Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the develop branch, built from commit 53a72f668d.

Monadic interface

The monadic interface of optional allows the application of functions to optional values without resorting to the usage of explicit if-statements.

Function map takes a function mapping type T onto type U and maps an optional<T> onto an optional<U> using the provided function.

int length(const string& s){ return s.size(); };

optional<string> null{}, thin{""}, word{"word"};
assert ( == none);
assert ( == 0);
assert ( == 4);

Function flat_map is similar, but it requires the function to return an optional<V> for some type V. This optional<V> becomes the return type of flat_map.

optional<char> first_char(const string& s) {
  if (s.empty()) return none;
  else           return s[0];

optional<string> null{}, thin{""}, word{"word"};
assert (null.flat_map(first_char) == none);
assert (thin.flat_map(first_char) == none);
assert (word.flat_map(first_char) == 'w');

These functions can be combined in one expression reflecting a chain of computations:

auto get_contents(path p) -> optional<string>;
auto trim(string) -> string;
auto length(string) -> int;

auto trimmed_size_of(optional<path> p) -> int
  return p.flat_map(get_contents)