example_test.go

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // These examples demonstrate more intricate uses of the flag package.
     6  package flag_test
     7  
     8  import (
     9  	"errors"
    10  	"flag"
    11  	"fmt"
    12  	"strings"
    13  	"time"
    14  )
    15  
    16  // Example 1: A single string flag called "species" with default value "gopher".
    17  var species = flag.String("species", "gopher", "the species we are studying")
    18  
    19  // Example 2: Two flags sharing a variable, so we can have a shorthand.
    20  // The order of initialization is undefined, so make sure both use the
    21  // same default value. They must be set up with an init function.
    22  var gopherType string
    23  
    24  func init() {
    25  	const (
    26  		defaultGopher = "pocket"
    27  		usage         = "the variety of gopher"
    28  	)
    29  	flag.StringVar(&gopherType, "gopher_type", defaultGopher, usage)
    30  	flag.StringVar(&gopherType, "g", defaultGopher, usage+" (shorthand)")
    31  }
    32  
    33  // Example 3: A user-defined flag type, a slice of durations.
    34  type interval []time.Duration
    35  
    36  // String is the method to format the flag's value, part of the flag.Value interface.
    37  // The String method's output will be used in diagnostics.
    38  func (i *interval) String() string {
    39  	return fmt.Sprint(*i)
    40  }
    41  
    42  // Set is the method to set the flag value, part of the flag.Value interface.
    43  // Set's argument is a string to be parsed to set the flag.
    44  // It's a comma-separated list, so we split it.
    45  func (i *interval) Set(value string) error {
    46  	// If we wanted to allow the flag to be set multiple times,
    47  	// accumulating values, we would delete this if statement.
    48  	// That would permit usages such as
    49  	//	-deltaT 10s -deltaT 15s
    50  	// and other combinations.
    51  	if len(*i) > 0 {
    52  		return errors.New("interval flag already set")
    53  	}
    54  	for _, dt := range strings.Split(value, ",") {
    55  		duration, err := time.ParseDuration(dt)
    56  		if err != nil {
    57  			return err
    58  		}
    59  		*i = append(*i, duration)
    60  	}
    61  	return nil
    62  }
    63  
    64  // Define a flag to accumulate durations. Because it has a special type,
    65  // we need to use the Var function and therefore create the flag during
    66  // init.
    67  
    68  var intervalFlag interval
    69  
    70  func init() {
    71  	// Tie the command-line flag to the intervalFlag variable and
    72  	// set a usage message.
    73  	flag.Var(&intervalFlag, "deltaT", "comma-separated list of intervals to use between events")
    74  }
    75  
    76  func Example() {
    77  	// All the interesting pieces are with the variables declared above, but
    78  	// to enable the flag package to see the flags defined there, one must
    79  	// execute, typically at the start of main (not init!):
    80  	//	flag.Parse()
    81  	// We don't call it here because this code is a function called "Example"
    82  	// that is part of the testing suite for the package, which has already
    83  	// parsed the flags. When viewed at pkg.go.dev, however, the function is
    84  	// renamed to "main" and it could be run as a standalone example.
    85  }
    86
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