c++ getline函数不允许我输入

Question

所以我有一个回文程序，代码如下:

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

void palindrome();
void compareTwoInt();
bool validation(const string&);

int main()
{
int selection;
cout << "\t\t\t MENU\n";     
cout << "\t\t\t ----\n";
cout << "\t\t\t1. Palindrome";
cout << "\n\t\t\t2. Compare Two Integers";
cout << "\n\t\t\t3. End program\n";
cout << "\n\t\t\tEnter your choice : ";
cin >> selection;

while (selection < 0 || selection > 4)
{
    cout << "\t\t\nInvalid entry. Please enter an appropriate entry.";
    cout << "\n\n \t\t\tEnter your choice: ";
    cin >> selection;
}
if (selection == 1)
{
    cout << "Enter a word, phrase, sentence: \n";
    string input;
    getline(cin, input);

    string input2;
    for (unsigned int i = 0; i < input.length(); i++)
    {
        if (isalnum(input[i]))
        {
            input2 += toupper(input[i]);
        }
    }

    cout << input2 << endl;

    if (validation(input2))
    {
        cout << "The phrase is a palindrome!" << endl;
        cout << "Press <Enter> key back to menu" << endl;

    }

    else
    {
        cout << "The phrase is not a palindrome!" << endl;
        cout << "Press <Enter> key back to menu" << endl;

    }
    fflush(stdin);
    cin.get();
    system("cls");
    return main();

}
else if (selection == 2)
{
    compareTwoInt();
    fflush(stdin);
    system("cls");
    return main();
}
else if (selection == 3)
{
    cout << "\t\t Good Bye. Press <Enter> key to End the program.\n";


}
fflush(stdin);
cin.get();
return 0;
}



void compareTwoInt()
{
int first, second;
cout << "\n\nEnter your positive integer : ";
cin >> first;
cout << "\nEnter your positive integer : ";
cin >> second;


fflush(stdin);
cin.get();


}

bool validation(const string& input)
{
    return input == string(input.rbegin(), input.rend());
}

由于某种原因，当我为回文选择 1 时，它不让我写单词，(换句话说，它不让我输入)

控制台只是说:

输入单词、词组、句子:

这个短语是回文!

按键返回菜单

有人知道如何解决这个问题吗？

提前致谢!

Answer 1

当您为回文选择 1 时，您按下回车键。因此，您的输入由数字 1 和换行符组成。你的cin >> selection;读取数字 1，然后是您的 getline(cin, input);读取换行符，将其解释为空行。您没有编写任何代码来对数字后的换行符输入做任何明智的事情，所以没有任何明智的事情发生。

尝试输入 1foof<enter>反而。您的代码会将其读作 1 后跟包含 foof 的行.