How to create custom format flags and options for fmt in Go?

To create custom format flags and options for the fmt package in Go, you need to implement the fmt.Formatter interface.

Here are the steps to create custom flags and options:

Define a new type that represents your custom format options. This type should hold any values or configuration specific to your custom formatting.

type MyFormatOptions struct {
    // Define fields for your custom format options
    Flag1 bool
    Flag2 int
    // ...
}

Implement the Format method of the fmt.Formatter interface for your custom type. This method will be called by the fmt package when formatting values with your custom format options.

func (opt MyFormatOptions) Format(f fmt.State, c rune) {
    // Implement the formatting logic based on the specified format
    // and the provided format state (f)
    // ...
}

Update the Format method to handle the different format verbs and flags. You can use the c parameter, which represents the format verb, to differentiate between different format verbs and implement the logic accordingly. The f parameter is of type fmt.State and contains information about the formatting state, such as flags and width.

func (opt MyFormatOptions) Format(f fmt.State, c rune) {
    switch c {
    case 'v':
        if f.Flag('+') {
            // Handle the "+" flag
        } else {
            // Default format
        }
    case 'd':
        // Handle integer format
    case 's':
        // Handle string format
    // ...
    }
}

Use the fmt.Fprintf function to format values using your custom format options. Pass an instance of your custom format options as an argument.

opt := MyFormatOptions{Flag1: true, Flag2: 10}
fmt.Fprintf(os.Stdout, "Value: %v\n", opt)  // Use %v to format with your custom options

With these steps, you can create custom format flags and options for the fmt package in Go. Make sure to handle different format verbs and flags in the Format method based on your requirements.