<ranges> concepts

Concepts are a C++20 language feature that constrain template parameters at compile time. They help prevent incorrect template instantiation, specify template argument requirements in a readable form, and provide more succinct template related compiler errors.

Consider the following example, which defines a concept to prevent instantiating a template with a type that doesn't support division:

// requires /std:c++20 or later
#include <iostream>

// Definition of dividable concept which requires 
// that arguments a & b of type T support division
template <typename T>
concept dividable = requires (T a, T b)
{
    a / b;
};

// Apply the concept to a template.
// The template will only be instantiated if argument T supports division.
// This prevents the template from being instantiated with types that don't support division.
// This could have been applied to the parameter of a template function, but because
// most of the concepts in the <ranges> library are applied to classes, this form is demonstrated.
template <class T> requires dividable<T>
class DivideEmUp
{
public:
    T Divide(T x, T y)
    {
        return x / y;
    }
};

int main()
{
    DivideEmUp<int> dividerOfInts;
    std::cout << dividerOfInts.Divide(6, 3); // outputs 2
    // The following line will not compile because the template can't be instantiated 
    // with char* because char* can be divided
    DivideEmUp<char*> dividerOfCharPtrs; // compiler error: cannot deduce template arguments 
}

When you pass the compiler switch /diagnostics:caret to Visual Studio 2022 version 17.4 preview 4, or later, the error that concept dividable<char*> evaluated to false will point directly to the expression requirement (a / b) that failed.

Range concepts are defined in the std::ranges namespace, and declared in the <ranges> header file. They're used in the declarations of range adaptors, views, and so on.

There are six categories of ranges. They're related to the categories of iterators listed in <iterator> concepts. In order of increasing capability, the categories are:

Range concept	Description
output_range input_range	Specifies a range that you can write to. Specifies a range that you can read from once.
forward_range	Specifies a range that you can read (and possibly write) multiple times.
bidirectional_range	Specifies a range that you can read and write both forwards and backwards.
random_access_range	Specifies a range that you can read and write by index.
contiguous_range	Specifies a range whose elements are sequential in memory, are the same size, and can be accessed using pointer arithmetic.

In the preceding table, concepts are listed in order of increasing capability. A range that meets the requirements of a concept generally meets the requirements of the concepts in the rows that precede it. For example, a random_access_range has the capability of a bidirectional_range, forward_range, input_range, and output_range. The exception is input_range, which can't be written to, so it doesn't have the capabilities of output_range.

Other range concepts include:

Range concept	Description
rangeC++20	Specifies a type that provides an iterator and a sentinel.
borrowed_rangeC++20	Specifies that the lifetime of the range's iterators aren't tied to the range's lifetime.
common_rangeC++20	Specifies that the type of the range's iterator and the type of the range's sentinel are the same.
Simple_ViewC++20	Not an official concept defined as part of the standard library, but used as a helper concept on some interfaces.
sized_rangeC++20	Specifies a range that can provide its number of elements efficiently.
viewC++20	Specifies a type that has efficient (constant time) move construction, assignment, and destruction.
viewable_rangeC++20	Specifies a type that either is a view or can be converted to one.

bidirectional_range

A bidirectional_range supports reading and writing the range forwards and backwards.

template<class T>
concept bidirectional_range =
    forward_range<T> && bidirectional_iterator<iterator_t<T>>;

Parameters

T
The type to test to see if it's a bidirectional_range.

Remarks

This kind of range supports bidirectional_iterator or greater. A bidirectional_iterator has the capabilities of a forward_iterator, but can also iterate backwards.

Some examples of a bidirectional_range are std::set, std::vector, and std::list.

borrowed_range

A type models borrowed_range if the validity of iterators you get from the object can outlive the lifetime of the object. That is, the iterators for a range can be used even when the range no longer exists.

template<class T>
concept borrowed_range =
    range<T> &&
    (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);

Parameters

T
The type to test to see if it's a borrowed_range.

Remarks

The lifetime of an rvalue range can end following a function call whether the range models borrowed_range or not. If it's a borrowed_range, you may be able to continue to use the iterators with well-defined behavior regardless of when the range's lifetime ends.

