Conveniently copy std::vector<unsigned char> to input stream (std::istream) object

Question

I'm trying to make use of a function which comes in a 3rd party lib and expects an input stream object in which binary file data is transported.

Signature looks like that:

doSomething(const std::string& ...,
          const std::string& ...,
          std::istream& aData,
          const std::string& ...,
          const std::map<std::string, std::string>* ...,
          long ...,
          bool ...);

Since I can't alter/change this 3rd party lib/function, I have to adapt in "my" code. In the calling place, I have a std::vector which contains the data which is expected to be passed in an istream object. Currently I copy the vector to the stream, by iterating over it and using the << operator to copy byte by byte.

I strongly suspect that there might be a more effective/convenient way but couldn't find anything useful up to now. Any help/your thoughts are greatly appreciated.

Best, JR

Answer 1

You can use a vector of characters as an underlying buffer for an input stream without copying the contents of the vector:

std::vector<unsigned char> my_vec;
my_vec.push_back('a');
my_vec.push_back('b');
my_vec.push_back('c');
my_vec.push_back('\n');

// make an imput stream from my_vec
std::stringstream is;
is.rdbuf()->pubsetbuf(reinterpret_cast<char*>(&my_vec[0]), my_vec.size());

// dump the input stream into stdout
std::cout << is.rdbuf();

---

@NeilKirk reports that the above method of using pubsetbuf is non-portable.

One portable way is to use boost::iostreams library. This is how to construct an input stream from a vector without copying its contents:

#include <iostream>
#include <vector>

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

int main() {
    std::vector<unsigned char> my_vec;
    my_vec.push_back('a');
    my_vec.push_back('b');
    my_vec.push_back('c');
    my_vec.push_back('\n');

    // Construct an input stream from the vector.
    boost::iostreams::array_source my_vec_source(reinterpret_cast<char*>(&my_vec[0]), my_vec.size());
    boost::iostreams::stream<boost::iostreams::array_source> is(my_vec_source);

    // Dump the input stream into stdout.
    std::cout << is.rdbuf();
}