optional

Single header implementation of std::optional with functional-style extensions and support for references.

Clang + GCC: MSVC:

std::optional is the preferred way to represent an object which may or may not have a value. Unfortunately, chaining together many computations which may or may not produce a value can be verbose, as empty-checking code will be mixed in with the actual programming logic. This implementation provides a number of utilities to make coding with optional cleaner.

For example, instead of writing this code:

std::optional<image> get_cute_cat (const image& img) {
    auto cropped = crop_to_cat(img);
    if (!cropped) {
      return std::nullopt;
    }

    auto with_tie = add_bow_tie(*cropped);
    if (!with_tie) {
      return std::nullopt;
    }

    auto with_sparkles = make_eyes_sparkle(*with_tie);
    if (!with_sparkles) {
      return std::nullopt;
    }

    return add_rainbow(make_smaller(*with_sparkles));
}

You can do this:

tl::optional<image> get_cute_cat (const image& img) {
    return crop_to_cat(img)
           .and_then(add_bow_tie)
           .and_then(make_eyes_sparkle)
           .map(make_smaller)
           .map(add_rainbow);
}

The interface is the same as std::optional, but the following member functions are also defined. Explicit types are for clarity.

• map: carries out some operation on the stored object if there is one.
  • tl::optional<std::size_t> s = opt_string.map(&std::string::size);
• and_then: like map, but for operations which return a tl::optional.
  • tl::optional<int> stoi (const std::string& s);
  • tl::optional<int> i = opt_string.and_then(stoi);
• or_else: calls some function if there is no value stored.
  • opt.or_else([] { throw std::runtime_error{"oh no"}; });
• map_or: carries out a map if there is a value, otherwise returns a default value.
  • tl::optional<std::size_t> s = opt_string.map_or(&std::string::size, 0);
• map_or_else: carries out a map if there is a value, otherwise returns the result of a given default function.
  • std::size_t get_default();
  • tl::optional<std::size_t> s = opt_string.map_or_else(&std::string::size, get_default);
• conjunction: returns the argument if a value is stored in the optional, otherwise an empty optional.
  • tl::make_optional(42).conjunction(13); //13
  • tl::optional<int>{}.conjunction(13); //empty
• disjunction: returns the argument if the optional is empty, otherwise the current value.
  • tl::make_optional(42).disjunction(13); //42
  • tl::optional<int>{}.disjunction(13); //13
• take: returns the current value, leaving the optional empty.
  • opt_string.take().map(&std::string::size); //opt_string now empty;

In addition to those member functions, optional references are also supported:

int i = 42;
tl::optional<int&> o = i;
*o == 42; //true
i = 12;
*o == 12; //true
&*o == &i; //true

Assignment has rebind semantics rather than assign-through semantics:

int j = 8;
o = j;

&*o == &j; //true

Compiler support

Tested on:

• Linux
  • clang 6.0.1
  • clang 5.0.2
  • clang 4.0.1
  • clang 3.9
  • clang 3.8
  • clang 3.7
  • clang 3.6
  • clang 3.5
  • g++ 8.0.1
  • g++ 7.3
  • g++ 6.4
  • g++ 5.5
  • g++ 4.9
  • g++ 4.8
• Windows
  • MSVC 2015
  • MSVC 2017

Standards Proposal

This library also serves as an implementation of WG21 standards paper P0798R0: Monadic operations for std::optional. This paper proposes adding map, and_then, and or_else to std::optional.

To the extent possible under law, Sy Brand has waived all copyright and related or neighboring rights to the optional library. This work is published from: United Kingdom.