Why is cin.ignore() necessary when using “getline” after “cin”, but is not required when using “cin” multiple times?

Question

In my knowledge when using getline() after cin, we need to flush the newline character in the buffer first, before calling getline(),

Answer 1

signed main(){
    /*
     * input is of next two lines
     * Stack Overflow<Enter>
     * Where developers learn.<Enter>
     * for more about cin.ignore visit http://www.cplusplus.com/reference/istream/istream/ignore/
     */
    string name, address;
    cin>>name;
    //cin.ignore();
    //cin.ignore(256,'\n');
    getline(cin,address);
    cout << address << endl;
    assert(address == " Overflow"); // breaks if using cin.ignore() or cin.ignore(256,'\n')
    assert(address == "Overflow"); // breaks if using cin.ignore(256,'\n') OR not using any
    assert(address == "Where developers learn."); // breaks if using cin.ignore() or not using ignore
}

enter the input

Stack Overflow

Where developers learn.

Answer 2

Because getline() reads until the next newline character from the given std::istream, while std::istream::operator>>() skips any whitespaces (Spaces, Tabs and newlines).

So when you read an integer or a floating point number, all trailing whitespaces are left in the input stream. When you read from a console or terminal, you type the data and hit Enter, the latter of which is left in the stream, and will be caught by getline() if you don't clear it.

You don't have to clear it because the next time you read a std::string, std::istream::operator>>() skips the whitespaces for you.

---

Consider this code segment;

std::string a, b;
std::cin >> a;
std::getline(std::cin, b);

and this input:

Stack Overflow<Enter>
Where developers learn.<Enter>

The first cin statement will read the word Stack, leaving a space and Overflow<Enter> behind. Then it'll be read by getline, so

assert(b == " Overflow");

If you insert a std::cin.ignore() before calling getline(), it will instead turn into

assert(b == "Where developers learn.");