User Manual {#mainpage} =========== \tableofcontents \section tutorial-preface Preface -------------------------------------------- Range library for C++14/17/20. This code is the basis of [the range support in C++20](http://eel.is/c++draft/#ranges). **Development Status:** This code is fairly stable, well-tested, and suitable for casual use, although currently lacking documentation. No promise is made about support or long-term stability. This code *will* evolve without regard to backwards compatibility. A notable exception is anything found within the `ranges::cpp20` namespace. Those components will change rarely or (preferably) never at all. \subsection tutorial-installation Installation -------------------------------------------- This library is header-only. You can get the source code from the [range-v3 repository](https://github.com/ericniebler/range-v3) on github. To compile with Range-v3, just `#include` the individual headers you want. This distribution actually contains three separate header-only libraries: * include/concepts/... contains the Concepts Portability Preprocessor, or CPP, which is a set of macros for defining and using concept checks, regardless of whether your compiler happens to support the C++20 concepts language feature or not. * include/meta/... contains the Meta Library, which is a set of meta-programming utilities for processing types and lists of types at compile time. * include/range/... contains the Range-v3 library, as described below. The Range-v3 library is physically structured in directories by feature group: * include/range/v3/actions/... contains _actions_, or composable components that operate eagerly on containers and return the mutated container for further actions. * include/range/v3/algorithms/... contains all the STL _algorithms_ with overloads that accept ranges, in addition to the familiar overloads that take iterators. * include/range/v3/functional/... contains many generally useful components that would be familiar to functional programmers. * include/range/v3/iterator/... contains the definitions of many useful iterators and iterator-related concepts and utilities. * include/range/v3/numeric/... contains numeric algorithms corresponding to those found in the standard `` header. * include/range/v3/range/... contains range-related utilities, such as `begin`, `end`, and `size`, range traits and concepts, and conversions to containers. * include/range/v3/utility/... contains a miscellaneous assortment of reusable code. * include/range/v3/view/... contains _views_, or composable components that operate lazily on ranges and that themselves return ranges that can be operated upon with additional view adaptors. \subsection tutorial-license License -------------------------------------------- Most of the source code in this project are mine, and those are under the Boost Software License. Parts are taken from Alex Stepanov's Elements of Programming, Howard Hinnant's libc++, and from the SGI STL. Please see the attached LICENSE file and the CREDITS file for the licensing and acknowledgements. \subsection tutorial-compilers Supported Compilers -------------------------------------------------------------------------------- The code is known to work on the following compilers: - clang 5.0 - GCC 6.5 - Clang/LLVM 6 (or later) on Windows - MSVC VS2019, with `/permissive-` and either `/std:c++latest`, `/std:c++20`, or `/std:c++17` \section tutorial-quick-start Quick Start -------------------------------------------------------------------------------- Range-v3 is a generic library that augments the existing standard library with facilities for working with *ranges*. A range can be loosely thought of a pair of iterators, although they need not be implemented that way. Bundling begin/end iterators into a single object brings several benefits: convenience, composability, and correctness. **Convenience** It's more convenient to pass a single range object to an algorithm than separate begin/end iterators. Compare: ~~~~~~~{.cpp} std::vector v{/*...*/}; std::sort( v.begin(), v.end() ); ~~~~~~~ with ~~~~~~~{.cpp} std::vector v{/*...*/}; ranges::sort( v ); ~~~~~~~ Range-v3 contains full implementations of all the standard algorithms with range-based overloads for convenience. **Composability** Having a single range object permits *pipelines* of operations. In a pipeline, a range is lazily adapted or eagerly mutated in some way, with the result immediately available for further adaptation or mutation. Lazy adaption is handled by *views*, and eager mutation is handled by *actions*. For instance, the below uses _views_ to filter a container using a predicate and transform the resulting range with a function. Note that the underlying data is `const` and is not mutated by the views. ~~~~~~~{.cpp} std::vector const vi{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; using namespace ranges; auto rng = vi | views::remove_if([](int i){ return i % 2 == 1; }) | views::transform([](int i){ return std::to_string(i); }); // rng == {"2","4","6","8","10"}; ~~~~~~~ In the code above, `rng` simply stores a reference to the underlying data and the filter and transformation functions. No work is done until `rng` is iterated. In contrast, _actions_ do their work eagerly, but they also compose. Consider the code below, which reads some data into a vector, sorts it, and makes it