验证struct变量是否为空

Question

First of all, look at the following code snippet:

package main

import (
    "fmt"
)

func main() {

    var para1 struct {
        email, addr string
    }

    para1.email = "[e-mail]"

    if para1 != nil {
        fmt.Println(para1)
    }

}

When I compile this code, I've got the compiler error:

./struct_func.go:15: cannot convert nil to type struct { email string; addr string }

How can I validate if my struct variable, if nil or not? Or I have to validate of property like

if para1.email != nil {
    fmt.Println(para1)
}

Answer 1

You can compare the struct to its zero value. You can test the string for its zero (empty) value "" or test for string length zero. For example,

package main

import (
    "fmt"
)

func main() {
    var para1 struct {
        email, addr string
    }
    para1Zero := para1
    para1.email = "[e-mail]"
    if para1 != para1Zero {
        fmt.Println(para1)
    }
    if para1.email != "" {
        fmt.Println(para1.email)
    }
    if len(para1.email) != 0 {
        fmt.Println(para1.email)
    }
}

Output:

{[e-mail] }
[e-mail]
[e-mail]

The Go Programming Language Specification

The zero value

When memory is allocated to store a value, either through a declaration or a call of make or new, and no explicit initialization is provided, the memory is given a default initialization. Each element of such a value is set to the zero value for its type: false for booleans, 0 for integers, 0.0 for floats, "" for strings, and nil for pointers, functions, interfaces, slices, channels, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified.