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)

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::stringname);// return either an int or a `not-an-int`intmain(){if(boost::optional\<int\>oi=getConfigParam("MaxValue"))// did I get a real int?runWithMax(\*oi);// use my intelserunWithNoMax();}

Last revised: December 02, 2021 at 06:49:56 GMT