unique. ~~~~~~~{.cpp} extern std::vector read_data(); using namespace ranges; std::vector vi = read_data() | actions::sort | actions::unique; ~~~~~~~ Unlike views, with actions each step in the pipeline (`actions::sort` and `actions::unique`) accepts a container _by value_, mutates it in place, and returns it. **Correctness** Whether you are using views or actions, you are operating on data in a pure functional, declarative style. You rarely need to trouble yourself with iterators, although they are there under the covers should you need them. By operating declaratively and functionally instead of imperatively, we reduce the need for overt state manipulation and branches and loops. This brings down the number of states your program can be in, which brings down your bug counts. In short, if you can find a way to express your solution as a composition of functional transformations on your data, you can make your code _correct by construction_. \subsection tutorial-views Views -------------------------------------------------------------------------------- As described above, the big advantage of ranges over iterators is their composability. They permit a functional style of programming where data is manipulated by passing it through a series of combinators. In addition, the combinators can be *lazy*, only doing work when the answer is requested, and *purely functional*, without mutating the original data. This makes it easier to reason about your code. A _view_ is a lightweight wrapper that presents a view of an underlying sequence of elements in some custom way without mutating or copying it. Views are cheap to create and copy and have non-owning reference semantics. Below are some examples that use views: Filter a container using a predicate and transform it. ~~~~~~~{.cpp} std::vector const vi{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; using namespace ranges; auto rng = vi | views::remove_if([](int i){return i % 2 == 1;}) | views::transform([](int i){return std::to_string(i);}); // rng == {"2","4","6","8","10"}; ~~~~~~~ Generate an infinite list of integers starting at 1, square them, take the first 10, and sum them: ~~~~~~~{.cpp} using namespace ranges; int sum = accumulate(views::ints(1) | views::transform([](int i){return i*i;}) | views::take(10), 0); ~~~~~~~ Generate a sequence on the fly with a range comprehension and initialize a vector with it: ~~~~~~~{.cpp} using namespace ranges; auto vi = views::for_each(views::ints(1, 10), [](int i) { return yield_from(views::repeat_n(i, i)); }) | to(); // vi == {1,2,2,3,3,3,4,4,4,4,5,5,5,5,5,...} ~~~~~~~ ### View const-ness Logically, a view is a factory for iterators, but in practice a view is often implemented as a state machine, with the state stored within the view object itself (to keep iterators small) and mutated as the view is iterated. Because the view contains mutable state, many views lack a `const`-qualified `begin()`/`end()`. When `const` versions of `begin()`/`end()` are provided, they are truly `const`; that is, thread-safe. Since views present the same interface as containers, the temptation is to think they behave like containers with regard to `const`-ness. This is not the case. Their behavior with regards to `const`-ness is similar to iterators and pointers. The `const`-ness of a view is not related to the `const`-ness of the underlying data. A non-`const` view may refer to elements that are themselves `const`, and _vice versa_. This is analogous to pointers; an `int* const` is a `const` pointer to a mutable `int`, and a `int const*` is a non-`const` pointer to a `const` `int`. Use non-`const` views whenever possible. If you need thread-safety, work with view copies in threads; don't share. ### View validity Any operation on the underlying range that invalidates its iterators or sentinels will also invalidate any view that refers to any part of that range. Additionally, some views (_e.g._, `views::filter`), are invalidated when the underlying elements of the range are mutated. It is best to recreate a view after any operation that may have mutated the underlying range. ### List of range views Below is a list of the lazy range combinators, or views, that Range-v3 provides, and a blurb about how each is intended to be used.
\link ranges::views::addressof_fn `views::addressof`\endlink
Given a source range of lvalue references, return a new view that is the result of taking `std::addressof` of each.
\link ranges::views::adjacent_filter_fn `views::adjacent_filter`\endlink
For each pair of adjacent elements in a source range, evaluate the specified binary predicate. If the predicate evaluates to false, the second element of the pair is removed from the result range; otherwise, it is included. The first element in the source range is always included. (For instance, `adjacent_filter` with `std::not_equal_to` filters out all the non-unique elements.)
\link ranges::views::adjacent_remove_if_fn `views::adjacent_remove_if`\endlink
For each pair of adjacent elements in a source range, evaluate the specified binary predicate. If the predicate evaluates to true, the first element of the pair is removed from the result range; otherwise, it is included. The last element in the source range is always included.
\link ranges::views::all_fn `views::all`\endlink
Return a range containing all the elements in the source. Useful for converting containers to ranges.
