---
title: "ClikaRT::cli::Arg"
sidebar_label: "Arg"
description: "A typed option or positional handle. Fluent setters return *this; each configures the underlying declared option through the parser. Non-owning; valid while the owning App lives."
---

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**Header:** `ClikaRT/cli/arg.h`

A typed option or positional handle. Fluent setters return `*this`; each configures the underlying declared option through the parser. Non-owning; valid while the owning [`App`](./App.md) lives.

## Member functions

### help() {#help}

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`Arg&` **`help`**(`std::string_view` `text`)

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The help line shown for this option; chainable.

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Declared in `ClikaRT/cli/arg.h`, line 83

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### default\_value() {#default_value}

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`Arg&` **`default_value`**(`std::string_view` `value`)

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Value used when the option is absent (parsed from this text at the option's declared type).

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Declared in `ClikaRT/cli/arg.h`, line 86

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### implicit\_value() {#implicit_value}

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`Arg&` **`implicit_value`**(`std::string_view` `value`)

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Value used when the option is given with no argument (e.g. `--jobs`).

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Declared in `ClikaRT/cli/arg.h`, line 88

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### required() {#required}

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`Arg&` **`required`**(`bool` `v` `=` `true`)

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Mark required; parsing fails when the option is absent; chainable.

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Declared in `ClikaRT/cli/arg.h`, line 90

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### metavar() {#metavar}

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`Arg&` **`metavar`**(`std::string_view` `name`)

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The value placeholder shown in usage (e.g. `PATH`); chainable.

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Declared in `ClikaRT/cli/arg.h`, line 92

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### env() {#env}

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`Arg&` **`env`**(`std::string_view` `name`)

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Environment variable consulted when the option is absent from argv.

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Declared in `ClikaRT/cli/arg.h`, line 94

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### multiple() {#multiple}

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`Arg&` **`multiple`**(`bool` `v` `=` `true`)

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Accept the option more than once (collect all occurrences).

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Declared in `ClikaRT/cli/arg.h`, line 96

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### delimiter() {#delimiter}

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`Arg&` **`delimiter`**(`char` `d`)

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Split a single argument into several values on `d` (e.g. `-I a,b,c`).

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Declared in `ClikaRT/cli/arg.h`, line 98

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### choices() {#choices}

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`Arg&` **`choices`**(`std::initializer_list<std::string_view>` `values`)

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Restrict accepted values to this set (compared at the declared type).

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Declared in `ClikaRT/cli/arg.h`, line 100

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### conflicts\_with() {#conflicts_with}

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`Arg&` **`conflicts_with`**(`std::string_view` `other`)

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Mutual exclusion with option `other`; both present is a usage error.

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Declared in `ClikaRT/cli/arg.h`, line 102

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### needs() {#needs}

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`Arg&` **`needs`**(`std::string_view` `other`)

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Require option `other` whenever this one is present.

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Declared in `ClikaRT/cli/arg.h`, line 104

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### hidden() {#hidden}

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`Arg&` **`hidden`**(`bool` `v` `=` `true`)

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Hide from help/usage (still parsed); chainable.

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Declared in `ClikaRT/cli/arg.h`, line 106

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### alias() {#alias}

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`Arg&` **`alias`**(`std::string_view` `name`)

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An additional accepted spelling; chainable.

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Declared in `ClikaRT/cli/arg.h`, line 108

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### validator() {#validator}

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`Arg&` **`validator`**([`Validator`](./index.md#Validator) `fn`)

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Reject values your callback disapproves of (e.g. a numeric range, a format). Runs after type-parsing and [`choices`](./Arg.md#choices). A throwing validator is contained caller-side (see detail::contain) and becomes a failure. An empty validator clears the one set before, so every value passes it.

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Declared in `ClikaRT/cli/arg.h`, line 113

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