如何在 C++ 中使用 fflush(stdin)

Question

I have a simple question (I think so) about fflush() in C++. Every time I write some code to input string in C++, I have to try many many ways because My program caused Error every time. So I will ask a very simple question. My code here:

void main()
{
    int n;
    string str;
    cin >> n;
    fflush(stdin);
    fflush(stdin);
    getline(cin, str);
    cout << n << endl << str << endl;
}

and the compiler does not allow me to enter the string str , how can I do that now?

And I don't want to talk about cin.ignore() here, just about only fflush()

Answer 1

flushing stdin is undefined behavior, since fflush is meant to be called on an output stream. This is taken from the C standard [7.21.5.2]/2 :

 int fflush(FILE *ostream);

If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

you can find the standard C here in the link below, go to page 139

ANSI/ISO 9899-1990