\link ranges::any_view `any_view(rng)`\endlink
Type-erased range of elements with value type `T`; can store _any_ range with this value type.
\link ranges::views::c_str_fn `views::c_str`\endlink
View a `\0`-terminated C string (e.g. from a `const char*`) as a range.
\link ranges::views::cache1_fn `views::cache1`\endlink
Caches the most recent element within the view so that dereferencing the view's iterator multiple times doesn't incur any recomputation. This can be useful in adaptor pipelines that include combinations of `view::filter` and `view::transform`, for instance. `views::cache1` is always single-pass.
\link ranges::views::cartesian_product_fn `views::cartesian_product`\endlink
Enumerates the n-ary cartesian product of `n` ranges, i.e., generates all `n`-tuples `(e1, e2, ... , en)` where `e1` is an element of the first range, `e2` is an element of the second range, etc.
\link ranges::views::chunk_fn `views::chunk`\endlink
Given a source range and an integer *N*, produce a range of contiguous ranges where each inner range has *N* contiguous elements. The final range may have fewer than *N* elements.
\link ranges::views::common_fn `views::common`\endlink
Convert the source range to a *common* range, where the type of the `end` is the same as the `begin`. Useful for calling algorithms in the `std::` namespace.
\link ranges::views::concat_fn `views::concat`\endlink
Given *N* source ranges, produce a result range that is the concatenation of all of them.
\link ranges::views::const_fn `views::const_`\endlink
Present a `const` view of a source range.
\link ranges::views::counted_fn `views::counted`\endlink
Given an iterator `it` and a count `n`, create a range that starts at `it` and includes the next `n` elements.
\link ranges::views::cycle_fn `views::cycle`\endlink
Returns an infinite range that endlessly repeats the source range.
\link ranges::views::delimit_fn `views::delimit`\endlink
Given a source range and a value, return a new range that ends either at the end of the source or at the first occurrence of the value, whichever comes first. Alternatively, `views::delimit` can be called with an iterator and a value, in which case it returns a range that starts at the specified position and ends at the first occurrence of the value.
\link ranges::views::drop_fn `views::drop`\endlink
Given a source range and an integral count, return a range consisting of all but the first *count* elements from the source range, or an empty range if it has fewer elements.
\link ranges::views::drop_last_fn `views::drop_last`\endlink
Given a source range and an integral count, return a range consisting of all but the last *count* elements from the source range, or an empty range if it has fewer elements.
\link ranges::views::drop_exactly_fn `views::drop_exactly`\endlink
Given a source range and an integral count, return a range consisting of all but the first *count* elements from the source range. The source range must have at least that many elements.
\link ranges::views::drop_while_fn `views::drop_while`\endlink
Remove elements from the front of a range that satisfy a unary predicate.
\link ranges::views::empty() `views::empty`\endlink
Create an empty range with a given value type.
\link ranges::views::enumerate() `views::enumerate`\endlink
Pair each element of a range with its index.
\link ranges::views::filter_fn `views::filter`\endlink
Given a source range and a unary predicate, filter the elements that satisfy the predicate. (For users of Boost.Range, this is like the `filter` adaptor.)
\link ranges::views::for_each_fn `views::for_each`\endlink
Lazily applies an unary function to each element in the source range that returns another range (possibly empty), flattening the result.
\link ranges::views::generate_fn `views::generate`\endlink
Given a nullary function, return an infinite range whose elements are generated with the function.
\link ranges::views::generate_n_fn `views::generate_n`\endlink
Given a nullary function and a count, return a range that generates the requested number of elements by calling the function.
\link ranges::views::chunk_by_fn `views::chunk_by`\endlink
Given a source range and a binary predicate, return a range of ranges where each range contains contiguous elements from the source range such that the following condition holds: for each element in the range apart from the first, when that element and the previous element are passed to the binary predicate, the result is true. In essence, `views::chunk_by` groups contiguous elements together with a binary predicate.
\link ranges::views::indirect_fn `views::indirect`\endlink
Given a source range of readable values (e.g. pointers or iterators), return a new view that is the result of dereferencing each.
\link ranges::views::intersperse_fn `views::intersperse`\endlink
Given a source range and a value, return a new range where the value is inserted between contiguous elements from the source.
