
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return strconv.AppendInt(buf, rv.Int(), 10), true
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		return strconv.AppendUint(buf, rv.Uint(), 10), true
	case reflect.Float32:
		return strconv.AppendFloat(buf, rv.Float(), 'g', -1, 32), true
	case reflect.Float64:
		return strconv.AppendFloat(buf, rv.Float(), 'g', -1, 64), true
	case reflect.Bool:
		return strconv.AppendBool(buf, rv.Bool()), true
	case reflect.String:
		s := rv.String()
		return append(buf, s...), true
	}
	return
}

var valuerReflectType = reflect.TypeOf((*driver.Valuer)(nil)).Elem()

callValuerValue returns vr.Value(), with one exception: If vr.Value is an auto-generated method on a pointer type and the pointer is nil, it would panic at runtime in the panicwrap method. Treat it like nil instead. Issue 8415. This is so people can implement driver.Value on value types and still use nil pointers to those types to mean nil/NULL, just like stringstring. This function is mirrored in the database/sql/driver package.

func callValuerValue(vr driver.Valuer) (v driver.Value, err error) {
	if rv := reflect.ValueOf(vr); rv.Kind() == reflect.Ptr &&
		rv.IsNil() &&
		rv.Type().Elem().Implements(valuerReflectType) {
		return nil, nil
	}
	return vr.Value()
}

decimal composes or decomposes a decimal value to and from individual parts. There are four parts: a boolean negative flag, a form byte with three possible states (finite=0, infinite=1, NaN=2), a base-2 big-endian integer coefficient (also known as a significand) as a []byte, and an int32 exponent. These are composed into a final value as "decimal = (neg) (form=finite) coefficient * 10 ^ exponent". A zero length coefficient is a zero value. The big-endian integer coefficient stores the most significant byte first (at coefficient[0]). If the form is not finite the coefficient and exponent should be ignored. The negative parameter may be set to true for any form, although implementations are not required to respect the negative parameter in the non-finite form. Implementations may choose to set the negative parameter to true on a zero or NaN value, but implementations that do not differentiate between negative and positive zero or NaN values should ignore the negative parameter without error. If an implementation does not support Infinity it may be converted into a NaN without error. If a value is set that is larger than what is supported by an implementation, an error must be returned. Implementations must return an error if a NaN or Infinity is attempted to be set while neither are supported. NOTE(kardianos): This is an experimental interface. See https://golang.org/issue/30870

type decimal interface {
	decimalDecompose
	decimalCompose
}

Decompose returns the internal decimal state in parts. If the provided buf has sufficient capacity, buf may be returned as the coefficient with the value set and length set as appropriate.

	Decompose(buf []byte) (form byte, negative bool, coefficient []byte, exponent int32)
}

Compose sets the internal decimal value from parts. If the value cannot be represented then an error should be returned.

	Compose(form byte, negative bool, coefficient []byte, exponent int32) error