在C ++中，如何控制cin到cout的流量

Question

Image a console program with a interactive prompt.

A user's command is logically split by semicolon.

Here is the simplified code.

#include <iostream>
#include <string>

using namespace std;

int main() {
  bool exit = false;
  string line;
  string input_str;

  do {
    cout << "propmt> " << flush;

    while (getline(cin, line) && !line.empty()) {
      if (!input_str.empty()) {
        input_str += " ";
      }
      input_str += line;

      auto size = input_str.find_first_of(';');

      // find a semicolon
      if (size != string::npos) {
        /* some code deal with part of string before semicolon */
        cout << "\nsample output\nsample output\nsample output\n" << endl;

        input_str.erase(0, size + 1);
        if (!input_str.empty()) {
          cout << "     -> " << flush;
        } else {
          cout << "propmt> " << flush;
        }
      }
    } // getline loop
  } while (!exit);

  return 0;
}

The problem is when input is like this (Note: only one return key in this input).

And users just copy and paste it into the command line, not input it by hand.

AAAAAAAAAAAAA; BBBBBBBBBB
BBBBBBBBBBBBB;

My program's output is:

propmt> AAAAAAAAAAAAA; BBBBBBBBBB
BBBBBBBBBBBBB;
sample output
sample output
sample output

     -> 

But I want the this part BBBBBBBBBBBBB; showed after my sample output.

It should be like this:

propmt> AAAAAAAAAAAAA; BBBBBBBBBB
sample output
sample output
sample output

     -> BBBBBBBBBBBBB;

And then when user input another Enter key, the program could deal with the whole B command and show the result.

Answer 1

You misunderstand why this happens: data does not flow from cin to cout . You can see this clearly if you run your program with standard input or standard output redirected from/to file.

Instead, the display of user input is a feature provided by the terminal (or whatever) your program is running in. The C++ standard library provides no functionality for controlling this behavior — you will instead need to use a third party library (eg ncurses, or whatever Windows uses) to tell the terminal not to echo user input.