# std::ranges::views::istream, std::ranges::basic_istream_view, std::ranges::istream_view, std::ranges::wistream_view

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< std::movable Val, class CharT, class Traits = std::char_traits<CharT> > requires std::default_initializable<Val> && /*stream-extractable*/<Val,CharT,Traits> class basic_istream_view : public ranges::view_interface<basic_istream_view<Val,CharT,Traits>> | (1) | (since C++20)
Helper templates |  |
template< class Val > using istream_view = ranges::basic_istream_view<Val, char>; | (2) | (since C++20)
template< class Val > using wistream_view = ranges::basic_istream_view<Val, wchar_t>; | (3) | (since C++20)
Customization point objects |  |
namespace views { template< class T > inline constexpr /*unspecified*/ istream = /*unspecified*/; } | (4) | (since C++20)
Helper concepts |  |
template< class Val, class CharT, class Traits > concept /*stream-extractable*/ = requires(std::basic_istream<CharT,Traits>& is, Val& t) { is >> t; }; | (5) | (exposition only*)

1) A range factory that generates a sequence of elements by repeatedly calling operator>>.
2-3) Convenience alias templates for character types char and wchar_t.
4) views::istream<T>(e) is expression-equivalent to (has the same effect as) ranges::basic_istream_view<T, typename U::char_type, typename U::traits_type>(e) for any suitable subexpressions e, where `U` is std::remove_reference_t<decltype(e)>. The program is ill-formed if `U` is not both publicly and unambiguously derived from std::basic_istream<typename U::char_type, typename U::traits_type>, which may result in a substitution failure.
5) The exposition-only concept /*stream-extractable*/<Val,CharT,Traits> is satisfied when lvalue of `Val` can be extracted from lvalue of std::basic_istream<CharT,Traits>.

The iterator type of `basic_istream_view` is move-only: it does not meet the LegacyIterator requirements, and thus does not work with pre-C++20 algorithms.

### Customization point objects

The name `views::istream<T>` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__istream_fn<T>`.

All instances of `__istream_fn<T>` are equal. The effects of invoking different instances of type `__istream_fn<T>` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `views::istream<T>` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `views::istream<T>` above, `__istream_fn<T>` models

- std::invocable<__istream_fn<T>, Args...>,
- std::invocable<const __istream_fn<T>, Args...>,
- std::invocable<__istream_fn<T>&, Args...>, and
- std::invocable<const __istream_fn<T>&, Args...>.

Otherwise, no function call operator of `__istream_fn<T>` participates in overload resolution.

### Member functions

(constructor) (C++20) | constructs a `basic_istream_view` (public member function)
begin (C++20) | returns an iterator (public member function)
end (C++20) | returns std::default_sentinel (public member function)
##### Inherited from std::ranges::view_interface
(none) | although `basic_istream_view` is derived from std::ranges::view_interface, it cannot use any of inherited member functions.

## std::ranges::basic_istream_view::basic_istream_view

constexpr explicit basic_istream_view( std::basic_istream<CharT, Traits>& stream ); |  | (since C++20)

Initializes the stored pointer to stream with std::addressof(stream), and value-initializes the stored value of `Val`.

## std::ranges::basic_istream_view::begin

constexpr auto begin(); |  | (since C++20)

Equivalent to *stream_ >> value_; return /*iterator*/{*this};, where `stream_` is the stored pointer to stream and `value_` is the stored value of `Val`.

## std::ranges::basic_istream_view::end

constexpr std::default_sentinel_t end() const noexcept; |  | (since C++20)

Equivalent to return std::default_sentinel;.

### Nested classes

iterator (C++20) | the iterator type of `basic_istream_view`, the name is exposition-only (exposition-only member class*)

### Example

Run this code

    #include <algorithm>
    #include <iomanip>
    #include <iostream>
    #include <iterator>
    #include <ranges>
    #include <sstream>
    #include <string>

    int main()
    {
        auto words = std::istringstream{"today is yesterday’s tomorrow"};
        for (const auto& s: std::views::istream<std::string>(words))
            std::cout << std::quoted(s, '/') << ' ';
        std::cout << '\n';

        auto floats = std::istringstream{"1.1  2.2\t3.3\v4.4\f55\n66\r7.7  8.8"};
        std::ranges::copy(
            std::views::istream<float>(floats),
            std::ostream_iterator<float>{std::cout, ", "}
        );
        std::cout << '\n';
    }

Output:

    /today/ /is/ /yesterday’s/ /tomorrow/
    1.1, 2.2, 3.3, 4.4, 55, 66, 7.7, 8.8,

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2325R3 | C++20 | default constructor was provided as `view` must be `default_initializable` | removed along with the requirement
LWG 3568 | C++20 | P2325R3 accidentally made the stored value default-initialized | restored to value-initialization
P2432R1 | C++20 | `ranges::istream_view` was a function template and did not follow the naming convention | made an alias template; customization point objects added

### See also

istream_iterator | input iterator that reads from std::basic_istream (class template)

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