在处理一些参数的时候，可能需要将参数转换为各种类型，这里实现一个通用的转换函数，实现各种类型之间的相互转换。
当然，如果源数据格式和目标数据类型不一致，是会返回错误的。例如将字符串“一二三”转换为数值类型则会报错，而将字符串“123”转换为数值类型则OK。
这段代码实际抄自go自带的“database/sql”库，只是源代码作为内部函数无法在外面调用，可以单独把需要的功能拎出来使用：
代码中有一个Scaner接口，可以自行实现，然后通过"convertAssign()"函数，作为dst参数传入。
package main
import (
"errors"
"fmt"
"reflect"
"strconv"
"time"
)
// RawBytes is a byte slice that holds a reference to memory owned by
// the database itself. After a Scan into a RawBytes, the slice is only
// valid until the next call to Next, Scan, or Close.
type RawBytes []byte
var errNilPtr = errors.New("destination pointer is nil") // embedded in descriptive error
// convertAssign 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.
func convertAssign(dest, src interface{}) error {
// 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 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
}
}
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 := 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 convertAssign(dv.Interface(), src)
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
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:
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:
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:
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
}
// Value is a value that drivers must be able to handle.
// It is either nil, a type handled by a database driver's NamedValueChecker
// interface, or an instance of one of these types:
//
// int64
// float64
// bool
// []byte
// string
// time.Time
type Value interface{}
type boolType struct{}
var Bool boolType
func (boolType) String() string { return "Bool" }
func (boolType) ConvertValue(src interface{}) (Value, error) {
switch s := src.(type) {
case bool:
return s, nil
case string:
b, err := strconv.ParseBool(s)
if err != nil {
return nil, fmt.Errorf("sql/driver: couldn't convert %q into type bool", s)
}
return b, nil
case []byte:
b, err := strconv.ParseBool(string(s))
if err != nil {
return nil, fmt.Errorf("sql/driver: couldn't convert %q into type bool", s)
}
return b, nil
}
sv := reflect.ValueOf(src)
switch sv.Kind() {
case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
iv := sv.Int()
if iv == 1 || iv == 0 {
return iv == 1, nil
}
return nil, fmt.Errorf("sql/driver: couldn't convert %d into type bool", iv)
case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
uv := sv.Uint()
if uv == 1 || uv == 0 {
return uv == 1, nil
}
return nil, fmt.Errorf("sql/driver: couldn't convert %d into type bool", uv)
}
return nil, fmt.Errorf("sql/driver: couldn't convert %v (%T) into type bool", src, src)
}
type Scanner interface {
// Scan assigns a value from a database driver.
//
// The src value will be of one of the following types:
//
// int64
// float64
// bool
// []byte
// string
// time.Time
// nil - for NULL values
//
// An error should be returned if the value cannot be stored
// without loss of information.
//
// Reference types such as []byte are only valid until the next call to Scan
// and should not be retained. Their underlying memory is owned by the driver.
// If retention is necessary, copy their values before the next call to Scan.
Scan(src interface{}) error
}
使用方法：
package main
import "log"
func main() {
var dst1 float64
var dst2 int
src := "123"
if err := convertAssign(&dst1, src); err != nil {
log.Fatalf("convert failed, %v", err)
}else {
log.Printf("convert ok: %f", dst1)
}
if err := convertAssign(&dst2, src); err != nil {
log.Fatalf("convert failed, %v", err)
}else {
log.Printf("convert ok: %d", dst2)
}
}
例子输出结果如下：
2019/02/15 16:59:39 convert ok: 123.000000
2019/02/15 16:59:39 convert ok: 123
怎么样，是不是很强大