Cases where this isn't true are, for example, for containers like vector or list because when the container's lifetime ends, the iterators would refer to elements that have been destroyed.

You can continue to use the iterators for a borrowed_range, for example, for a view like iota_view<int>{0, 42} whose iterators are over set of values that aren't subject to being destroyed because they're generated on demand.

common_range

The type of the iterator for a common_range is the same as the type of the sentinel. That is, begin() and end() return the same type.

template<class T>
concept common_range =
   ranges::range<T> && std::same_as<ranges::iterator_t<T>, ranges::sentinel_t<T>>;

Parameters

T
The type to test to see if it's a common_range.

Remarks

Getting the type from std::ranges::begin() and std::ranges::end() is important for algorithms that calculate the distance between two iterators, and for algorithms that accept ranges denoted by iterator pairs.

The standard containers (for example, vector) meet the requirements of common_range.

contiguous_range

The elements of a contiguous_range are stored sequentially in memory and can be accessed using pointer arithmetic. For example, an array is a contiguous_range.

template<class T>
concept contiguous_range =
    random_access_range<T> && contiguous_iterator<iterator_t<T>> &&
    requires(T& t) {{ ranges::data(t) } -> same_as<add_pointer_t<range_reference_t<T>>>;};

Parameters

T
The type to test to see if it's a contiguous_range.

Remarks

A contiguous_range can be accessed by pointer arithmetic because the elements are laid out sequentially in memory and are the same size. This kind of range supports continguous_iterator, which is the most flexible of all the iterators.

Some examples of a contiguous_range are std::array, std::vector, and std::string.

Example: contiguous_range

The following example shows using pointer arithmetic to access a contiguous_range:

// requires /std:c++20 or later
#include <ranges>
#include <iostream>
#include <vector>

int main()
{
    // Show that vector is a contiguous_range
    std::vector<int> v = {0,1,2,3,4,5};
    std::cout << std::boolalpha << std::ranges::contiguous_range<decltype(v)> << '\n'; // outputs true

    // Show that pointer arithmetic can be used to access the elements of a contiguous_range
    auto ptr = v.data();
    ptr += 2;
    std::cout << *ptr << '\n'; // outputs 2
}

true
2

forward_range

A forward_range supports reading (and possibly writing) the range multiple times.

template<class T>
concept forward_range = input_range<T> && forward_iterator<iterator_t<T>>;

Parameters

T
The type to test to see if it's a forward_range.

Remarks

This kind of range supports forward_iterator or greater. A forward_iterator can iterate over a range multiple times.

input_range

An input_range is a range that can be read from once.

template<class T>
concept input_range = range<T> && input_iterator<iterator_t<T>>;

Parameters

T
The type to test to see if it's an input_range.

Remarks

When a type meets the requirements of input_range:

• The ranges::begin() function returns an input_iterator. Calling begin() more than once on an input_range results in undefined behavior.
• You can dereference an input_iterator repeatedly, which yields the same value each time. An input_range isn't multi-pass. Incrementing an iterator invalidates any copies.
• It can be used with ranges::for_each.
• It supports input_iterator or greater.

output_range

An output_range is a range that you can write to.

template<class R, class T>
concept output_range = range<R> && output_iterator<iterator_t<R>, T>;

Parameters

R
The type of the range.

T
The type of the data to write to the range.

Remarks

The meaning of output_iterator<iterator_t<R>, T> is that the type provides an iterator that can write values of type T to a range of type R. In other words, it supports output_iterator or greater.

random_access_range

A random_access_range can read or write a range by index.

template<class T>
concept random_access_range =
bidirectional_range<T> && random_access_iterator<iterator_t<T>>;

Parameters

T
The type to test to see if it's a sized_range.

Remarks

This kind of range supports random_access_iterator or greater. A random_access_range has the capabilities of an input_range, output_range, forward_range, and bidirectional_range. A random_access_range is sortable.

