Can I init a iostream from a buffer without copying it?

Question

I have a buffer unsigned char *buffer filled with size bytes. I wanna init a stream from it.

I can do it as follow, which copys the buffer bytes:

string s(bytes, bytes + size);
stringstream ss(s);

I wonder if I can init a stream without that copy?

why would you want to do that? There's no way to do that AFAIK — Tony The Lion, Mar 10 '13 at 12:11
have you proved that this is a problem in your code? How large is your buffer? — Tony The Lion, Mar 10 '13 at 12:13
The real question is why you're using a character buffer in the first place. If you're writing C++ code, everything should be using the `string` class in the first place, which would solve all of your problems (and more). — Cody Gray - on strike, Mar 10 '13 at 12:13
@CodyGray Because I need to call the third lib to read data from a zip file. — smilingpoplar, Mar 10 '13 at 12:14
@TonyTheLion The copy shouldn't be a problem in my app. I just wonder if there is a better way. — smilingpoplar, Mar 10 '13 at 12:15
To do that, you'll probably have to derive your own `streambuf` class and construct an `istream` from it. — jrok, Mar 10 '13 at 12:35

TAS · Accepted Answer · 2013-03-10T19:04:14.987

3

You need to implement a custom streambuf that you then pass to the istream constructor. This will allow you to access the array data as any other stream. See the following links for more details:

Boost provides the iostreams library for helping with this. In particular, it already provides the array_source class for wrapping standard arrays. The following code sample illustrates how to accomplish this:

#include <cstdlib>
#include <iostream>
#include <string>

#include <boost/iostreams/stream_buffer.hpp>
#include <boost/iostreams/device/array.hpp>


namespace io = boost::iostreams;

int main(int argc, char* argv[]) {
    const char buffer[] = "buffer data\n";

    io::array_source src{ buffer, strlen(buffer) };
    io::stream_buffer<io::array_source> strbuf{ src };

    std::istream stream{ &strbuf };
    std::string line;
    std::getline(stream, line);
    std::cout << line << std::endl;
    return EXIT_SUCCESS;
}

edited Mar 10 '13 at 19:04

answered Mar 10 '13 at 13:08

TAS

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boost::iostreams [copies the data into a buffer](http://www.boost.org/doc/libs/1_55_0/libs/iostreams/doc/concepts/source.html) – Marco Kinski Nov 20 '13 at 14:27
@MarcoKinski Not sure how the link you posted gives this information. Could you provide more detail? – TAS Nov 21 '13 at 06:11
@TAS boost::iostreams::stream_buffer delegates the work to a "Device". "Source" is a "Device" for input purposes and this interface gives you no alternative as to copy the data into a buffer. – Marco Kinski Nov 25 '13 at 12:19

Can I init a iostream from a buffer without copying it?

1 Answers1