\link ranges::views::ints_fn `views::ints`\endlink
Generate a range of monotonically increasing `int`s. When used without arguments, it generates the quasi-infinite range [0,1,2,3...]. It can also be called with a lower bound, or with a lower and upper bound (exclusive). An inclusive version is provided by `closed_ints`.
\link ranges::views::iota_fn `views::iota`\endlink
A generalization of `views::ints` that generates a sequence of monotonically increasing values of any incrementable type. When specified with a single argument, the result is an infinite range beginning at the specified value. With two arguments, the values are assumed to denote a half-open range.
\link ranges::views::join_fn `views::join`\endlink
Given a range of ranges, join them into a flattened sequence of elements. Optionally, you can specify a value or a range to be inserted between each source range.
\link ranges::views::keys_fn `views::keys`\endlink
Given a range of `pair`s (like a `std::map`), return a new range consisting of just the first element of the `pair`.
\link ranges::views::linear_distribute_fn `views::linear_distribute`\endlink
Distributes `n` values linearly in the closed interval `[from, to]` (the end points are always included). If `from == to`, returns `n`-times `to`, and if `n == 1` it returns `to`.
\link ranges::views::move_fn `views::move`\endlink
Given a source range, return a new range where each element has been cast to an rvalue reference.
\link ranges::views::partial_sum_fn `views::partial_sum`\endlink
Given a range and a binary function, return a new range where the *N*th element is the result of applying the function to the *N*th element from the source range and the (N-1)th element from the result range.
\link ranges::views::remove_fn `views::remove`\endlink
Given a source range and a value, filter out those elements that do not equal value.
\link ranges::views::remove_if_fn `views::remove_if`\endlink
Given a source range and a unary predicate, filter out those elements that do not satisfy the predicate. (For users of Boost.Range, this is like the `filter` adaptor with the predicate negated.)
\link ranges::views::repeat_fn `views::repeat`\endlink
Given a value, create a range that is that value repeated infinitely.
\link ranges::views::repeat_n_fn `views::repeat_n`\endlink
Given a value and a count, create a range that is that value repeated *count* number of times.
\link ranges::views::replace_fn `views::replace`\endlink
Given a source range, a source value and a target value, create a new range where all elements equal to the source value are replaced with the target value.
\link ranges::views::replace_if_fn `views::replace_if`\endlink
Given a source range, a unary predicate and a target value, create a new range where all elements that satisfy the predicate are replaced with the target value.
\link ranges::views::reverse_fn `views::reverse`\endlink
Create a new range that traverses the source range in reverse order.
\link ranges::views::sample_fn `views::sample`\endlink
Returns a random sample of a range of length `size(range)`.
\link ranges::views::single_fn `views::single`\endlink
Given a value, create a range with exactly one element.
\link ranges::views::slice_fn `views::slice`\endlink
Give a source range a lower bound (inclusive) and an upper bound (exclusive), create a new range that begins and ends at the specified offsets. Both the begin and the end can be integers relative to the front, or relative to the end with "`end-2`" syntax.
\link ranges::views::sliding_fn `views::sliding`\endlink
Given a range and a count `n`, place a window over the first `n` elements of the underlying range. Return the contents of that window as the first element of the adapted range, then slide the window forward one element at a time until hitting the end of the underlying range.
\link ranges::views::split_fn `views::split`\endlink
Given a source range and a delimiter specifier, split the source range into a range of ranges using the delimiter specifier to find the boundaries. The delimiter specifier can be an element or a range of elements. The elements matching the delimiter are excluded from the resulting range of ranges.
\link ranges::views::split_when_fn `views::split_when`\endlink
Given a source range and a delimiter specifier, split the source range into a range of ranges using the delimiter specifier to find the boundaries. The delimiter specifier can be a predicate or a function. The predicate should take a single argument of the range's reference type and return `true` if and only if the element is part of a delimiter. The function should accept an iterator and sentinel indicating the current position and end of the source range and return `std::make_pair(true, iterator_past_the_delimiter)` if the current position is a boundary; otherwise `std::make_pair(false, ignored_iterator_value)`. The elements matching the delimiter are excluded from the resulting range of ranges.
\link ranges::views::stride_fn `views::stride`\endlink
Given a source range and an integral stride value, return a range consisting of every *N*th element, starting with the first.
\link ranges::views::tail_fn `views::tail`\endlink
Given a source range, return a new range without the first element. The range must have at least one element.
\link ranges::views::take_fn `views::take`\endlink
Given a source range and an integral count, return a range consisting of the first *count* elements from the source range, or the complete range if it has fewer elements. (The result of `views::take` is not a `sized_range`.)
