...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Copyright © 2003-2007 Fernando Luis Cacciola Carballal
Copyright © 2014-2016 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)
Table of Contents
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.
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.
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: May 05, 2016 at 21:43:25 GMT |