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PositionalArgument/NamedArgument

NamedArgument UDA is used to declare an argument that has a name (usually starting with - or --).

PositionalArgument UDA is used to declare an argument that has specific position in the command line.

Signature

PositionalArgument() PositionalArgument(string placeholder) PositionalArgument(uint position) PositionalArgument(uint position, string placeholder) NamedArgument(string[] names...) NamedArgument(string[] shortNames, string[] longNames)

Parameters

  • position

    Zero-based position of the argument. If it's omitted then positional arguments have an order of their declaration.

  • placeholder

    Name of this argument that is shown in help text. By default, the name of data member is used.

  • names

    Name(s) of this argument that can be used in command line. By default, the name of data member is used.

  • shortNames

    Short name(s) of this argument that can be used in command line.

  • longNames

    Long name(s) of this argument that can be used in command line.

Public members

Description

Description can be used to provide a description of the argument. This text is printed next to the argument in the argument list section of a help message.

Signature

Description(auto ref ... argument, string text) Description(auto ref ... argument, string function() text)

Parameters

  • text

    Text that contains argument description or a function that returns such text.

Usage example

struct my_command { @(NamedArgument.Description("custom description")) int a; }

Hidden

Hidden can be used to indicate that the argument should not be printed in help message or returned in shell completion.

Signature

Hidden(auto ref ... argument, bool hide = true)

Parameters

  • hide

    If true then argument is not printed in help message.

Usage example

struct my_command { @(NamedArgument.Hidden) int a; }

Placeholder

Placeholder provides custom text that is used to indicate the value of the argument in help message.

Signature

Placeholder(auto ref ... argument, string value)

Parameters

  • value

    Text that is used as a placeholder for a value of an argument in help message.

Usage example

struct my_command { @(NamedArgument.Placeholder("VALUE")) int a; }

Required

Mark an argument as required so if it is not provided in command line, argparse will error out.

By default all positional arguments are required.

Signature

Required(auto ref ... argument)

Usage example

struct my_command { @(NamedArgument.Required) int a; }

Optional

Mark an argument as optional so it can be omitted in command line without causing errors.

By default all named arguments are optional.

Signature

Optional(auto ref ... argument)

Usage example

struct my_command { @(NamedArgument.Optional) int a; }

NumberOfValues

NumberOfValues is used to limit number of values that an argument can accept.

Signature

NumberOfValues(auto ref ... argument, size_t min, size_t max) NumberOfValues(auto ref ... argument, size_t num)

Parameters

  • min

    Minimum number of values.

  • max

    Maximum number of values.

  • num

    Exact number of values.

Usage example

struct my_command { @(NamedArgument.NumberOfValues(1,3)) int[] a; @(NamedArgument.NumberOfValues(2)) int[] b; }

MinNumberOfValues

MinNumberOfValues is used to set minimum number of values that an argument can accept.

Signature

MinNumberOfValues(auto ref ... argument, size_t min)

Parameters

  • min

    Minimum number of values.

Usage example

struct my_command { @(NamedArgument.MinNumberOfValues(2)) int[] a; }

MaxNumberOfValues

MaxNumberOfValues is used to set maximum number of values that an argument can accept.

Signature

MaxNumberOfValues(auto ref ... argument, size_t max)

Parameters

  • max

    Maximum number of values.

Usage example

struct my_command { @(NamedArgument.MinNumberOfValues(3)) int[] a; }

AllowNoValue

AllowNoValue allows an argument to not have a value in the command line - in this case, the value provided to this function will be used.

Signature

AllowNoValue(VALUE)(auto ref ... argument, VALUE valueToUse)

Parameters

  • valueToUse

    Value that is used when argument has no value specified in command line.

Usage example

struct my_command { @(NamedArgument.AllowNoValue(10)) int a; }

ForceNoValue

ForceNoValue forces an argument to have no value in the command line. The argument is behaving like a boolean flag but instead of true/false values, there can be either a value provided to ForceNoValue or a default one (.init).

Signature

ForceNoValue(VALUE)(auto ref ... argument, VALUE valueToUse)

Parameters

  • valueToUse

    Value that is used when argument is specified in command line.

Usage example

struct my_command { @(NamedArgument.ForceNoValue(10)) int a; }

AllowedValues

AllowedValues can be used to restrict what value can be provided in the command line for an argument.

Signature

AllowedValues(TYPE)(auto ref ... argument, TYPE[] values...)

Parameters

  • values

    List of values that an argument can have.

Usage example

struct my_command { @(NamedArgument.AllowedValues(1,4,16,8)) int a; }

Counter

Counter can be used to mark an argument that tracks the number of times it's specified in the command line.

Signature

Counter(auto ref ... argument)

Usage example

struct my_command { @(NamedArgument.Counter) int a; }

PreValidation

PreValidation can be used to customize the validation of raw string values.

Signature

PreValidation(auto ref ... argument, RETURN function(VALUE value) func)

Parameters

Usage example

struct my_command { @(NamedArgument.PreValidation((string s) { return s.length > 0;})) int a; }

Parse

Parse can be used to provide custom conversion from raw string to a value.

Signature

Parse(auto ref ... argument, RECEIVER function(VALUE value) func) Parse(auto ref ... argument, RETURN function(ref RECEIVER receiver, RawParam param) func)

Parameters

Usage example

struct my_command { @(NamedArgument.Parse((string s) { return s[1]; })) char a; }

Validation

Validation can be used to validate parsed value.

Signature

Validation(auto ref ... argument, RETURN function(VALUE value) func)

Parameters

Usage example

struct my_command { @(NamedArgument.Validation((int a) { return a >= 0 && a <= 9; })) int a; }

Action

Action can be used to customize a logic of how "destination" should be changed based on parsed argument value.

Signature

Action(auto ref ... argument, RETURN function(ref RECEIVER receiver, VALUE value) func)

Parameters

Usage example

struct my_command { @(NamedArgument.Action((ref int a, int v) { a += v; }) int a; }
Last modified: 21 October 2025
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