When Should You Use `%v` for Printing in Go?

Using %v for Printing: Potential Issues to Consider

While the %v verb allows for printing both integers (%d) and strings (%s), its usage should be considered cautiously to avoid unintended formatting issues.

Default Format vs. Specific Format

Unlike the %d verb, which instructs the fmt package to print the value as a number (with base 10), the %v verb uses the default format. This default format can be overridden by implementing the fmt.Stringer interface (with a String() string method) for your custom type.

Example

Consider the following code using a custom type MyInt that implements the fmt.Stringer interface:

type MyInt int

func (mi MyInt) String() string {
    return fmt.Sprintf("*%d*", int(mi))
}

func main() {
    var mi MyInt = 2
    fmt.Printf("%d %v", mi, mi)
}

Output:

2 *2*

When using %v, the fmt package checks if the value implements fmt.Stringer and calls its String() method. This results in the value being displayed with the overridden format ("2" instead of "2").

Formatting Rules

The fmt package follows a specific set of formatting rules when using %v:

1. It检查接口实现：

   • reflect.Value: 替换为实际值。
   • Formatter：使用自定义格式器进行格式化。
   • GoStringer（带有 #v）：调用 GoStringer 方法。
2. String() 方法：如果实现 String() string，将其值转换为字符串。
3. Error 方法：如果实现 error 接口，使用 Error 方法将其值转换为字符串。

Conclusion

While it's technically possible to always use %v for printing, it's generally recommended to use specific verbs like %d and %s for integers and strings. Using %v may lead to unexpected formatting behavior if your types implement certain interfaces, particularly fmt.Stringer.

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