c++ memory allocation howto

I am just starting C++ and I can't understand how my code works:

Ok I allocate memory, but at the time of the allocation nobody knows the size of the memory to be allocated. But still the code works. How much memory is allocated? How the compiler knows how much memory I will need?

EDIT:

Sorry if my question was not clear. Let me please try clarify it. So I dynamically allocate some memory in the heap by using my pointer. But since there is no text in the sting variable, in my opinion it is quite difficult to know how much text (bytes) I will enter via getline.

I tried asking the size of two different text literals, and yes they are different in size.

sizeof("") // is 1 (because of the ending 0 maybe?) sizeof("sometext") // is 9 

But for the string: the sizeof gives me 4 both times. It's clear that the sizeof() gives me the length of the pointer pointing to the string.

How can I allocate memory? If I allocate memory for a new string, only allocates to a pointer pointing to the memory address of the first character in the string? Obviously the characters I enter must be stored somewhere. And I first allocate the memory, and then I load some text into it.

Edit 2: make the edited code to look code, not plain text.

//Edit: string a,b = "sometext"; cout << sizeof(a) << endl; //4 cout << sizeof(b); //4 //-------------------------------------------------------- #include <iostream> #include <string> #include <exception> using namespace std; int main() { //Defining struct struct musicCD { string artist, title; // artist of the CD }; //Memory allocation musicCD *ptr; try{ptr = new musicCD;} catch(bad_alloc){cerr << "Out of memory :(";return -1;} catch(...){cerr << "Something bad happened :O !";return -1; } //Get the data to store: cout << "Please enter the data for the CD!' " << endl; cout << "Please enter the artist: ";getline(cin, ptr->artist); cout << endl; //Write out the data cout << "The data entered: " << endl; cout << "The artist performing is: \t" << ptr->artist << endl; delete ptr; return 0; }