what(）: basic_string::_M_construct 空无效错误

#include<iostream>
#include<string>
using namespace std;
string generateKey(string str, string key)
{
int x = str.size();
for(int i=0; ;i++)
{
if(x==i)
i=0;
if(key.size() == str.size())
break;
key.push_back(key[i]);
}
return key;
}
string cipherText(string str, string key)
{
string cipher_text;
for(int i=0; i<str.size(); i++)
{
int x=(str[i]+key[i])%26;
x+='A';
cipher_text.push_back(x);
}
return cipher_text;
}
string originalText(string cipher_text, string key)
{
string orig_text;
for(int i=0;i<cipher_text.size(); i++)
{
int x=(cipher_text[i]-key[i] +26)%26;
x +='A';
orig_text.push_back(x);
}
return orig_text;
}
int main()
{
int x;
cout<<"Enter your choice"<<endl;
cout<<"1. Mono-alphabeticn2. Poly-alphabeticn";
cin>>x;
switch(x)
{
case 1:
cout<<"Mono-alphabetic cipher"<<endl;
cout<<"Enter your choice n 1.Encryptionn 2.Decryptionn";
int b;
cin>>b;
switch(b)
{
case 1:
int k,i;
char plainText[100];
cout<<"Encryptionn";
cout<<"Enter Plain text"<<endl;
cin>>plainText;
cout<<"Enter Encryption key"<<endl;
cin>>k;
for(i=0;(i<100 && plainText[i]!='');i++)
plainText[i]=plainText[i]+k;
cout<<"n Encrypted text:"<<plainText<<endl;
break;
case 2:
int d;
char encrypedText[100];
cout<<"Decryptionn";
cout<<"Enter Encrypted text"<<endl;
cin>>encrypedText;
cout<<"Enter Decryption key"<<endl;
cin>>d;
for(i=0;(i<100 && encrypedText[i]!='');i++)
encrypedText[i]=encrypedText[i]-d;
cout<<"n Decrypted text:"<<encrypedText<<endl;
break;
default:
cout<<"WRONG Choice Entered!!!n";
break;
}
case 2:
{
cout<<"Poly-alphabetic"<<endl;
string str ="PLANTTHATBOMBTHERE";
string keyword="AMAZON";
string key =generateKey(str,key);
string cipher_text = cipherText(str,key);
cout<<"Ciphertext:"<<cipher_text<<endl;
cout<<"Original text:"<<originalText(cipher_text, key);
break;
break;
}
}
}

此代码用于替换密码，并且多字母密码部分未运行，显示 std：：logic_error 错误和 what((： basic_string：：_M_construct 空无效错误请帮助第一部分运行正常，没有任何错误，但多字母密码部分没有显示任何编译错误，它只显示运行时错误