An article explains anonymous variables in Golang in detail

In Golang, anonymous variables can be used to ignore unwanted return values ​​or placeholders. Anonymous variables are special types of variables that simplify code and improve readability. This article will introduce in detail the definition, characteristics and usage methods of anonymous variables.

What is anonymous variable

In Golang, anonymous variables are variables that do not explicitly declare names, and are usually used in cases where a value is temporarily stored but does not need to be used in subsequent code. The way anonymous variables are declared is to use the underscore "_" as the variable name. See an example:

_ = functionName(arg1, arg2, ...)

where functionName is the function name to be called, arg1, arg2, ... is the parameter list passed to the function. In this syntax structure, an underscore is used as the variable name to indicate that the return value of the function is not concerned. In this way, the result of the function call can be discarded directly, thereby achieving the purpose of ignorance. Let’s look at a few different examples:

package main
import "fmt"
func main() {
    // Declare an anonymous variable and initialize it to 10    _ = 10
    // Declare an anonymous variable and assign the return value of a function to it    _, _ = ("Hello, World!")
    // Multiple anonymous variables can be declared in the same statement    _, _ = 10, 20
    // Some return values ​​can be ignored using anonymous variables when a function returns multiple values    _, ignoredValue := someFunction()
    (ignoredValue)
}
func someFunction() (int, string) {
    return 10, "Ludoshin's Blog"
}

In the example above, anonymous variables are used to ignore the return value of the function, temporarily store the value, and use it if some return values ​​need to be ignored. Please note that anonymous variables cannot be reassigned or used elsewhere, and their functions are limited to the declared location.

Function in guide packets

Anonymous variables are a common use in Golang for anonymous variables to import a package without using its exportable functions, types, variables, etc. (or just using an init function). For example:

package main  
import (  
    _ "fmt"  
)  
func main() {  
    // Here you can use the function of the fmt package, but it will not introduce its public interface}

In this example, anonymous variables are used to import the fmt package, avoiding exportable variables, types, functions, etc. in this package.

Function in implementing interfaces

Let's take a look at an example first./open-telemetry/opentelemetry-go/blob/exporters/trace/zipkin/v0.20.1/exporters/trace/zipkin/There is the following line of code in this file:

var (
    _  = &Exporter{}
)

This line of code creates an anonymous variable (named _) and tries to assign &Exporter{} to it. Because the type of _ is , if Exporter does not implement all methods of the interface, this line of code will fail at compile time, thus providing a static type checking mechanism for checking whether Exporter implements the interface at compile time.

This is a common pattern, especially when dealing with interfaces and types, which ensures that your type implements the interface you expect. The advantage of this approach is that it does not introduce additional runtime overhead, because the _ variable is a special identifier in Go and does not reference or allocate memory. This pattern does not create or use any actual variables, but is only checked at compile time, so there is no need to worry about impacting the program's runtime performance.

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