Let's write and use a converter function that converts an a std::string to an int. It is possible that for a given string (e.g. "cat") there exist no value of type int capable of representing the conversion result. We do not consider such situation an error. We expect that the converter can be used only to check if the conversion is possible. A natural signature for this function can be:

#include <boost/optional.hpp>
boost::optionl<int> convert(const std::string& text);

All necessary functionality can be included with one header <boost/optional.hpp>. The above function signature means that the function can either return a value of type int or a flag indicating that no value of int is available. This does not indicate an error. It is like one additional value of int. This is how we can use our function:

const std::string& text = /*... */;
boost::optionl<int> oi = convert(text); // move-construct
if (oi)                                 // contextual conversion to bool
  int i = *oi;                          // operator*

In order to test if optional contains a value, we use the contextual conversion to type bool. Because of this we can combine the initialization of the optional object and the test into one instruction:

if (boost::optionl<int> oi = convert(text))
  int i = *oi;

We extract the contained value with operator* (and with operator-> where it makes sense). An attempt to extract the contained value of an uninitialized optional object is an undefined behaviour (UB). This implementation guards the call with BOOST_ASSERT. Therefore you should be sure that the contained value is there before extracting. For instance, the following code is reasonably UB-safe:

int i = *convert("100");

This is because we know that string value "100" converts to a valid value of int. If you do not like this potential UB, you can use an alternative way of extracting the contained value:

try {
  int j = convert(text).value();
}
catch (const boost::bad_optional_access&) {
  // deal with it
}

This version throws an exception upon an attempt to access a non-existent contained value. If your way of dealing with the missing value is to use some default, like 0, there exists a yet another alternative:

int k = convert(text).value_or(0);

This uses the atoi-like approach to conversions: if text does not represent an integral number just return 0. Now, let's consider how function convert can be implemented.

boost::optionl<int> convert(const std::string& text)
{
  std::stringstream s(text);
  int i;
  if ((s >> i) && s.get() == std::char_traits<char>::eof())
    return i;
  else
    return boost::none;
}

Observe the two return statements. return i uses the converting constructor that can create optional<T> from T. Thus constructed optional object is initialized and its value is a copy of i. The other return statement uses another converting constructor from a special tag boost::none. It is used to indicate that we want to create an uninitialized optional object.

Suppose we want to implement a lazy load optimization. This is because we do not want to perform an expensive initialization of our Resource until (if at all) it is really used. We can do it this way:

class Widget
{
  boost::optional<Resource> resource_;

public:
  Widget() {}

  Resource& getResource() // not thread-safe
  {
    if (resource_ == boost::none)
        resource_.emplace("resource", "arguments");

    return *resource_;
  }
};

optional's default constructor creates an uninitialized optional. No call to Resource's default constructor is attempted. Resource doesn't have to be Default Constructible. In function getResource we first check if resource_ is initialized. This time we do not use the contextual conversion to bool, but a comparison with boost::none. These two ways are equivalent. Function emplace initializes the optional in-place by perfect-forwarding the arguments to the constructor of Resource. No copy- or move-construction is involved here. Resource doesn't even have to be MoveConstructible.

	Note
	Function emplace is only available on compilers that support rvalue references and variadic templates. If your compiler does not support these features and you still need to avoid any move-constructions, use In-Place Factories.

Suppose we have class Date, which does not have a default constructor: there is no good candidate for a default date. We have a function that returns two dates in form of a boost::tuple:

boost::tuple<Date, Date> getPeriod();

In other place we want to use the result of getPeriod, but want the two dates to be named: begin and end. We want to implement something like 'multiple return values':

Date begin, end; // Error: no default ctor!
boost::tie(begin, end) = getPeriod();

The second line works already, this is the capability of Boost.Tuple library, but the first line won't work. We could set some initial invented dates, but it is confusing and may be an unacceptable cost, given that these values will be overwritten in the next line anyway. This is where optional can help:

boost::optional<Date> begin, end;
boost::tie(begin, end) = getPeriod();

It works because inside boost::tie a move-assignment from T is invoked on optional<T>, which internally calls a move-constructor of T.