Some examples of a random_access_range are std::vector, std::array, and std::deque.

range

Defines the requirements a type must meet to be a range. A range provides an iterator and a sentinel, so that you can iterate over its elements.

template<class T>
concept range = requires(T& rg)
{
  ranges::begin(rg);
  ranges::end(rg);
};

Parameters

T
The type to test to see if it's a range.

Remarks

The requirements of a range are:

• It can be iterated using std::ranges::begin() and std::ranges::end()
• ranges::begin() and ranges::end() run in amortized constant time and don't modify the range. Amortized constant time doesn't mean O(1), but that the average cost over a series of calls, even in the worst case, is O(n) rather than O(n^2) or worse.
• [ranges::begin(), ranges::end()) denotes a valid range.

Simple_View

A Simple_View is an exposition-only concept used on some ranges interfaces. It isn't defined in the library. It's only used in the specification to help describe the behavior of some range adaptors.

template<class V>
  concept Simple_View = // exposition only
    ranges::view<V> && ranges::range<const V> &&
    std::same_as<std::ranges::iterator_t<V>, std::ranges::iterator_t<const V>> &&
    std::same_as<std::ranges::sentinel_t<V>, std::ranges::sentinel_t<const V>>;

Parameters

V
The type to test to see if it's a Simple_View.

Remarks

A view V is a Simple_View if all of the following are true:

• V is a view
• const V is a range
• Both v and const V have the same iterator and sentinel types.

sized_range

A sized_range provides the number of elements in the range in amortized constant time.

template<class T>
  concept sized_range = range<T> &&
    requires(T& t) { ranges::size(t); };

Parameters

T
The type to test to see if it's a sized_range.

Remarks

The requirements of a sized_range are that calling ranges::size on it:

• Doesn't modify the range.
• Returns the number of elements in amortized constant time. Amortized constant time doesn't mean O(1), but that the average cost over a series of calls, even in the worst case, is O(n) rather than O(n^2) or worse.

Some examples of a sized_range are std::list and std::vector.

Example: sized_range

The following example shows that a vector of int is a sized_range:

// requires /std:c++20 or later
#include <ranges>
#include <iostream>
#include <vector>

int main()
{
    std::cout << std::boolalpha << std::ranges::sized_range<std::vector<int>> << '\n'; // outputs "true"
}    

view

A view has constant time move construction, assignment, and destruction operations--regardless of the number of elements it has. Views don't need to be copy constructible or copy assignable, but if they are, those operations must also run in constant time.

Because of the constant time requirement, you can efficiently compose views. For example, given a vector of int called input, a function that determines if a number is divisible by three, and a function that squares a number, the statement auto x = input | std::views::filter(divisible_by_three) | std::views::transform(square); efficiently produces a view that contains the squares of the numbers in input that are divisible by three. Connecting views together with | is referred to as composing the views. If a type satisfies the view concept, then it can be composed efficiently.

template<class T>
concept view = ranges::range<T> && std::movable<T> && ranges::enable_view<T>;

Parameters

T
The type to test to see if it's a view.

Remarks

The essential requirement that makes a view composable is that it's cheap to move/copy. This is because the view is moved/copied when it's composed with another view. It must be a movable range.

ranges::enable_view<T> is a trait used to claim conformance to the semantic requirements of the view concept. A type can opt in by:

• publicly and unambiguously deriving from a specialization of ranges::view_interface
• publicly and unambiguously deriving from the empty class ranges::view_base, or
• specializing ranges::enable_view<T> to true

Option 1 is preferred because view_interface also provides default implementation that saves some boilerplate code you have to write.

Failing that, option 2 is a little simpler than option 3.

The advantage of option 3 is that it's possible without changing the definition of the type.

viewable_range

A viewable_range is a type that either is a view or can be converted to one.

template<class T>
  concept viewable_range =
    range<T> && (borrowed_range<T> || view<remove_cvref_t<T>>);

Parameters

T
The type to test to see if it either is a view or can be converted to one.

Remarks

Use std::ranges::views::all() to convert a range to a view.

See also

<ranges>
Range adaptors
View classes