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optional

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

Documentation Status Clang + GCC: Linux Build Status MSVC: Windows Build Status

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.


CC0

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.

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C++11/14/17 std::optional with functional-style extensions and reference support

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