Optional parameters with defaults in Go struct constructors

问题

I've found myself using the following pattern as a way to get optional parameters with defaults in Go struct constructors:

package main

import (
    "fmt"
)

type Object struct {
    Type int
    Name string
}

func NewObject(obj *Object) *Object {
    if obj == nil {
        obj = &Object{}
    }
    // Type has a default of 1
    if obj.Type == 0 {
        obj.Type = 1
    }
    return obj
}

func main() {
    // create object with Name="foo" and Type=1
    obj1 := NewObject(&Object{Name: "foo"})
    fmt.Println(obj1)

    // create object with Name="" and Type=1
    obj2 := NewObject(nil)
    fmt.Println(obj2)

    // create object with Name="bar" and Type=2
    obj3 := NewObject(&Object{Type: 2, Name: "foo"})
    fmt.Println(obj3)
}

Is there a better way of allowing for optional parameters with defaults?

回答1:

The approach seems reasonable to me. However, you have a bug. If I explicitly set Type to 0, it will get switched to 1.

My suggested fix: Use a struct literal for the default value: http://play.golang.org/p/KDNUauy6Ie

Or perhaps extract it out: http://play.golang.org/p/QpY2Ymze3b

回答2:

You could use the ... operator.

instead of writing ToCall(a=b) like in python you write, ToCall("a",b)

See the Go Play Example

func GetKwds(kwds []interface{}) map[string]interface{} {
    result := make(map[string]interface{})

    for i := 0; i < len(kwds); i += 2 {
        result[kwds[i].(string)] = kwds[i+1]
    }

    return result
}

func ToCall(kwds ...interface{}) {
    args := GetKwds(kwds)
    if value, ok := args["key"]; ok {
        fmt.Printf("key: %#v\n", value)
    }
    if value, ok := args["other"]; ok {
        fmt.Printf("other: %#v\n", value)
    }
}

func main() {
    ToCall()
    ToCall("other", &map[string]string{})
    ToCall("key", "Test", "other", &Object{})

}

回答3:

Take a look at "Allocation with new" in Effective Go. They explain about making zero-value structs a useful default.

If you can make Object.Type (and your other fields) have a default of zero, then Go struct literals already give you exactly the feature you're requesting.

From the section on composite literals:

The fields of a composite literal are laid out in order and must all be present. However, by labeling the elements explicitly as field:value pairs, the initializers can appear in any order, with the missing ones left as their respective zero values.

That means you can replace this:

obj1 := NewObject(&Object{Name: "foo"})
obj2 := NewObject(nil)
obj3 := NewObject(&Object{Type: 2, Name: "foo"})

with this:

obj1 := &Object{Name: "foo"}
obj2 := &Object{}
obj3 := &Object{Type: 2, Name: "foo"}

If it is not possible to make the zero value the default for all of your fields, the recommended approach is a constructor function. For example:

func NewObject(typ int, name string) *Object {
    return &Object{Type: typ, Name: name}
}

If you want Type to have a nonzero default, you can add another constructor function. Suppose Foo objects are the default and have Type 1.

func NewFooObject(name string) *Object {
    return &Object{Type: 1, Name: name}
}

You only need to make one constructor function for each set of nonzero defaults you use. You can always reduce that set by changing the semantics of some fields to have zero defaults.

Also, note that adding a new field to Object with a zero default value doesn't require any code changes above, because all struct literals use labeled initialization. That comes in handy down the line.

回答4:

https://play.golang.org/p/SABkY9dbCOD

Here's an alternative that uses a method of the object to set defaults. I've found it useful a few times, although it's not much different than what you have. This might allow better usage if it's part of a package. I don't claim to be a Go expert, maybe you'll have some extra input.

package main

import (
    "fmt"
)

type defaultObj struct {
    Name      string
    Zipcode   int
    Longitude float64
}

func (obj *defaultObj) populateObjDefaults() {
    if obj.Name == "" {
        obj.Name = "Named Default"
    }
    if obj.Zipcode == 0 {
        obj.Zipcode = 12345
    }
    if obj.Longitude == 0 {
        obj.Longitude = 987654321
    }
}

func main() {
    testdef := defaultObj{Name: "Mr. Fred"}
    testdef.populateObjDefaults()
    fmt.Println(testdef)

    testdef2 := defaultObj{Zipcode: 90210}
    testdef2.populateObjDefaults()
    fmt.Println(testdef2)

    testdef2.Name = "Mrs. Fred"
    fmt.Println(testdef2)

    testdef3 := defaultObj{}
    fmt.Println(testdef3)
    testdef3.populateObjDefaults()
    fmt.Println(testdef3)
}

Output:

{Mr. Fred 12345 9.87654321e+08}
{Named Default 90210 9.87654321e+08}
{Mrs. Fred 90210 9.87654321e+08}
{ 0 0}
{Named Default 12345 9.87654321e+08}

回答5:

Dave Cheney offered a nice solution to this where you have functional options to overwrite defaults:

https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

So your code would become:

package main

import (
    "fmt"
)

type Object struct {
    Type int
    Name string
}

func NewObject(options ...func(*Object)) *Object {
    // Setup object with defaults 
    obj := &Object{Type: 1}
    // Apply options if there are any
    for _, option := range options {
        option(obj)
    }
    return obj
}

func WithName(name string) func(*Object) {
    return func(obj *Object) {
        obj.Name = name
    }
}

func WithType(newType int) func(*Object) {
    return func(obj *Object) {
        obj.Type = newType
    }
}

func main() {
    // create object with Name="foo" and Type=1
    obj1 := NewObject(WithName("foo"))
    fmt.Println(obj1)

    // create object with Name="" and Type=1
    obj2 := NewObject()
    fmt.Println(obj2)

    // create object with Name="bar" and Type=2
    obj3 := NewObject(WithType(2), WithName("foo"))
    fmt.Println(obj3)
}

https://play.golang.org/p/pGi90d1eI52

来源：https://stackoverflow.com/questions/20150628/optional-parameters-with-defaults-in-go-struct-constructors

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