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

From cppreference.com
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 <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.

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 , it cannot use any of inherited member functions.

(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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