Golang function defer statement usage analysis

Golang is an object-oriented programming language that supports concurrency and is compiled into machine code. It has simple syntax, efficient performance and a rich standard library. In Golang, the defer statement is used to delay execution of a function. This language feature is very useful when writing code. This article will explain the use and analysis of the defer statement of Golang functions.

1. Basic syntax of defer statement

In Golang, the defer statement can be used to postpone the execution of a function or method until the function returns. The syntax of the defer statement is very simple. The syntax format is:

defer 函数名(参数列表)

where defer is a keyword in the Golang language. The defer statement can be used anywhere, but it is best to declare it at the beginning of the function or method, so that the execution process of the function or method can be displayed more clearly.

2. Execution principle of defer statement

When executing a function, if there are defer statements inside the function, then these defer statements will be executed in reverse order according to the order of definition, that is to say, they are defined last The defer statement is executed first, and the defer statement defined first is executed last. The defer statement has last-in-first-out logic.

For example, the following code implements a simple defer statement example:

package main

import (
    "fmt"
)

func main() {
    defer fmt.Println("defer 1")
    defer fmt.Println("defer 2")
    defer fmt.Println("defer 3")

    fmt.Println("Hello, Golang!")
}

The output result is:

Hello, Golang!
defer 3
defer 2
defer 1

As you can see, fmt.Println("Hello , Golang!"), and then executed three defer statements based on the last-in-first-out logic.

3. Application scenarios of defer statement

The defer statement is very commonly used in Golang language and can be used in the following scenarios:

1. Close the file

When using Golang to operate files, you need to close the file immediately after the file opening operation is completed. If you use the Close() function directly, the file may not be closed when an unexpected situation occurs when the program is running. At this time, you can Use the defer statement to delay the execution of the Close() function until the end of the function to ensure that the file can be closed normally. The following is a relevant code example:

file, err := os.Open("test.txt")
if err != nil {
    fmt.Println(err)
}
defer file.Close()

2. Unlocking operation

In Golang, use sync.Mutex to control the mutex lock and release the lock at the end of the function. You can use the defer statement to avoid deadlock in the program. The following is a sample code:

var mutex sync.Mutex

func sample() {
    mutex.Lock()
    defer mutex.Unlock()

    // 操作代码
}

3. Calculate function execution time

When testing the performance of a Golang function, you can record the time before and after the function is executed, and calculate the time difference to obtain the execution time of the function. If the time is calculated directly inside the function, the timestamp may be obtained in different places due to a lot of code logic, making debugging complicated. At this time, you can use the defer statement to calculate the time difference at the end of the function to obtain the function execution time.

import (
    "time"
)

func calcExecTime() {
    startTime := time.Now().UnixNano()
    defer func() {
        fmt.Println("time", float32(time.Now().UnixNano()-startTime)/1000000.0)
    }()

    // 操作代码
}

4. Precautions for the defer statement

When using the defer statement, you need to pay attention to the following points:

1. The execution of the defer statement will be in the current function or It is executed before the method exits, so any code in the function or method that modifies the internal state of the function or method will still take effect when defer is executed.
2. The defer statement is usually used to clean up some resources, such as closing files or releasing memory, so be sure to use the defer statement at the right time.
3. When using the defer statement, you should avoid using functions containing expensive code, such as large loops or connecting to the database, which may affect the performance of the program.

5. Summary

In Golang, the defer statement can be used to postpone the execution of a function or method until the function returns. The defer statement is very commonly used in the Golang language and can be used in closing files, unlocking operations, and calculating function execution time. When using the defer statement, you should avoid using functions containing expensive code to prevent affecting the performance of the program.

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