vedgy / optional

optional (nullable) objects for C++11

Optional

A single-header header-only library for representing optional (nullable) objects for C++11 and passing them by value. This is the reference implementation of proposal N793 (see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3793.html). Optional is now accepted into Library Fundamentals Technical Specification (see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3848.html). The interface is based on Fernando Cacciola's Boost.Optional library (see http://www.boost.org/doc/libs/1_52_0/libs/optional/doc/html/index.html)

Usage

optional<int> readInt(); // this function may return int or a not-an-int

if (optional<int> oi = readInt()) // did I get a real int
  cout << "my int is: " << *oi;   // use my int
else
  cout << "I have no int";

For more usage examples and the overview see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3527.html

Supported compilers

Clang 3.2, Clang 3.4, G++ 4.7.2, G++ 4.8.1. Tested only with libstdc++, versions 20130531, 20120920, 20110428. Others have reported it also works with libc++.

Known Issues

• Currently, the reference (and the only known) impementation of certain pieces of functionality explore what C++11 identifies as undefined behavior (see national body comment FI 15: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3770.html#FI15). This is mostly why Optional was removed from C++14 and put into Library Fundamentals TS. Luckily what the Standard identifies as UB is well defined on all known platforms. We expect that the C++14 wil fix this problem, so that our trick becomes well-defined.
• In libstdc++ versions below 20130531 the constructor taking initializer_list argument is not constexpr. This is because initializer_list operations are not constexpr in C++11. This works however in version 20130531. It is also not enabled for libc++ because I do not have access to it and do nto know if it provides constexpr initializer_list.
• In G++ 4.7.2 and 4.8.0 member function value_or does not have rvalue reference overload. These compilers do not support rvalue overloding on *this.