英文:
Generic Programming in Go. Avoiding hard coded type assertion
问题
我正在编写一个通用缓存机制，并且需要在一个结构体中设置一些属性，只知道它们的reflect.Type、属性名称和要设置的reflect.Value，但是我无法避免类型断言，这使得我的代码不够通用...
func main() {
addressNew := Address{"新地址描述！"}
// 在实际问题中，我知道值的reflect.Type，但是
// 结构体以interface{}的形式传递给我，就像这个方法一样
// 以通用的方式从redis返回多种类型的值，
// (Customer, Order, Address, Product, SKU等)，但也返回值的reflect.Type。
interfaceSomeValue := getMyValue()
fmt.Printf("%v", interfaceSomeValue)
fmt.Println("")
// 这部分代码来自我的缓存机制，它是其他项目使用的库
// 我的缓存库真的不知道所有其他类型的结构体来执行类型断言，
// 但是缓存机制知道接口的reflect.Type。
// 如果你尝试这种方式，将会导致尝试在接口中访问FieldByName时发生panic，
// 因为Customer来自getMyValue并且变成了interface{}，
// 现在需要进行类型断言 -> http://play.golang.org/p/YA8U9_KzC9
newCustomerNewAttribute := SetAttribute(&interfaceSomeValue, "Local", interface{}(addressNew), reflect.TypeOf(Customer{}))
fmt.Printf("%v", newCustomerNewAttribute)
fmt.Println("")
}
func SetAttribute(object interface{}, attributeName string, attValue interface{}, objectType reflect.Type) interface{} {
if reflect.ValueOf(object).Kind() != reflect.Ptr {
panic("需要一个指针")
}
value := reflect.ValueOf(object).Elem()
field := value.FieldByName(attributeName)
valueForAtt := reflect.ValueOf(attValue)
field.Set(valueForAtt)
return value.Interface()
}
问题的Go Playground（通过硬编码的类型断言解决）
英文:
I'm programming a generic cache mechanism and i need to set some attributes in a struct knowing only their reflect.Type, attribute name and reflect.Value to be setted in the attribute, but i can't avoid the type assertion, that makes my code not generic...
func main() {
addressNew := Address{"New Address description!"}
// In the real problem, i know the reflect.Type of value, but
// the struct came to me as a interface{}, just like this method
// Return many kinds of values from redis as interface{},
// (Customer, Order, Address, Product, SKU etc), in a generic way,
// but returns the reflect.Type of value also.
interfaceSomeValue := getMyValue()
fmt.Printf("%v", interfaceSomeValue)
fmt.Println("")
// This portion of code comes from my cache mechanism, that is a library
// used by other projects. My cache lib really can't know all others
// type structs to perform the type assertion, but the cache mechanism know
// the reflect.Type of the interface.
// If you try at this way, will cause a panic by try to access a FieldByName
// in a interface, because the Customer comes from getMyValue and
// becomes a interface{}, and now a type assertion is
// required -> http://play.golang.org/p/YA8U9_KzC9
newCustomerNewAttribute := SetAttribute(&interfaceSomeValue, "Local", interface{}(addressNew), reflect.TypeOf(Customer{}))
fmt.Printf("%v", newCustomerNewAttribute)
fmt.Println("")
}
func SetAttribute(object interface{}, attributeName string, attValue interface{}, objectType reflect.Type) interface{} {
if reflect.ValueOf(object).Kind() != reflect.Ptr {
panic("need a pointer")
}
value := reflect.ValueOf(object).Elem()
field := value.FieldByName(attributeName)
valueForAtt := reflect.ValueOf(attValue)
field.Set(valueForAtt)
return value.Interface()
}
Go Playground for the problem (works by hard coded type assertion)...
Go Playground for the problem (doesn't work with an unknown interface)
答案1
得分: -1
最后我找到了一种方法来实现这个。请按照下面的Go Playground和代码片段进行操作：
//新的方法SetAttribute，不需要任何硬编码的类型断言，只需基于objectType参数
func SetAttribute(myUnknownTypeValue *interface{}, attributeName string, attValue interface{}, objectType reflect.Type) {
// 为旧值创建值
oldValue := reflect.ValueOf(*myUnknownTypeValue)
// 基于类型创建一个新值
newValue := reflect.New(objectType).Elem()
// 将旧值设置为新值，进行隐式类型断言，而不是硬编码
newValue.Set(oldValue)
// 将值属性设置为新结构体的副本
field := newValue.FieldByName(attributeName)
valueForAtt := reflect.ValueOf(attValue)
field.Set(valueForAtt)
// 从reflect.Value中获取新值
newValInterface := newValue.Interface()
// 将新值设置为指针
*myUnknownTypeValue = newValInterface
}
英文:
Finally i find a way to do that. Follow the Go Playground and code snippet below:
GO Playground: Avoiding Hard Coded Type Assertion
<pre><code>
//new approach SetAttribute, without any hard coded type assertion, just based on objectType parameter
func SetAttribute(myUnknownTypeValue *interface{}, attributeName string, attValue interface{}, objectType reflect.Type) {
// create value for old val
oldValue := reflect.ValueOf(*myUnknownTypeValue)
//create a new value, based on type
newValue := reflect.New(objectType).Elem()
// set the old value to the new one, making the
// implicit type assertion, not hard coding that.
newValue.Set(oldValue)
//set value attribute to the new struct, copy of the old one
field := newValue.FieldByName(attributeName)
valueForAtt := reflect.ValueOf(attValue)
field.Set(valueForAtt)
//capture the new value from reflect.Value
newValInterface := newValue.Interface()
//set the new value to the pointer
*myUnknownTypeValue = newValInterface
}
</code></pre>
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