Get an istream from a char*

Question

I have a char* and the data length that I'm receiving from a library, and I need to pass the data to a function that takes an istream.

I know I can create a stringstream but that will copy all the data. And also, the data will surely have 0s since it's a zip file, and creating a stringstream will take the data until the first 0 I think.

Is there any way to create an istream from a char* and it's size without copying all the data?

Answer 1

Here's a non-deprecated method found on the web, has you derive your own std::streambuf class, but easy and seems to work:

#include <iostream>
#include <istream>
#include <streambuf>
#include <string>

struct membuf : std::streambuf
{
    membuf(char* begin, char* end) {
        this->setg(begin, begin, end);
    }
};

int main()
{
    char buffer[] = "I'm a buffer with embedded nulls\0and line\n feeds";

    membuf sbuf(buffer, buffer + sizeof(buffer));
    std::istream in(&sbuf);
    std::string line;
    while (std::getline(in, line)) {
        std::cout << "line: " << line << "\n";
    }
    return 0;
}

Which outputs:

line: I'm a buffer with embedded nullsand line
line:  feeds

Answer 2

A non deprecated solution using Boost:

#include <boost/iostreams/stream.hpp>
#include <boost/iostreams/device/array.hpp>
using namespace boost::iostreams;

basic_array_source<char> input_source(my_ptr_to_char, byte_count);
stream<basic_array_source<char> > input_stream(input_source);

or even simpler:

#include <boost/interprocess/streams/bufferstream.hpp>
using namespace boost::interprocess;

bufferstream input_stream(my_ptr_to_char, byte_count);

Answer 3

I needed a solution that supports tellg and seekg and didn't require boost.

char_array_buffer from A beginner's guide to writing a custom stream buffer (std::streambuf) gave a got starting point.

byte_array_buffer.h:

#include <cstdio>
#include <string>
#include <list>
#include <fstream>
#include <iostream>

//
// http://www.mr-edd.co.uk/blog/beginners_guide_streambuf
//

class byte_array_buffer : public std::streambuf
{
public:
    byte_array_buffer(const uint8_t *begin, const size_t size);

private:
    int_type underflow();
    int_type uflow();
    int_type pbackfail(int_type ch);
    std::streamsize showmanyc();
    std::streampos seekoff ( std::streamoff off, std::ios_base::seekdir way,
                            std::ios_base::openmode which = std::ios_base::in | std::ios_base::out );
    std::streampos seekpos ( std::streampos sp,
                            std::ios_base::openmode which = std::ios_base::in | std::ios_base::out);

    // copy ctor and assignment not implemented;
    // copying not allowed
    byte_array_buffer(const byte_array_buffer &);
    byte_array_buffer &operator= (const byte_array_buffer &);

private:
    const uint8_t * const begin_;
    const uint8_t * const end_;
    const uint8_t * current_;
};

byte_array_buffer.cpp:

#include "byte_array_buffer.h"

#include <cassert>


byte_array_buffer::byte_array_buffer(const uint8_t *begin, const size_t size) :
begin_(begin),
end_(begin + size),
current_(begin_)
{
    assert(std::less_equal<const uint8_t *>()(begin_, end_));
}

byte_array_buffer::int_type byte_array_buffer::underflow()
{
    if (current_ == end_)
        return traits_type::eof();

    return traits_type::to_int_type(*current_);
}

byte_array_buffer::int_type byte_array_buffer::uflow()
{
    if (current_ == end_)
        return traits_type::eof();

    return traits_type::to_int_type(*current_++);
}

byte_array_buffer::int_type byte_array_buffer::pbackfail(int_type ch)
{
    if (current_ == begin_ || (ch != traits_type::eof() && ch != current_[-1]))
        return traits_type::eof();

    return traits_type::to_int_type(*--current_);
}

std::streamsize byte_array_buffer::showmanyc()
{
    assert(std::less_equal<const uint8_t *>()(current_, end_));
    return end_ - current_;
}


std::streampos byte_array_buffer::seekoff ( std::streamoff off, std::ios_base::seekdir way,
                                           std::ios_base::openmode which )
{
    if (way == std::ios_base::beg)
    {
        current_ = begin_ + off;
    }
    else if (way == std::ios_base::cur)
    {
        current_ += off;
    }
    else if (way == std::ios_base::end)
    {
        current_ = end_ + off;
    }

    if (current_ < begin_ || current_ > end_)
        return -1;


    return current_ - begin_;
}

std::streampos byte_array_buffer::seekpos ( std::streampos sp,
                                           std::ios_base::openmode which )
{
    current_ = begin_ + sp;

    if (current_ < begin_ || current_ > end_)
        return -1;

    return current_ - begin_;
}

Answer 4

The only (simple) portable way includes making the copy:

std::istringstream ss(std::string(buf,len));

In fact, this is likely to copy the data twice, once to create the string and once to create the istringstream. (Maybe C++11 can avoid one of the copies via a move constructor; I am not sure.)

However, if you are lucky, your C++ implementation will let you do this:

std::istringstream ss;
ss.rdbuf()->pubsetbuf(buf,len);

Under GNU C++ (and, I believe, some other implementations), this will create the stringstream without copying the data. But this is "implementation-defined" behavior according to the spec. (See also this question.)

By including the len parameter, you ensure that both of these will have no problem with null characters.

The only portable way to do what you want is to implement your own subclass of stringbuf and use it to initialize the stringstream. Not for the faint of heart.

Answer 5

An extension to the accepted answer that supports tellg and seekg:

struct membuf : std::streambuf
{
    membuf(char* begin, char* end)
    {
        this->setg(begin, begin, end);
    }

    pos_type seekoff(off_type off, std::ios_base::seekdir dir, std::ios_base::openmode which = std::ios_base::in) override
    {
        if (dir == std::ios_base::cur)
            gbump(off);
        else if (dir == std::ios_base::end)
            setg(eback(), egptr() + off, egptr());
        else if (dir == std::ios_base::beg)
            setg(eback(), eback() + off, egptr());
        return gptr() - eback();
    }

    pos_type seekpos(pos_type sp, std::ios_base::openmode which) override
    {
        return seekoff(sp - pos_type(off_type(0)), std::ios_base::beg, which);
    }
};

Usage of this class stays the same.

Answer 6

Have you tried std::istrstream? http://stdcxx.apache.org/doc/stdlibref/istrstream.html

Technically, I think that it is deprecated, but still part of the standard.

Answer 7

Try the Boost.Iostreams array source and sink classes.

http://www.boost.org/doc/libs/1_47_0/libs/iostreams/doc/index.html