Golang标准库中定义的那些预定义错误变量如何使用

io.EOF

os.ErrNotExist

errors.Is

errors.As

package main

import (
    "errors"
    "fmt"
    "io"
    "os"
)

func readData(r io.Reader) ([]byte, error) {
    data := make([]byte, 10)
    n, err := r.Read(data)
    if err != nil {
        if errors.Is(err, io.EOF) {
            fmt.Println("Reached end of file")
            return data[:n], nil // 允许返回部分数据
        }
        return nil, fmt.Errorf("read error: %w", err)
    }
    return data, nil
}

func checkFile(filename string) error {
    _, err := os.Stat(filename)
    if err != nil {
        if errors.Is(err, os.ErrNotExist) {
            return fmt.Errorf("file does not exist: %w", err)
        }
        return fmt.Errorf("file stat error: %w", err)
    }
    return nil
}

func main() {
    // 示例1: 使用 io.EOF
    r := io.LimitReader(strings.NewReader("hello"), 3) // 创建一个限制读取的Reader
    data, err := readData(r)
    if err != nil {
        fmt.Println("Error:", err)
    } else {
        fmt.Println("Data:", string(data))
    }

    // 示例2: 使用 os.ErrNotExist
    filename := "nonexistent_file.txt"
    err = checkFile(filename)
    if err != nil {
        fmt.Println("Error:", err)
    }
}

import "strings"

errors.Is

errors.Is(err, io.EOF)

true

err

io.EOF

package main

import (
    "errors"
    "fmt"
    "io"
    "os"
)

func readFile(filename string) error {
    f, err := os.Open(filename)
    if err != nil {
        return fmt.Errorf("failed to open file: %w", err)
    }
    defer f.Close()

    _, err = io.ReadAll(f)
    if err != nil {
        if errors.Is(err, io.EOF) {
            fmt.Println("end of file reached")
            return nil
        }
        return fmt.Errorf("failed to read file: %w", err)
    }
    return nil
}

func main() {
    err := readFile("my_file.txt")
    if err != nil {
        fmt.Println(err)
    }
}

package main

import (
    "context"
    "errors"
    "fmt"
    "os"
    "time"
)

func checkPermission(filename string) error {
    _, err := os.Open(filename)
    if err != nil {
        if errors.Is(err, os.ErrPermission) {
            return fmt.Errorf("permission denied: %w", err)
        }
        return fmt.Errorf("failed to open file: %w", err)
    }
    return nil
}

func doSomething(ctx context.Context) error {
    select {
    case <-ctx.Done():
        if errors.Is(ctx.Err(), context.Canceled) {
            return fmt.Errorf("operation cancelled: %w", ctx.Err())
        }
        if errors.Is(ctx.Err(), context.DeadlineExceeded) {
            return fmt.Errorf("deadline exceeded: %w", ctx.Err())
        }
        return ctx.Err()
    case <-time.After(1 * time.Second):
        fmt.Println("Operation completed")
        return nil
    }
}

func main() {
    // 示例1: 使用 os.ErrPermission
    err := checkPermission("protected_file.txt")
    if err != nil {
        fmt.Println(err)
    }

    // 示例2: 使用 context.Canceled 和 context.DeadlineExceeded
    ctx, cancel := context.WithTimeout(context.Background(), 500*time.Millisecond)
    defer cancel()

    err = doSomething(ctx)
    if err != nil {
        fmt.Println(err)
    }
}

errors.Is

Is

package main

import (
    "errors"
    "fmt"
    "io"
)

type CustomEOFError struct {
    Message string
}

func (e *CustomEOFError) Error() string {
    return fmt.Sprintf("custom EOF error: %s", e.Message)
}

func (e *CustomEOFError) Is(target error) bool {
    return errors.Is(io.EOF, target) // 检查目标是否是 io.EOF 或包装了 io.EOF
}

func readDataCustom(r io.Reader) ([]byte, error) {
    data := make([]byte, 10)
    n, err := r.Read(data)
    if err != nil {
        if errors.Is(err, io.EOF) {
            return nil, &CustomEOFError{Message: "Reached end of file with custom error"}
        }
        return nil, fmt.Errorf("read error: %w", err)
    }
    return data, nil
}

func main() {
    r := io.LimitReader(strings.NewReader("hello"), 3)
    _, err := readDataCustom(r)
    if err != nil {
        if errors.Is(err, io.EOF) {
            fmt.Println("Custom EOF error encountered")
        }
        fmt.Println("Error:", err)
    }
}

import "strings"