\link ranges::views::take_exactly_fn `views::take_exactly`\endlink
Given a source range and an integral count, return a range consisting of the first *count* elements from the source range. The source range must have at least that many elements. (The result of `views::take_exactly` is a `sized_range`.)
\link ranges::views::take_last_fn `views::take_last`\endlink
Given a source range and an integral count, return a range consisting of the last *count* elements from the source range. The source range must be a `sized_range`. If the source range does not have at least *count* elements, the full range is returned.
\link ranges::views::take_while_fn `views::take_while`\endlink
Given a source range and a unary predicate, return a new range consisting of the elements from the front that satisfy the predicate.
\link ranges::views::tokenize_fn `views::tokenize`\endlink
Given a source range and optionally a submatch specifier and a `std::regex_constants::match_flag_type`, return a `std::regex_token_iterator` to step through the regex submatches of the source range. The submatch specifier may be either a plain `int`, a `std::vector`, or a `std::initializer_list`.
\link ranges::views::transform_fn `views::transform`\endlink
Given a source range and a unary function, return a new range where each result element is the result of applying the unary function to a source element.
\link ranges::views::trim_fn `views::trim`\endlink
Given a source bidirectional range and a unary predicate, return a new range without the front and back elements that satisfy the predicate.
\link ranges::views::unbounded_fn `views::unbounded`\endlink
Given an iterator, return an infinite range that begins at that position.
\link ranges::views::unique_fn `views::unique`\endlink
Given a range, return a new range where all consecutive elements that compare equal save the first have been filtered out.
\link ranges::views::values_fn `views::values`\endlink
Given a range of `pair`s (like a `std::map`), return a new range consisting of just the second element of the `pair`.
\link ranges::views::zip_fn `views::zip`\endlink
Given *N* ranges, return a new range where *M*th element is the result of calling `make_tuple` on the *M*th elements of all *N* ranges.
\link ranges::views::zip_with_fn `views::zip_with`\endlink
Given *N* ranges and a *N*-ary function, return a new range where *M*th element is the result of calling the function on the *M*th elements of all *N* ranges.
\subsection tutorial-actions Actions -------------------------------------------------------------- When you want to mutate a container in-place, or forward it through a chain of mutating operations, you can use actions. The following examples should make it clear. Read data into a vector, sort it, and make it unique. ~~~~~~~{.cpp} extern std::vector read_data(); using namespace ranges; std::vector vi = read_data() | actions::sort | actions::unique; ~~~~~~~ Do the same to a `vector` that already contains some data: ~~~~~~~{.cpp} vi = std::move(vi) | actions::sort | actions::unique; ~~~~~~~ Mutate the container in-place: ~~~~~~~{.cpp} vi |= actions::sort | actions::unique; ~~~~~~~ Same as above, but with function-call syntax instead of pipe syntax: ~~~~~~~{.cpp} actions::unique(actions::sort(vi)); ~~~~~~~ ### List of range actions Below is a list of the eager range combinators, or actions, that Range-v3 provides, and a blurb about how each is intended to be used.
\link ranges::actions::drop_fn `actions::drop`\endlink
Removes the first `N` elements of the source range.
\link ranges::actions::drop_while_fn `actions::drop_while`\endlink
Removes the first elements of the source range that satisfy the unary predicate.
`actions::erase`
Removes all elements in the sub-range of the source (range version) or all elements after position.
`actions::insert`
Inserts all elements of the range into the source at position.
\link ranges::actions::join_fn `actions::join`\endlink
Flattens a range of ranges.
`actions::push_back`
Appends elements to the tail of the source.
`actions::push_front`
Appends elements before the head of the source.
\link ranges::actions::remove_if_fn `actions::remove_if`\endlink
Removes all elements from the source that satisfy the predicate.
\link ranges::actions::remove_fn `actions::remove`\endlink
Removes all elements from the source that are equal to value.
\link ranges::actions::reverse_fn `actions::reverse`\endlink
Reverses all the elements in the container.
\link ranges::actions::shuffle_fn `actions::shuffle`\endlink
Shuffles the source range.
\link ranges::actions::slice_fn `actions::slice`\endlink
Removes all elements from the source that are not part of the sub-range.
\link ranges::actions::sort_fn `actions::sort`\endlink
Sorts the source range (unstable).
