Runtime Struct: 运行时结构体构造方法

参考:

https://github.com/itsubaki/gostruct

https://pkg.go.dev/reflect#example-StructOf

reflect.New(typ reflect.Type) reflect.Value

New returns a Value representing a pointer to a new zero value for the specified type. That is, the returned Value’s Type is PtrTo(typ).

因此,给定一个结构体类型的type,我们就可以构造出value

reflect.StructOf(fields []reflect.StructField) reflect.Type

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 generate wrapper methods for embedded fields and panics if passed unexported StructFields. These limitations may be lifted in a future version.

因此,给定[]reflect.StructField,就可以构造出type

注: 其他类型同理,比如reflect.ChanOf,reflect.ArrayOf,reflect.SliceOf

reflect.StructField

// A StructField describes a single field in a struct.
type StructField struct {
	// Name is the field name.
	Name string
	// PkgPath is the package path that qualifies a lower case (unexported)
	// field name. It is empty for upper case (exported) field names.
	// See https://golang.org/ref/spec#Uniqueness_of_identifiers
	PkgPath string

	Type      Type      // field type
	Tag       StructTag // field tag string
	Offset    uintptr   // offset within struct, in bytes
	Index     []int     // index sequence for Type.FieldByIndex
	Anonymous bool      // is an embedded field
}

根据reflect.StructOf的文档和自身的注释(见上文),Offset,Index都不需要指定,Anonymous也不支持(go1.13),name必须capital,PkgPath也为空

总结来说,我们需要指定:

iField := reflect.StructField{
    Name: "Id",
    Type: reflect.TypeOf(uint64(0)),
    Tag:  `json:"id"`, // optional
}

Build

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()

一般来说,使用时,比如将其返回,要转换成Interface{}

为了方便的用string访问字段,而不是index,我们可以自己包装一层,然后加上Interface(),Addr()两个方法

type myStruct struct {
	internal reflect.Value
	index    map[string]int
}

func (i *myStruct) Field(name string) reflect.Value {
	return i.internal.Field(i.index[name])
}

func (i *myStruct) Interface() interface{} {
	return i.internal.Interface()
}

func (i *myStruct) Addr() interface{} {
	return i.internal.Addr().Interface()
}

调用Addr()方法,可用于取地址,常用于调用指针接收者的方法

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.

用处

母鸡,暂时不太知道运行时构建结构体的用处

我唯一用到反射的地方,就只有解析结构体字段了,比如把struct转成map这种

但是以此来作为熟悉refelct包,还是不错的