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(&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

Dependencies

Requires Standardese for generating documentation.

Requires Catch for testing. This is bundled in the test directory.

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.

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