\link ranges::actions::split_fn `actions::split`\endlink
Split a range into a sequence of subranges using a delimiter (a value, a sequence of values, a predicate, or a binary function returning a `pair`).
\link ranges::actions::stable_sort_fn `actions::stable_sort`\endlink
Sorts the source range (stable).
\link ranges::actions::stride_fn `actions::stride`\endlink
Removes all elements whose position does not match the stride.
\link ranges::actions::take_fn `actions::take`\endlink
Keeps the first `N`-th elements of the range, removes the rest.
\link ranges::actions::take_while_fn `actions::take_while`\endlink
Keeps the first elements that satisfy the predicate, removes the rest.
\link ranges::actions::transform_fn `actions::transform`\endlink
Replaces elements of the source with the result of the unary function.
\link ranges::actions::unique_fn `actions::unique`\endlink
Removes adjacent elements of the source that compare equal. If the source is sorted, removes all duplicate elements.
\link ranges::actions::unstable_remove_if_fn `actions::unstable_remove_if`\endlink
Much faster (each element remove has constant time complexity), unordered version of `remove_if`. Requires bidirectional container.
\subsection tutorial-utilities Utilities ---------------------------------------------- Below we cover some utilities that range-v3 provides for creating your own view adaptors and iterators. #### Create Custom Views with view_facade Range-v3 provides a utility for easily creating your own range types, called \link ranges::view_facade `ranges::view_facade`\endlink. The code below uses `view_facade` to create a range that traverses a null-terminated string: ~~~~~~~{.cpp} #include // A range that iterates over all the characters in a // null-terminated string. class c_string_range : public ranges::view_facade { friend ranges::range_access; char const * sz_ = ""; char const & read() const { return *sz_; } bool equal(ranges::default_sentinel_t) const { return *sz_ == '\0'; } void next() { ++sz_; } public: c_string_range() = default; explicit c_string_range(char const *sz) : sz_(sz) { assert(sz != nullptr); } }; ~~~~~~~ The `view_facade` class generates an iterator and begin/end member functions from the minimal interface provided by `c_string_range`. This is an example of a very simple range for which it is not necessary to separate the range itself from the thing that iterates the range. Future examples will show examples of more sophisticated ranges. With `c_string_range`, you can now use algorithms to operate on null-terminated strings, as below: ~~~~~~~{.cpp} #include #include int main() { c_string_range r("hello world"); // Iterate over all the characters and print them out ranges::for_each(r, [](char ch){ std::cout << ch << ' '; }); // prints: h e l l o w o r l d } ~~~~~~~ #### Create Custom Views with view_adaptor Often, a new range type is most easily expressed by adapting an existing range type. That's the case for many of the range views provided by the Range-v3 library; for example, the `views::remove_if` and `views::transform` views. These are rich types with many moving parts, but thanks to a helper class called \link ranges::view_adaptor `ranges::view_adaptor`\endlink, they aren't hard to write. Below in roughly 2 dozen lines of code is the `transform` view, which takes one range and transforms all the elements with a unary function. ~~~~~~~{.cpp} #include #include // A class that adapts an existing range with a function template class transform_view : public ranges::view_adaptor, Rng> { friend ranges::range_access; ranges::semiregular_box_t fun_; // Make Fun model semiregular if it doesn't class adaptor : public ranges::adaptor_base { ranges::semiregular_box_t fun_; public: adaptor() = default; adaptor(ranges::semiregular_box_t const &fun) : fun_(fun) {} // Here is where we apply Fun to the elements: auto read(ranges::iterator_t it) const -> decltype(fun_(*it)) { return fun_(*it); } }; adaptor begin_adaptor() const { return {fun_}; } adaptor end_adaptor() const { return {fun_}; } public: transform_view() = default; transform_view(Rng && rng, Fun fun) : transform_view::view_adaptor{std::forward(rng)} , fun_(std::move(fun)) {} }; template transform_view transform(Rng && rng, Fun fun) { return {std::forward(rng), std::move(fun)}; } ~~~~~~~ Range transformation is achieved by defining a nested `adaptor` class that handles the transformation, and then defining `begin_adaptor` and `end_adaptor` members that return adaptors for the begin iterator and the end sentinel, respectively. The `adaptor` class has a `read` member that performs the transformation. It is passed an iterator to the current