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

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< cpp‎ | ranges
C++
Ranges library
Range adaptors
std::ranges::basic_istream_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 >
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 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 type Val can be extracted from lvalue of type 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 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>

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.

Data members

Member Definition
std::basic_istream <CharT, Traits> * stream_ a pointer to the input stream
(exposition-only member object*
Val value_ the stored value
(exposition-only member object*

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
(C++23)
 returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(C++23)
 returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)

Although basic_istream_view is derived from std::ranges::view_interface

(until C++23)

std::ranges::basic_istream_view::basic_istream_view

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

Initializes stream_ with std::addressof(stream) , and value-initializes value_

std::ranges::basic_istream_view::begin

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

Equivalent to * stream_ >> value_ ; return iterator {*this};

std::ranges::basic_istream_view::end

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

Returns std::default_sentinel.

Nested classes

 the iterator type of basic_istream_view
(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
LWG 3568 C++20 P2325R3 accidentally made the stored value default-initialized restored to value-initialization
P2325R3 C++20 default constructor was provided as
view must be default_initializable 		removed along with
the requirement
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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