Boost C++ Libraries

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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
Optional return values
Optional automatic variables
Optional data members
Bypassing unnecessary default construction
Storage in containers
Tutorial
Motivation
Design Overview
When to use Optional
Relational operators
Optional references
Rebinding semantics for assignment of optional references
In-Place Factories
A note about optional<bool>
Exception Safety Guarantees
Type requirements
Reference
Synopsis
Detailed Semantics
Dependencies and Portability
Dependencies
Optional Reference Binding
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 different. Also suppose that any possible value of type int is a valid value for "MaxValue", so we cannot jut 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: July 10, 2014 at 11:41:49 GMT


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