Return boost streambuf from function - c++

Question

I am trying to wrap up a code that read gz files into a function, source code is taken from https://techoverflow.net/2013/11/03/c-iterating-lines-in-a-gz-file-using-boostiostreams/
My try
boost::iostreams::filtering_streambuf<boost::iostreams::input> func(std::string filename);
boost::iostreams::filtering_streambuf<boost::iostreams::input> func(std::string filename)
{
std::ifstream file(filename, std::ios_base::in | std::ios_base::binary);
boost::iostreams::filtering_streambuf<boost::iostreams::input> inbuf;
inbuf.push(boost::iostreams::gzip_decompressor());
inbuf.push(file);
return inbuf;
}
void mymainfunc(std::string filename)
{
//Convert streambuf to istream
std::istream ifstrm( func( filename));
std::string line;
while(std::getline(ifstrm, line)) {
std::cout << line << std::endl;
}
}
Code runs fine if not run through the function, i am doing something wrong in the return type I think.
Error: https://pastebin.com/kFpjYG0M

Answer 1

Streams are not copyable. In fact, this filteringstreambuf is not even movable.
So in this case, you will want to dynamically allocate and return by smart-pointer. However, even just returning the filtering streambuf will not work, because it would hold a reference to the ifstream. And that's a local.
So, maybe you need to package it up:
Live On Coliru
#include <boost/iostreams/filtering_streambuf.hpp>
#include <boost/iostreams/filter/gzip.hpp>
#include <fstream>
namespace bio = boost::iostreams;
struct MySource {
using fisb = bio::filtering_istreambuf;
struct State {
State(std::string filename) : ifs(filename, std::ios::binary) {
buf.push(bio::gzip_decompressor());
buf.push(ifs);
}
fisb buf;
std::ifstream ifs;
std::istream is { &buf };
};
std::unique_ptr<State> _state;
operator std::istream&() const { return _state->is; }
};
MySource func(std::string filename) {
auto inbuf = std::make_unique<MySource::State>(filename);
return {std::move(inbuf)};
}
#include <iostream>
void mymainfunc(std::string filename)
{
auto source = func(filename);
std::istream& is = source;
std::string line;
while(std::getline(is, line)) {
std::cout << line << std::endl;
}
}
int main(){
mymainfunc("test.cpp.gz");
}
Alternative #1
You can simplify that:
Live On Coliru
struct MySource {
struct State {
State(std::string filename) : ifs(filename, std::ios::binary) {
is.push(bio::gzip_decompressor());
is.push(ifs);
}
std::ifstream ifs;
bio::filtering_istream is;
};
std::unique_ptr<State> _state;
operator std::istream&() const { return _state->is; }
};
Not dealing with the streambuffer separately makes it simpler.
Alternative #2
Not copying the whole thing has its own elegance:
Live On Coliru
#include <boost/iostreams/filtering_stream.hpp>
#include <boost/iostreams/filter/gzip.hpp>
#include <fstream>
#include <iostream>
void mymainfunc(std::istream& is) {
std::string line;
while(std::getline(is, line)) {
std::cout << line << std::endl;
}
}
namespace bio = boost::iostreams;
int main(){
std::ifstream ifs("test.cpp.gz", std::ios::binary);
bio::filtering_istream is;
is.push(bio::gzip_decompressor());
is.push(ifs);
mymainfunc(is);
}