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std::istreambuf_iterator

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< cpp‎ | iterator
 
 
Iterator library
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(C++20)
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(C++17)(C++20)
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(C++17)
 
 
Defined in header <iterator>
template< class CharT, class Traits = std::char_traits<CharT> >

class istreambuf_iterator
    : public std::iterator<std::input_iterator_tag,
                           CharT, typename Traits::off_type,

                           /* unspecified */, CharT>
(until C++17)
template< class CharT, class Traits = std::char_traits<CharT> >
class istreambuf_iterator;
(since C++17)

std::istreambuf_iterator is a single-pass input iterator that reads successive characters from the std::basic_streambuf object for which it was constructed.

The default-constructed std::istreambuf_iterator is known as the end-of-stream iterator. When a valid std::istreambuf_iterator reaches the end of the underlying stream, it becomes equal to the end-of-stream iterator. Dereferencing or incrementing it further invokes undefined behavior.

std::istreambuf_iterator has a trivial copy constructor, a constexpr default constructor, and a trivial destructor.

(since C++11)

Contents

[edit] Member types

Member type Definition
iterator_category std::input_iterator_tag
value_type CharT
difference_type typename Traits::off_type
pointer /* unspecified */
reference CharT
char_type CharT
traits_type Traits
int_type typename Traits::int_type
streambuf_type std::basic_streambuf<CharT, Traits>
istream_type std::basic_istream<CharT, Traits>
/* proxy */ Implementation-defined class type. The name proxy is for exposition only.
A proxy object holds a char_type character and a streambuf_type* pointer.
Dereferencing a proxy object with operator* yields the stored character.

Member types iterator_category, value_type, difference_type, pointer and reference are required to be obtained by inheriting from std::iterator<std::input_iterator_tag, CharT, typename Traits::off_type, /* unspecified */, CharT>.

(until C++17)

The member type pointer is usually CharT* (see below).

[edit] Member functions

constructs a new istreambuf_iterator
(public member function) [edit]
(destructor)
(implicitly declared)
destructs an istreambuf_iterator
(public member function) [edit]
obtains a copy of the current character
(public member function) [edit]
advances the iterator
(public member function) [edit]
tests if both istreambuf_iterators are end-of-stream or if both are valid
(public member function) [edit]

[edit] Non-member functions

(removed in C++20)
compares two istreambuf_iterators
(function template) [edit]

[edit] Notes

The resolution of LWG issue 659 introduced operator->. It is expected that given an std::istreambuf_iterator i, the expressions (*i).m and i->m have the same effect.

However, the resolution does not provide a formal specification of its behavior. Thus it is implemented differently, including returning nullptr, returning the address of a temporary, or does even provide the member at all. Its intended behavior can hardly be achieved, and it is removed by the resolution of LWG issue 2790.

The resolution of LWG issue 659 also made the member type pointer unspecified in order to allow operator-> to return a proxy. This is to allow operator-> to compile when CharT is not a class type.

[edit] Example

#include <iostream>
#include <iterator>
#include <sstream>
#include <string>
 
int main()
{
    // typical use case: an input stream represented as a pair of iterators
    std::istringstream in{"Hello, world"};
    std::istreambuf_iterator<char> it{in}, end;
    std::string ss{it, end};
    std::cout << "ss has " << ss.size() << " bytes; "
                 "it holds \"" << ss << "\"\n";
 
    // demonstration of the single-pass nature
    std::istringstream s{"abc"};
    std::istreambuf_iterator<char> i1{s}, i2{s};
    std::cout << "i1 returns '" << *i1 << "'\n"
                 "i2 returns '" << *i2 << "'\n";
 
    ++i1;
    std::cout << "after incrementing i1, but not i2:\n"
                 "i1 returns '" << *i1 << "'\n"
                 "i2 returns '" << *i2 << "'\n";
 
    ++i2;
    std::cout << "after incrementing i2, but not i1:\n"
                 "i1 returns '" << *i1 << "'\n"
                 "i2 returns '" << *i2 << "'\n";
}

Output:

ss has 12 bytes; it holds "Hello, world"
i1 returns 'a'
i2 returns 'a'
after incrementing i1, but not i2:
i1 returns 'b'
i2 returns 'b'
after incrementing i2, but not i1:
i1 returns 'c'
i2 returns 'c'

[edit] 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 659 C++98 1. std::istreambuf_iterator did not have operator->
2. the member type pointer was specified as CharT*
1. added
2. made unspecified
LWG 2790 C++98 the operator-> added by LWG issue 659 was not useful removed

[edit] See also

output iterator that writes to std::basic_streambuf
(class template) [edit]
input iterator that reads from std::basic_istream
(class template) [edit]