How to read and modify boost::beast::flat_buffer

I need to read data from boost::beast::flat_buffer into a std::string, then modify the string object and write the result back into boost::beast::flat_buffer.

To convert boost::beast::flat_buffer into a string, I suppose I can use the following code

boost::beast::flat_buffer buf;
// ...
std::string buf_data(boost::asio::buffers_begin(buf.data()), boost::asio::buffers_end(buf.data()));
buf_data = "new data";
// how to change boost::beast::flat_buffer buf;?

Then I need to modify flat_buffer so that its contents are equivalent to the string buf_data. How can this be implemented?

Generically, use asio::buffer_copy:

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#include <boost/asio.hpp>
#include <boost/beast/core.hpp>
namespace asio  = boost::asio;
namespace beast = boost::beast;

int main() {
    beast::flat_buffer buf;

    // read into buf
    std::string buf_data = beast::buffers_to_string(buf.data());

    // something else
    buf_data = "new data";

    buf.clear();
    asio::buffer_copy(buf.data(), asio::buffer(buf_data));
}