How to write to a boost::asio::mutable_buffer?

I have some code that provides me with a pointer to a buffer, and the buffer's size that I need to fill with data. I represent this buffer with a boost::asio::mutable_buffer instance, but how do I properly use this buffer (e.g. write a string to it, ...) and have boost enforce the buffer boundaries?

Here's some pseudo code:

size_t some_callback(void *ptr, size_t) {
    // this function is called by 3rd party
    return our_handler(boost::asio::mutable_buffer(ptr, size));
}

size_t our_handler(const boost::asio::mutable_buffer &buffer)
{
    const std::string test("test");
    // How do I write this string into my buffer?
    return test.size();
}

boost::asio::buffer_cast<>() is what you should use to get access to the pointer used by the buffer. boost::asio::buffer_size() is what you should use to get access to the size used.

e.g.

const std::string test("test");
const size_t len = std::min(boost::asio::buffer_size(mybuf), test.length());
memcpy(boost::asio::buffer_cast<void *>(mybuf),
    test.c_str(),
    len);
const std::string test2("test");
boost::asio::mutable_buffer offset = mybuf + len;
const size_t len2 = std::min(boost::asio::buffer_size(offset), test2.length());
memcpy(boost::asio::buffer_cast<void *>(offset),
    test.c_str(),
    len2);

boost::asio::mutable_buffer offset2 = offset + len2;