Boost C++ Libraries

Let's write and use a converter function that converts a std::string to an int. It is possible that for a given string (e.g. "cat") there exists 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::optional<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::optional<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::optional<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 nonexistent 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. Finally, you can provide a callback to be called when trying to access the contained value fails:

int fallback_to_default()
{
  cerr << "could not convert; using -1 instead" << endl;
  return -1;
}

int l = convert(text).value_or_eval(fallback_to_default);

This will call the provided callback and return whatever the callback returns. The callback can have side effects: they will only be observed when the optional object does not contain a value.

Now, let's consider how function convert can be implemented.

boost::optional<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.