istream from file_descriptor_source (boost::iostreams) or file - c++

Question

I need to do something like this for my program's input:
stream input;
if (decompressed)
input.open(filepath);
else {
file_descriptor=_popen("decompressor "+filepath,"r");
input.open(file_descriptor);
}
input.read(...)
...
I can see one solution - to use _popen in both cases and just copy the file to stdout if it's already decompressed, but this doesn't seem very elegant.
Funny how difficult this is compared with C - I guess the standard library missed it. Now I am lost in the cryptic boost::iostreams documentation. Example code would be great if anyone knows how.

Answer 1

Is this what you're after:
#include <cstdio>
#include <string>
#include <iostream>
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>
namespace io = boost::iostreams;
int main()
{
bool flag = false;
FILE* handle = 0;
if (flag)
{
handle = _popen("dir", "r");
}
else
{
handle = fopen ("main.cpp", "r");
}
io::stream_buffer<io::file_descriptor_source> fpstream (fileno(handle));
std::istream in (&fpstream);
std::string line;
while (in)
{
std::getline (in, line);
std::cout << line << std::endl;
}
return 0;
}

Answer 2

Adding to jon-hanson's answer, here's a simple example demonstrating how to use file_descriptor_source with a pipe.
How to build:
g++ -m32 -DBOOST_IOSTREAMS_NO_LIB -isystem ${BOOST_PATH}/include \
${BOOST_SRC_PATH}/libs/iostreams/src/file_descriptor.cpp blah.cc -o blah
The code:
#include <fcntl.h>
#include <stdio.h>
#include <boost/iostreams/device/file_descriptor.hpp>
#include <boost/iostreams/stream.hpp>
int main( int argc, char* argv[] ) {
// if you just do 'using namespace...', there's a
// namespace collision with the global 'write'
// function used in the child
namespace io = boost::iostreams;
int pipefd[] = {0,0};
pipe( pipefd, 0 ); // If you use O_NONBLOCK, you'll have to
// add some extra checks to the loop so
// it will wait until the child is finished.
if( 0 == fork() ) {
// child
close( pipefd[0] ); // read handle
dup2( pipefd[1], FILENO_STDOUT );
printf( "This\nis\na\ntest\nto\nmake sure that\nit\nis\working as expected.\n" );
return 0; // ya ya, shoot me ;p
}
// parent
close( pipefd[1] ); // write handle
char *buff = new char[1024];
memset( buff, 0, 1024 );
io::stream<io::file_descriptor_source> fds(
io::file_descriptor_source( pipefd[0], io::never_close_handle ) );
// this should work with std::getline as well
while( fds.getline( buff, 1024 )
&& fds.gcount() > 0 // this condition is not enough if you use
// O_NONBLOCK; it should only bail if this
// is false AND the child has exited
) {
printf( "%s,", buff );
}
printf( "\n" );
}