Understanding of the Golang interface

This is a creation in Article, where the information may have evolved or changed.

Understanding Golang's interface mainly lies in the following two points:

• Interface is a collection of methods
• Interface is a type

Simple example

package mainimport "fmt"type Animal interface {    Speak() string}type Cat struct{}func (c Cat) Speak() string {    return "cat"}type Dog struct{}func (d Dog) Speak() string {    return "dog"}func Test(params interface{}) {    fmt.Println(params)}func main() {    animals := []Animal{Cat{}, Dog{}}    for _, animal := range animals {        fmt.Println(animal.Speak())    }        Test("string")    Test(123)    Test(true)}

In the above code, animal is defined as an interface, and cat and dog two structures implement the method defined in the interface respectively. When interface{} is used as a function parameter, different types of parameters can be accepted.

Pointers and interface

If the code above is:

func (c Cat) Speak() string {    return "cat"}

Modified to:

func (c *Cat) Speak() string {    return "cat"}

The following error occurs when you run the source code again:

cannot use Cat literal (type Cat) as type Animal in array or slice literal:Cat does not implement Animal (Speak method has pointer receiver)

This is because the program thinks that Cat does not implement the Speak () method, but is implemented by *cat. This means that the struct does not implicitly convert the type when implementing an interface method.

Interface array

interface{} is very different as a function parameter and []interface{} as a formal parameter, as shown in the following example:

package mainimport (    "fmt")func PrintAll(vals []interface{}) {    for _, val := range vals {        fmt.Println(val)    }}func main() {    names := []string{"stanley", "david", "oscar"}    PrintAll(names)}

The above code is not working correctly and the error message is:

cannot use names (type []string) as type []interface {} in argument to PrintAll

This indicates that there must be more than one type conversion operation before assigning a value to an array of interfaces, the correct code is as follows:

package mainimport (    "fmt")func PrintAll(vals []interface{}) {    for _, val := range vals {        fmt.Println(val)    }}func main() {    names := []string{"stanley", "david", "oscar"}    vals := make([]interface{}, len(names))    for i, v := range names {        vals[i] = v    }    PrintAll(vals)}

The above is the basic content about the use of interface.