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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.

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:

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.