代码 * 忽略 * 每个字符串后面的第一个字符，因为您 * 告诉它忽略 * 输入中的字符......调用cin.ignore()。该调用不在循环 * 内部 ，而是在循环之前，以便忽略cin >> number提取后留在输入流中的换行符。
注意，对getline的调用不会在流的缓冲区中留下换行符;参见this cppreference page：
......下一个可用输入字符是delim，如Traits::eq(c, delim)所测试的，在这种情况下，从输入中提取定界符字符*，但不将其附加到str。
下面是经过适当修改的代码版本：

#include <string>
#include <iostream>
using std::cin, std::cout;

int main()
{
    std::string sentence;
    int countv = 0, countc = 0, countspace = 0, number, s = 1;
    cout << "How many sentence would you like to check? - ";
    cin >> number;
    cin.ignore(); // Call ignore HERE (once) to skip the newline left in the buffer from the above!

    while (s <= number)
    {
        cout << "\nSentence " << s << ":";
     //  cin.ignore(); WRONG!
        std::getline(cin, sentence);

        for (size_t i = 0; i < sentence.length(); i++)
        {
            if (sentence[i] == 'a' || sentence[i] == 'e' || sentence[i] == 'i' || sentence[i] == 'o' || sentence[i] == 'u')
                countv++;
            else if (isspace(sentence[i]))
                countspace++;
            else
                countc++;
        }
        cout << "\nSentence " << s << " result:";
        cout << "\nThere are " << countv << " vowels in the sentence.";
        cout << "\nThere are " << countc << " consonants in the sentence.";
        cout << "\nThere are " << countspace << " whitespace in the sentence.";
        countc = countv = countspace = 0; // Nicer than the dodgy use of the comma!
        s++;
        cout << "\n";
    }
}