setg

// I memory map a file using mapped_file_source from boost which has these two members ...

class mapped_file_source {
    // ...

    size_type size() const;
    const char* data() const;    // ...
};

// I'm trying to put data into an istream so that I can parse the data line by line.

    struct membuf : streambuf
    {
        membuf(char* begin, char* end) {
            this->setg(begin, begin, end);
        }
    };

    // ....

    membuf sbuf(const_cast<char *>(map.data()), map.size());
    istream in(&sbuf);

    while (getline(in, line)) {
        // do stuff w/ line
    }

/*
test.cpp:49:54: error: invalid conversion from ‘boost::iostreams::mapped_file_source::size_type {aka long unsigned int}’ to ‘char*’ [-fpermissive]
  membuf sbuf(const_cast<char *>(map.data()), map.size());
                                                      ^

The problem is that setg takes char *'s and map.size() returns size_type (a.k.a. long unsigned int).

Other thoughts on how to get data into an istream from a 'const char*' that is as efficient as possible?
*/