NanoRange

NanoRange is a C++17 implementation of the C++20 Ranges proposals (formerly the Ranges TS). It provides SFINAE-based implementations of all the proposed Concepts, and constrained and range-based versions the algorithms in the <algorithm> standard library header.

It is intended for users who want range-based goodness in their C++, but don't want to (or can't) use the full-blown Range-V3. It also aims to provide an easy upgrade path to standard ranges when they arrive.

NanoRange is compatible with all three major C++ compilers, including the latest version of Microsoft Visual C++.

Usage

The easiest way to use NanoRange is to simply download the latest, automatically-generated single-header version and include it in your own sources like any other header. This is currently the recommended way to use the library.

Alternatively, you can clone this repository and use the individual headers in the include/nanorange directory. This may give a slight boost to compile times, although there doesn't seem to be too much difference at present. (In any case, the single-header version is similar to what you'll get when you #include <algorithm> in C++20).

If you use Conan you can find the latest testing package at bintray or just use the nanorange/20191001 package in conan-center.

Finally, if you use vcpkg, you can install the latest version of NanoRange from master using

vcpkg install nanorange --head

Compatibility

NanoRange requires a conforming C++17 compiler, and is tested with GCC 7 and Clang 4.0 and newer. Older versions may work in some cases, but this is not guaranteed.

In addition, NanoRange works with MSVC 2017 version 15.9. Note that the /permissive- switch is required for correct two-phase lookup.

Documentation

There is currently no reference documentation for NanoRange itself, but the Ranges TS on which it is based is partially documented on cppreference.

What it provides

Concepts

NanoRange provides all of the concepts from the ranges papers in the form of constexpr bool variable templates. You can use these to constrain your own templates via std::enable_if, or in if constexpr statements in C++17. For example

template <typename Rng>
void foo(Rng&& rng) {
    if constexpr (nano::RandomAccessRange<Rng>) {
         // do something
    } else if constexpr (nano::BidirectionalRange<Rng>>) {
        // do something else
    } else if constexpr (nano::ForwardRange<Rng>) {
        // do a third thing
    }
}

Iterator adaptors

The One Ranges proposal P0896 adds two new iterator adaptors to the standard library, namely common_iterator and counted_iterator, which are both implemented in NanoRange.

Additionally, P0896 modifies the existing iterator adaptors for compatibility with the new concepts. NanoRange therefore provides drop-in replacements for these, specifically:

reverse_iterator
front_insert_iterator
back_insert_iterator
insert_iterator
istream_iterator
ostream_iterator
istreambuf_iterator
ostreambuf_iterator

Lastly, NanoRange provides an implementation of subrange from P0896. This can be used to turn an iterator/sentinel pair into in range, or as a span-like view of a subset of another range.

Algorithms

Range-based overloads

NanoRange provides range-based overloads of all the algorithms in <algorithm>. This means that you can finally say

std::vector<int> vec{5, 4, 3, 2, 1};
nano::sort(vec);

and it will Just Work.

Constraint checking

In the existing STL, the algorithm calls are unconstrained. This means that if you pass an argument which doesn't meet the requirements of the function, the compiler will go ahead and try to instantiate the template anyway, usually resulting in pages of error messages with template backtraces for which C++ is infamous.

For example, the following program has an error: a std::list iterator is not random-access, and so doesn't meet the requirements of std::sort():

int main()
{
    std::list<int> list{5, 4, 3, 2, 1};
    std::sort(list.begin(), list.end());
}

Compiling this two-line example with Clang 6.0 results in over 350 lines of error messages!

Calling nano::sort() instead of std::sort() on the other hand means that constraints are checked before the template is instantated. This time, the entire error output is:

example.cpp:9:5: error: no matching function for call to object of type 'const nano::ranges::detail::sort_fn'
    nano::sort(list.begin(), list.end());
    ^~~~~~~~~~
include/nanorange/algorithm/stl/sort.hpp:26:5: note: candidate template ignored: requirement 'RandomAccessIterator<std::__1::__list_iterator<int, void *> >' was not satisfied [with I = std::__1::__list_iterator<int, void *>, Comp = nano::ranges::less<void>]
    operator()(I first, I last, Comp comp = Comp{}) const
    ^
include/nanorange/algorithm/stl/sort.hpp:37:5: note: candidate template ignored: substitution failure [with Rng = std::__1::__list_iterator<int, void *>, Comp = std::__1::__list_iterator<int, void *>]: no matching function for call to object of type 'const nano::ranges::detail::begin_::fn'
    operator()(Rng&& rng, Comp comp = Comp{}) const
    ^
1 error generated.

which makes it clear that the first overload was rejected because list::iterator doesn't meet the requirements of RandomAccessIterator, and the second overload was rejected because list::iterator is not a range (you can't call nano::begin() on it). Much better.

Function objects

The algorithms in NanoRange are implemented as non-template function objects with templated function call operators. This means that you can pass them around without needing to specify template arguments or wrap them in lambdas. For example:

std::vector<std::vector<int>> vec_of_vecs = ...

nano::for_each(vec_of_vecs, nano::sort); // sorts each inner vector

Projections

The fully reimplemented algorithms (see below) offer support for projections. A projection is a unary callable which modifies ("projects") the view of the data that the algorithm sees. Because projections are routed through (an implementation of) std::invoke(), it's possible to use pointers to member functions and pointers to member data as projections. For example:

struct S {
    int i;
    float f;
};

std::vector<S> vec = ...

auto iter = nano::find(vec, 10, &S::i);
// iter points to the first member of vec for which i == 10

`constexpr` support

In P0896, almost all the algorithms are marked as constexpr (the exceptions being those which can potentially allocate temporary storage). NanoRange fully supports this, meaning the vast majority of algorithms can be called at compile-time. For example

auto sort_copy = [](auto rng) {
    nano::sort(rng);
    return rng;
};

int main()
{
    constexpr std::array a{5, 4, 3, 2, 1};
    constexpr auto b = sort_copy(a);

    static_assert(nano::is_sorted(b));
}

Ranges papers

The Ranges proposals have been consolidated into two main papers:

P0898R3 Standard library concepts
P0896R2 The One Ranges Proposal

NanoRange fully implements the first, and implements most of the second (except for Views).

Stability

NanoRange aims to track the various C++20 ranges proposals, and will be updated as new papers are published. As such, there are no API stability guarantees at this time.

Licence

NanoRange is provided under the Boost licence. See LICENSE_1_0.txt for details.

Acknowledgements

Many thanks to the following:

Eric Niebler and Casey Carter for the Ranges TS, Range-V3 and CMCSTL2. You guys are awesome.
Orson Peters for pdqsort
Phil Nash for the fantastic Catch testing framework
The editors of cppreference.com for painstakingly detailing the existing requirements of standard library algorithms, and more generally for maintaining the C++ programmer's bible.

kqbi/NanoRange