base::ranges

This directory aims to implement a C++14 version of the new std::ranges algorithms that were introduced in C++20. These implementations are added to the ::base::ranges namespace, and callers can access them by including base/ranges/algorithm.h.

Similarities with C++20:

Automatically deducing begin() and end()

As probably one of the most important changes for readability and usability, all algorithms in base::ranges have overloads for ranges of elements, which allow callers to no longer specify begin() and end() iterators themselves.

Before:

bool HasEvens(const std::vector<int>& vec) {
  return std::any_of(vec.begin(), vec.end(), [](int i) { return i % 2 == 0; });
}

After:

bool HasEvens(const std::vector<int>& vec) {
  return base::ranges::any_of(vec, [](int i) { return i % 2 == 0; });
}

Furthermore, these overloads also support binding to temporaries, so that applying algorithms to return values is easier:

std::vector<int> GetNums();

Before:

bool HasEvens() {
  std::vector<int> nums = GetNums();
  return std::any_of(nums.begin(), nums.end(),
                     [](int i) { return i % 2 == 0; });
}

After:

bool HasEvens() {
  return base::ranges::any_of(GetNums(), [](int i) { return i % 2 == 0; });
}

Support for Projections

In addition to supporting automatically deducing the begin() and end() iterator for ranges, the base::ranges:: algorithms also support projections, that can be applied to arguments prior to passing it to supplied transformations or predicates. This is especially useful when ordering a collection of classes by a specific data member of the class. Example:

Before:

std::sort(suggestions->begin(), suggestions->end(),
          [](const autofill::Suggestion& a, const autofill::Suggestion& b) {
            return a.match < b.match;
          });

After:

base::ranges::sort(*suggestions, /*comp=*/{}, &autofill::Suggestion::match);

Anything that is callable can be used as a projection. This includes FunctionObjects like function pointers or functors, but also pointers to member function and pointers to data members, as shown above. When not specified a projection defaults to base::ranges::identity, which simply perfectly forwards its argument.

Projections are supported in both range and iterator-pair overloads of the base::ranges:: algorithms, for example base::ranges::all_of has the following signatures:

template <typename InputIterator, typename Pred, typename Proj = identity>
bool all_of(InputIterator first, InputIterator last, Pred pred, Proj proj = {});

template <typename Range, typename Pred, typename Proj = identity>
bool all_of(Range&& range, Pred pred, Proj proj = {});

Differences from C++20:

To simplify the implementation of the base::ranges:: algorithms, they dispatch to the std:: algorithms found in C++14. This leads to the following list of differences from C++20. Since most of these differences are differences in the library and not in the language, they could be addressed in the future by adding corresponding implementations.

Lack of Constraints

Due to the lack of support for concepts in the language, the algorithms in base::ranges do not have the constraints that are present on the algorithms in std::ranges. Instead, they support any type, much like C++14's std:: algorithms. In the future this might be addressed by adding corresponding constraints via SFINAE, should the need arise.

Lack of Range Primitives

Due to C++14‘s lack of std::ranges concepts like sentinels and other range primitives, algorithms taking a [first, last) pair rather than a complete range, do not support different types for first and last. Since they rely on C++14’s implementation, the type must be the same. This could be addressed in the future by implementing support for sentinel types ourselves.

Lack of constexpr

The base::ranges algorithms can only be used in a constexpr context when they call underlying std:: algorithms that are themselves constexpr. Before C++20, only std::min, std::max and std::minmax are annotated appropriately, so code like constexpr bool foo = base::ranges::any_of(...); will fail because the compiler will not find a constexpr std::any_of. This could be addressed by either upgrading Chromium's STL to C++20, or implementing constexpr versions of some of these algorithms ourselves.

Lack of post C++14 algorithms

Since most algorithms in base::ranges dispatch to their C++14 equivalent, some std:: algorithms that are not present in C++14 have no implementation in base::ranges. This list of algorithms includes the following:

• std::sample (added in C++17)

Return Types

Some of the algorithms in std::ranges:: have different return types than their equivalent in std::. For example, while std::for_each returns the passed-in Function, std::ranges::for_each returns a std::ranges::for_each_result, consisting of the last iterator and the function.

In the cases where the return type differs, base::ranges:: algorithms will continue to return the old return type.

No blocking of ADL

The algorithms defined in std::ranges are not found by ADL, and inhibit ADL when found by unqualified name lookup. This is done to be able to enforce the constraints specified by those algorithms and commonly implemented by using function objects instead of regular functions. Since we don‘t support constrained algorithms yet, we don’t implement the blocking of ADL either.