cin缓冲区清理,主体程序被强制结束循环。

在做练习题的时候，准备将两个程序写成函数，再用主程序调用。
通过选择调用函数后，由cin输入字符，完成输出后，缓冲区还存有多余字符（尤其是空字符开头）。
导致主程序调用时判断循环结束，停止程序。

#include <iostream>
#include <cstring>
#include <string>
using namespace std;

void p5_8();
void p5_9();

int main()
{
    int num;
    cout << "code to use: ";
    cin >> num;
    while (num != 0)
    {
        if (num == 1)
            p5_8();
        else if (num == 2)
            p5_9();
        cout << "code to use: ";
        cin >> num;
    }
    return 0;
}

void p5_8()
{
    char word[100];
    int count = 0;
    cout << "Enter words (to stop, type the word done):" << endl;
    cin >> word;

    while (strcmp(word, "done") != 0)
    {
        if (bool(cin >> word) == true)
            count++;
    }

    cout << endl << "You entered a total of " << count << " words." << endl;
        //cin.sync();
}

void p5_9()
{

    string word;
    int count = 0;
    cout << "Enter words (to stop, type the word done):" << endl;
    cin >> word;

    while (word != "done")
    {
        if (bool(cin >> word) == true)
            count++;
    }

    cout << "You entered a total of " << count << " words." << endl;
        //cin.sync();
}

尝试使用sync(),clear(),都未成功。

cin.clear();
cin.sync(); //或者用cin.ignore();

请看代码：

#include <iostream>
#include <cstring>
#include <string>
using namespace std;

void p5_8();
void p5_9();

int main()
{
    int num;
    cout << "code to use: ";
    cin >> num;
    while (num != 0)
    {
        if (num == 1)
            p5_8();
        else if (num == 2)
            p5_9();
        cout << "code to use: ";
        cin >> num;
    }
    return 0;
}

void p5_8()
{
    char word[100];
    int count = 0;
    cout << "Enter words (to stop, type the word done):" << endl;
    cin >> word;

    while (strcmp(word, "done") != 0)
    {
        if (bool(cin >> word) == true)
            count++;
    }

    cout << endl << "You entered a total of " << count << " words." << endl;
    cin.clear();
    cin.sync();
}

void p5_9()
{

    string word;
    int count = 0;
    cout << "Enter words (to stop, type the word done):" << endl;
    cin >> word;

    while (word != "done")
    {
        if (bool(cin >> word) == true)
            count++;
    }

    cout << "You entered a total of " << count << " words." << endl;
    cin.clear();
    cin.sync();
}