Reindexed View¶

In practice, people often want to work on a selected subset of elements of a sequence. A typical approach is to copy those elements to another container, as

std::vector<int> source{1, 2, 3, 4, 5, 6};
std::vector<size_t> selected_inds{5, 1, 4};

std::vector<int> selected;
selected.reserve(selected_inds.size());
for (size_t i: selected_inds) {
    selected.push_back(source[i]);
}

This approach is cumbersome and inefficient. CLUE++ introduces a class template reindexed_view to tackle this problem. An reindexed view is an object that refers to the selected elements while providing container-like API to work with them. Below is an example:

#include <clue/reindexed_view.hpp>

using namespace clue;

std::vector<int> source{1, 2, 3, 4, 5, 6};
std::vector<size_t> selected_inds{5, 1, 4};

for (auto v: reindexed(source, selected_inds)) {
    // do something on v
}

Below is the documentation of this class template and relevant functions.

The `reindexed_view` class template¶

class reindexed_view¶

Formal:

template<class Container, class Indices>
class reindexed_view;

Parameters:	Container – The type of the element container. Indices – The type of the indices container.

Both Container and Indices need to be random access containers.

Note

Here, the Container type can be a const type, e.g. const vector<int>. Using a constant container type as the template argument would lead to a read-only view, which are often very useful.

Member types¶

The class reindexed_view<Container, Indices> contains a series of member typedefs as follows:

types	definitions
`container_type`	`std::remove_cv<Container>::type`
`indices_type`	`std::remove_cv<Indices>::type`
`value_type`	`container_type::value_type`
`size_type`	`indices_type::size_type`
`difference_type`	`indices_type::difference_type`
`const_reference`	`container_type::const_reference`
`const_pointer`	`container_type::const_pointer`
`const_iterator`	`container_type::const_iterator`

There are also other member typedefs, whose definitions depend on the constness of Container.

type reference¶: Defined as container_type::const_reference when Container is a const type, or container_type::reference otherwise.

type pointer¶: Defined as container_type::const_pointer when Container is a const type, or container_type::pointer otherwise.

type iterator¶: Defined as container_type::const_iterator when Container is a const type, or container_type::iterator otherwise.

Construction¶

constexpr reindexed_view(Container &container, Indices &indices) noexcept¶: Construct a reindexed view, with the given source container and index sequence.

Note

A reindexed view only maintains references to container and indices. It is the caller’s responsibility to ensure that the container and indices remain valid while using the view. Otherwise, undefined behaviors may result.

A convenient function reindexed is provided for creating reindexed views, without requiring the user to explicitly specify the container type and the indices type:

constexpr reindexed_view<Container, Indices> reindexed(Container &c, Indices &inds)¶

Construct a reindexed view, with the given source container and index sequence, where the types Container and Indices are deduced from arguments.

Note:	If `c` is a const reference, then `Container` will be deduced to a const type. The same also applies to `indices`.

Basic properties and element access¶

constexpr bool empty() const noexcept¶: Get whether the view is empty (i.e. contains no selected elements). It is equal to indices.empty().

constexpr size_type size() const noexcept¶: Get the number of selected elements. It is equal to indices.size().

constexpr size_type max_size() const noexcept¶: Get the maximum number of elements that a view can possibly refer to.

constexpr const_reference front() const¶: Get a const reference to the first element within the view.

reference front()¶: Get a reference to the first element within the view.

constexpr const_reference back() const¶: Get a const reference to the last element within the view.

reference back()¶: Get a reference to the last element within the view.

constexpr const_reference operator[](size_type pos) const¶: Get a const reference to the element at position pos, without bounds checking.

reference operator[](size_type pos)¶: Get a reference to the element at position pos, without bounds checking.

constexpr const_reference at(size_type pos) const¶: Get a const reference to the element at position pos, with bounds checking.

reference at(size_type pos)¶: Get a reference to the element at position pos, with bounds checking.

Iterators¶

constexpr const_iterator cbegin() const¶: Get a const iterator to the beginning.

constexpr const_iterator cend() const¶: Get a const iterator to the end.

constexpr const_iterator begin() const¶: Get a const iterator to the beginning, equivalent to cbegin().

constexpr const_iterator end() const¶: Get a const iterator to the end, equivalent to cend().

iterator begin()¶: Get an iterator to the beginning.

iterator end()¶: Get an iterator to the end.