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

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

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

Member functions

constructs a basic_istream_view
(public member function)
returns an iterator
(public member function)
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

(C++20)
the iterator type of basic_istream_view
(exposition-only member class*)

Example

#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

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

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