element. Other members are available for customization: `equal`, `next`, `prev`, `advance`, and `distance_to`; but the transform adaptor accepts the defaults defined in \link ranges::adaptor_base `ranges::adaptor_base`\endlink. With `transform_view`, we can print out the first 20 squares: ~~~~~~~{.cpp} int main() { auto squares = ::transform(views::ints(1), [](int i){return i*i;}); for(int i : squares | views::take(20)) std::cout << i << ' '; std::cout << '\n'; // prints 1 4 9 16 25 36 49 64 81 100 121 144 169 196 225 256 289 324 361 400 } ~~~~~~~ The `transform_view` defined above is an input range when it is wrapping an input range, a forward range when it's wrapping a forward range, etc. That happens because of smart defaults defined in the `adaptor_base` class that frees you from having to deal with a host of niggly detail when implementing iterators. *(Note: the above `transform_view` always stores a copy of the function in the sentinel. That is only necessary if the underlying range's sentinel type models bidirectional_iterator. That's a finer point that you shouldn't worry about right now.)* ##### view_adaptor in details Each `view_adaptor` contains `base()` member in view and iterator. `base()` - allow to access "adapted" range/iterator: ~~~~~~~{.cpp} std::vector vec; auto list = vec | views::transfom([](int i){ return i+1; }); assert( vec.begin() == list.begin().base() ); assert( vec.begin() == list.base().begin() ); ~~~~~~~ Like `basic_iterator`'s `cursor`, `view_adaptor`'s `adaptor` can contain mixin class too, to inject things into the public interface of the iterator: ~~~~~~~{.cpp} class adaptor : public ranges::adaptor_base { template struct mixin : BaseMixin { // everything inside this class will be accessible from iterator using BaseMixin::BaseMixin; auto& base_value() const { return *this->base(); } int get_i() const { return this->get().i; } }; int i = 100; }; ~~~~~~~ From within mixin you can call: * `get()` - to access adaptor internals * `base()` - to access adaptable iterator Iterator/sentinel adaptor may "override" the following members: ~~~~~~~{.cpp} class adaptor : public ranges::adaptor_base { // !For begin_adaptor only! template constexpr auto begin(Rng &rng) { return ranges::begin(rng.base()); } // !For end_adaptor only! template constexpr auto end(Rng &rng) { return ranges::end(rng.base()); } template bool equal(I const &this_iter, I const &that_iter) const { return this_iter == that_iter; } // or template bool equal(I const &this_iter, I const &that_iter, adaptor const &that_adapt) const { return *this.some_value == that_adapt.some_value && this_iter == that_iter; } // !For end_adaptor only! // Same as equal, but compare iterator with sentinel. // Not used, if iterator same as sentinel, and both have the same adaptor. template constexpr bool empty(I const &it, S const &end) const { return it == end; } // or template constexpr bool empty(I const &it, S const &end, SA const &end_adapt) const { return *this.some_value == end_adapt.some_value && it == end; } template reference_t read(I const &it) { return *it; } template void next(I &it) { ++it; } // !For bidirectional iterator only! template void prev(I &it) { --it; } // !For random access iterator only! template void advance(I &it, difference_type_t n) { it += n; } // !For "sized" iterators only! template difference_type_t distance_to(I const &this_iter, I const &that_iter) { return that_iter - this_iter; } // or template difference_type_t distance_to (I const &this_iter, I const &that_iter, adaptor const &that_adapt) { return that_iter - this_iter; } } ~~~~~~~ As you can see, some "overrides" have effect only for `begin_adaptor` or `end_adaptor`. In order to use full potential of adaptor, you need to have separate adaptors for begin and end: ~~~~~~~{.cpp} struct adaptor : adaptor_base { int n = 0; void next(iterator_t& it) { ++n; ++it; } }; struct sentinel_adaptor : adaptor_base { int stop_at; bool empty(const iterator_t&, const adaptor& ia, const sentinel_t& s) const { return ia.n == stop_at; } }; adaptor begin_adaptor() const { return {}; } sentinel_adaptor end_adaptor() const { return {100}; } ~~~~~~~ Sometimes, you can use the same adaptor for both `begin_adaptor` and `end_adaptor`: ~~~~~~~{.cpp} struct adaptor : adaptor_base { int n = 0; void next(iterator_t& it) { ++n; ++it; } // pay attention, we use equal, not empty. empty() will never trigger. template bool equal(I const &this_iter, I const &that_iter, adaptor const &that_adapt) const { return *this.n == that_adapt.n; } }; adaptor begin_adaptor() const { return {}; } adaptor end_adaptor() const { return {100}; } ~~~~~~~ Note that all the data