streambuf - initializing a C++ std::istringstream from an in memory buffer?

Question

I have a memory block (opaque), that I want to store in a Blob in mySQL through their C++ adapter. The adapter expects a istream:

virtual void setBlob(unsigned int parameterIndex, std::istream * blob) = 0;

So my question is: how can I create a std::istream from this memory block (typed as char*). It's not a string as it is not null-terminated (but I know its length of course).

I could not find a way to do it without copying my memory block for example in a std::string. I think this is a bit wasteful. Something like this doesn't work:

    std::streambuf istringbuf(blockPtr, blockLength);
    std::istringstream tmp_blob(&istringbuf);

because std::streambuf doesnt have such a constructor. I saw the following suggestion.

    std:: istringstream       tmp_blob;
    tmp_blob.rdbuf()->pubsetbuf(blockPtr, blockLength);

Is that the correct way?

Answer 1

It's actually pretty trivial to write a one-shot std::streambuf that uses the buffer in place as the default behaviour of all the virtual functions of std::streambuf does 'the right thing'. You can just setg the read area in construction and underflow and uflow can safely be left to return traits_type::eof() as the end of the initial get area is the end of the stream.

e.g.:

#include <streambuf>
#include <iostream>
#include <istream>
#include <ostream>

struct OneShotReadBuf : public std::streambuf
{
    OneShotReadBuf(char* s, std::size_t n)
    {
        setg(s, s, s + n);
    }
};

char hw[] = "Hello, World!
";

int main()
{
    // In this case disregard the null terminator
    OneShotReadBuf osrb(hw, sizeof hw - 1);
    std::istream istr(&osrb);

    istr >> std::cout.rdbuf();
}