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();
}