you store in the adaptor will become part of the iterator. If you will not "override" `begin_adaptor()` or/and `end_adaptor()` in your view_adaptor, default ones will be used. #### Create Custom Iterators with basic_iterator Here is an example of Range-v3 compatible random access proxy iterator. The iterator returns a key/value pair, like the `zip` view. ~~~~~~~{.cpp} #include #include using KeyIter = typename std::vector::iterator; using ValueIter = typename std::vector::iterator; struct cursor { // basic_iterator derives from "mixin", if present, so it can be used // to inject things into the public interface of the iterator struct mixin; // This is for dereference operator. using value_type = std::pair; ranges::common_pair read() const { return { *key_iterator, *value_iterator }; } bool equal(const cursor& other) const { return key_iterator == other.key_iterator; } void next() { ++key_iterator; ++value_iterator; } // prev optional. Required for Bidirectional iterator void prev() { --key_iterator; --value_iterator; } // advance and distance_to are optional. Required for random access iterator void advance(std::ptrdiff_t n) { key_iterator += n; value_iterator += n; } std::ptrdiff_t distance_to(const cursor& other) const { return other.key_iterator - this->key_iterator; } cursor() = default; cursor(KeyIter key_iterator, ValueIter value_iterator) : key_iterator(key_iterator) , value_iterator(value_iterator) {} KeyIter key_iterator; ValueIter value_iterator; }; struct cursor::mixin : ranges::basic_mixin { using ranges::basic_mixin::basic_mixin; // It is necessary to expose constructor in this way mixin(KeyIter key_iterator, ValueIter value_iterator) : mixin{ cursor(key_iterator, value_iterator) } {} KeyIter key_iterator() { return this->get().key_iterator; } ValueIter value_iterator() { return this->get().value_iterator; } }; using iterator = ranges::basic_iterator; void test() { std::vector keys = {1}; std::vector values = {10}; iterator iter(keys.begin(), values.begin()); ranges::common_pair pair = *iter; Key& key = pair.first; Value& value = pair.second; assert(&key == &keys[0]); assert(&value == &values[0]); auto key_iter = iter.key_iterator(); assert(key_iter == keys.begin()); } ~~~~~~~ `read()` returns references. But the default for `value_type`, which is `decay_t`, is `common_pair`. That is not correct in our case. It should be `pair`, so we explicitly specify `value_type`. `ranges::common_pair` has conversions: > `ranges::common_pair` ↔ `ranges::common_pair`. All `ranges::common_pair`s converts to their `std::pair` equivalents, also. For more information, see [http://wg21.link/P0186#basic-iterators-iterators.basic](http://wg21.link/P0186#basic-iterators-iterators.basic) \subsection tutorial-concepts Concept Checking -------------------------------------------------------------------------------- The Range-v3 library makes heavy use of concepts to constrain functions, control overloading, and check type constraints at compile-time. It achieves this with the help of a Concepts emulation layer that works on any standard-conforming C++14 compiler. The library provides many useful concepts, both for the core language and for iterators and ranges. You can use the concepts framework to constrain your own code. For instance, if you would like to write a function that takes an iterator/sentinel pair, you can write it like this: ~~~~~~~{.cpp} CPP_template(class Iter, class Sent, class Comp = /*...some_default..*/) (requires sentinel_for) void my_algorithm(Iter first, Sent last, Comp comp = Comp{}) { // ... } ~~~~~~~ You can then add an overload that take a Range: ~~~~~~~{.cpp} CPP_template(class Rng, class Comp = /*...some_default..*/) (requires range) void my_algorithm(Rng && rng, Comp comp = Comp{}) { return my_algorithm(ranges::begin(rng), ranges::end(rng)); } ~~~~~~~ With the type constraints expressed with the `CPP_template` macro, these two overloads are guaranteed to not be ambiguous. When compiling with C++20 concepts support, this uses real concept checks. On legacy compilers, it falls back to using `std::enable_if`. \subsection tutorial-future Range-v3 and the Future -------------------------------------------------------------------------------- Range-v3 formed the basis for the [Technical Specification on Ranges](https://www.iso.org/standard/70910.html), which has since been merged into the working draft and shipped with C++20 in the `std::ranges` namespace. More range adaptors are slated for inclusion in C++23 and beyond. The actions, as well as various utilities, have not yet been reviewed by the Committee, although the basic direction has already passed an initial review.