ncurses field_buffer returns unchanged initial string

Question

After editing text in field I press Enter, I expected to get edited value, but unchanged string was received. I was tried initialize field by non c_string buffer with spaces at free characters, same results. I have no clue why it happens. Similar questions not found.

Latests Cygwin, Debian, equal results.

#include <unistd.h>
#include <cstring>

#include <curses.h>
#include <form.h>

#include <iostream>
using namespace std;

int main()
{
    string str = "hello";


    initscr();
    noecho();
    keypad(stdscr, TRUE);
    cbreak();

    int field_width = 30;

    FIELD* fields[] =
    {
        new_field( /*H*/1, /*W*/field_width, /*Y*/0,/*X*/0,  0, 0),
        NULL
    };

    set_field_back(fields[0], A_UNDERLINE);
    field_opts_off(fields[0], O_AUTOSKIP);

    FORM* form = new_form( fields );
    post_form( form );

    refresh();

    set_field_buffer( fields[0], 0, str.c_str() );

    form_driver(form,REQ_END_LINE);

    // Loop through to get user requests
    int ch;
    while((ch = getch()) != KEY_F(1))
    {
        switch(ch)
        {
             default:  // paste char
                form_driver(form, ch);
                break;

             case '\n':
             case KEY_ENTER:
             {
                 char * field_buff = field_buffer( fields[0], 0 );
//               Iknow about spaces at the end of line
                 str = field_buff;

                  unpost_form(form);
                  free_form(form);
                  free_field(fields[0]);
                  endwin();
                  //
                  cout << "edited string: " << str << endl;
                  sleep(2);
                  //
                  return 0;
             }
             // end  case Enter
        }
        // end  switch
    }
    // end while
}
// end main

Answer 1

The field_buffer manual page says

The buffer contents are set with set_field_buffer, or as a side effect of any editing operations on the corresponding field.

The editing operations are requests to the form_driver, as listed in that manual page.