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std::istreambuf_iterator - cppreference.com

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 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.

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>.

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

(destructor)

(implicitly declared)

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.

#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'

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

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