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## Boost.Optional

### Fernando Luis Cacciola Carballal

Copyright © 2003-2007 Fernando Luis Cacciola Carballal

Copyright © 2014-2017 Andrzej Krzemieński

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

## Introduction

Class template `optional` is a wrapper for representing 'optional' (or 'nullable') objects who may not (yet) contain a valid value. Optional objects offer full value semantics; they are good for passing by value and usage inside STL containers. This is a header-only library.

#### Problem

Suppose we want to read a parameter form a config file which represents some integral value, let's call it `"MaxValue"`. It is possible that this parameter is not specified; such situation is no error. It is valid to not specify the parameter and in that case the program is supposed to behave slightly differently. Also, suppose that any possible value of type `int` is a valid value for `"MaxValue"`, so we cannot just use `-1` to represent the absence of the parameter in the config file.

#### Solution

This is how you solve it with `boost::optional`:

    #include <boost/optional.hpp>

    boost::optional<int> getConfigParam(std::string name);  // return either an int or a `not-an-int`

    int main()
    {
      if (boost::optional<int> oi = getConfigParam("MaxValue")) // did I get a real int?
        runWithMax(*oi);                                        // use my int
      else
        runWithNoMax();
    }

Last revised: April 11, 2018 at 13:55:26 GMT |