Proper character by character input? - c++

Question

To better familiarize myself with C++, I'm redoing an old college OS assignment: program your own shell. The first thing I tackled was accepting commands but my approach leaves some features to be desired. Here's what I have:
string GetLine() {
string line;
char input = _getch();
while (input != 13) {
switch (input) {
case 8: // backspace
if (line.length() != 0) {
line = line.substr(0, line.length() - 1);
cout << "\b \b";
}
break;
case -32: // all arrows' first byte
input = _getch(); // distinctly different arrow byte
switch (input) {
case 72: // up
break;
case 80: // down
break;
case 75: // left
break;
case 77: // right
break;
}
break;
case 9: // tab
break;
default:
line += input;
cout << input;
break;
}
input = _getch();
}
cout << endl;
return line;
}
It works pretty well and I know I have more to do with the arrow keys (if I'm even distinguishing between them correctly as is). The main problem I'm asking about is a certain situation where the cursor has wrapped to the next line on the terminal. Aiming to emulate cmd.exe I print a prompt and wait at the end of it for the user's input. When this prompt is long, it only takes a few characters before my command runs across two lines. If I just type it out and let it wrap around and press enter everything works fine. If I wrap to the second line and want to backspace back to the first line, the cursor stops at the first character on it's new line and won't "unwrap" back to the first line as I'd hope. What can I do here to achieve this unwrapping behaviour?
I think the problem is that trying to print '\b' won't unwrap, but I don't have an idea for an alternative and I am hoping that I don't have to treat this as a special case and that code that will backspace in the middle of a line will also work at the beginning of a line.

Answer 1

You need to use the winapi, SetConsoleCursorPosition will help you. Here is a minimal example that moves the cursor up one row from the current position:
#include <iostream>
#include <windows.h>
int main() {
std::cout << "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
"aaaaaaaaaaaaaaa";
HANDLE hStdout = GetStdHandle(STD_OUTPUT_HANDLE);
CONSOLE_SCREEN_BUFFER_INFO csbiInfo;
GetConsoleScreenBufferInfo(hStdout, &csbiInfo);
csbiInfo.dwCursorPosition.Y -= 1; // Move Y coordinate up by one
SetConsoleCursorPosition(hStdout, csbiInfo.dwCursorPosition);
std::cin.get();
}
Also, Console Functions has an entire list of all the necessary functions to control the console. You will want to use these in your program.
You will need to make your own screen buffer with CreateConsoleScreenBuffer and then set it as the buffer first. Then after that handle everything written and read from the console yourself.