getline() isn't working in code - c++

Question

Yesterday I started developing a encrypting machine, that works like a Caesar cipher. You put a message (e.g., HELLO), choose a key value (e.g., 3) and the result is KHOOR (3 letters forward).
The problem is that, if I use "cin >> msg;" I can only codify one word. If I use "getline (cin, msg);", the code doesn't work. Maybe it's a simple problem, but I can't solve it... :(
string msg;
int a, b, i, key_value;
char c;
cout << "WRITE YOUR MESSAGE:" << endl;
cin >> msg; //HERE IS THE PROBLEM!!!
system ("cls");
cout << "PUT A KEY VALUE:" << endl;
cin >> key_value;
system ("cls");
cout << "THE CODIFIED MESSAGE IS:" << endl;
for (i=0; i < msg.length(); i++) {
if (msg[i] == 'A') a = 1;
if (msg[i] == 'B') a = 2;
if (msg[i] == 'C') a = 3;
if (msg[i] == 'D') a = 4;
if (msg[i] == 'E') a = 5;
if (msg[i] == 'F') a = 6;
if (msg[i] == 'G') a = 7;
if (msg[i] == 'H') a = 8;
if (msg[i] == 'I') a = 9;
if (msg[i] == 'J') a = 10;
if (msg[i] == 'K') a = 11;
if (msg[i] == 'L') a = 12;
if (msg[i] == 'M') a = 13;
if (msg[i] == 'N') a = 14;
if (msg[i] == 'O') a = 15;
if (msg[i] == 'P') a = 16;
if (msg[i] == 'Q') a = 17;
if (msg[i] == 'R') a = 18;
if (msg[i] == 'S') a = 19;
if (msg[i] == 'T') a = 20;
if (msg[i] == 'U') a = 21;
if (msg[i] == 'V') a = 22;
if (msg[i] == 'W') a = 23;
if (msg[i] == 'X') a = 24;
if (msg[i] == 'Y') a = 25;
if (msg[i] == 'Z') a = 26;
b = a + key_value;
if (b > 26) b -= 26;
if (b == 1) c = 'A';
if (b == 2) c = 'B';
if (b == 3) c = 'C';
if (b == 4) c = 'D';
if (b == 5) c = 'E';
if (b == 6) c = 'F';
if (b == 7) c = 'G';
if (b == 8) c = 'H';
if (b == 9) c = 'I';
if (b == 10) c = 'J';
if (b == 11) c = 'K';
if (b == 12) c = 'L';
if (b == 13) c = 'M';
if (b == 14) c = 'N';
if (b == 15) c = 'O';
if (b == 16) c = 'P';
if (b == 17) c = 'Q';
if (b == 18) c = 'R';
if (b == 19) c = 'S';
if (b == 20) c = 'T';
if (b == 21) c = 'U';
if (b == 22) c = 'V';
if (b == 23) c = 'W';
if (b == 24) c = 'X';
if (b == 25) c = 'Y';
if (b == 26) c = 'Z';
cout << c;
}

Answer 1

So your code works fine: http://ideone.com/lBhD78
If you're trying to accept more than 1 word the simple extraction operator will not work however. Which is probably what you're asking about. In this case you will want to use getline. Like this for instance:
getline(cin, msg, '\n');
So I'm not certain what you're complaining about, however this could be much improved by:
Being case-insensitive
Only modifying alpha-characters
You could accomplish this by doing something like:
transform(cbegin(msg), cend(msg), ostream_iterator<char>(cout), [&](unsigned char i){
if(isalpha(i)) {
const auto a = islower(i) ? 'a' : 'A';
i = (i - a + key_value) % 26 + a;
}
return i; });
I've written a Live Example complete with getline.

Answer 2

You have to be carefull to write questions.
BUT, without being clear what you mean, the answer is:
When you read cin>>s, by default, you are considering the data are separated by "white spaces". So, it skips spaces and read the string until a new space or end of input.
If you have problems with getline, probably the problem is before the code you are shown. If you read a value with, for example, cin>>integer, you push newline key at the end of the input, but this character is still in the stream. If the following line in your code es getline, probably you will read an empty line.
In the first item you find why you current code works, in the second one, why your getline-version doesn't.