Checking for valid type input using std::cin (C++)

问题

I am working on the game 'Hex' for a C++ class, and am having trouble understanding how to make sure cin is reading the correct type. I have done some research and I'm close, but here is my problem:

    int i, j;
    while(true){
        cin >> i;
        if(cin.fail() ){ //if type wasn't right
            cin.clear(); //clear stream
            cin.ignore(); //ignore left over data
            cout << "First value is invalid input! Valid coordinates are in range 0 to " << size - 1 << endl;
            continue;
        }
        cin >> j;
        if(cin.fail() ){ //if type wasn't right
            cin.clear(); //clear stream
            cin.ignore(); //ignore left over data
            cout << "Second value is invalid input! Valid coordinates are in range 0 to " << size - 1 << endl;
            continue;
        }
        if(i < 0 or i >= size or j < 0 or j >= size){
            //prompt that input is wrong
            cout << "Invalid input! Valid coordinates are in range 0 to " << size - 1 << endl;
            continue;
        }

There are break statements there, it does get out okay. My problem is with runs such as this:

Player 1's turn
Enter the row and column coordinate with only a space inbetween
a b
First value is invalid input! Valid coordinates are in range 0 to 10
First value is invalid input! Valid coordinates are in range 0 to 10
u 8
First value is invalid input! Valid coordinates are in range 0 to 10
8 u

It doesn't detect that the second value was incorrect when they both were, and when only the second is incorrect, it doesn't detect it as an error. I have looked at http://www.cplusplus.com/reference/iostream/cin/?kw=cin and it doesn't even have the documentation for the fail, clear, and ignore functions, so I don't fully understand them. Thanks in advance!

回答1:

u 8
First value is invalid input! Valid coordinates are in range 0 to 10
8 u

As cin >> i; fails because of u, the loop is repeated, but only the first character following in the cin stream, a space, is consumed by cin.ignore();. As the loop starts again, cin >> i; is executed using what's left in the stream, in this case 8. This means that after the error message First value is invalid input! Valid coordinates are in range 0 to 10, the program is actually waiting on cin >> j;. When 8 u is entered, it reads 8 first and assigns its value to j, meaning there is no failure. The loop ends, and u is left in the stream.

To fix this, change cin.ignore(); by cin.ignore(numeric_limits<streamsize>::max(), '\n');. Instead of ignoring only the next character, which is a space in this example, it'll ignore the rest of the line, effectively resetting the cin stream. You'll need to include the <limits> header.

来源：https://stackoverflow.com/questions/16726657/checking-for-valid-type-input-using-stdcin-c

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c++

cin