Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the develop branch, built from commit c0b10385b9.
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Boost.Optional

Fernando Luis Cacciola Carballal

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)

Table of Contents

Introduction
Quick Start
Tutorial
Reference
Dependencies and Portability
Release Notes
Acknowledgements

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();
}

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