When and why do I need to use cin ignore in C

Question

In C++, I developed a simple application that requested the user to enter a number and then a string. 

Surprisingly, when I ran the application, it never paused to ask for the string. 

It simply ignored it. 

After conducting some research on StackOverflow, I discovered that I needed to include the following line:

cin.ignore(256, '\n');

before the line with the string input 

That addressed the problem and allowed the software to run. 

My issue is why C++ need the cin.ignore() line, and how can I forecast when I will need to use it.

Here's the software I created:

#include <iostream>
#include <string>

using namespace std;

int main()
{
    double num;
    string mystr;

    cout << "Please enter a number: " << "\n";
    cin >> num;
    cout << "Your number is: " << num << "\n";
    cin.ignore(256, '\n'); // Why do I need this line?
    cout << "Please enter your name: \n";
    getline (cin, mystr);
    cout << "So your name is " << mystr << "?\n";
    cout << "Have a nice day. \n";

}

Answer 1

Ignore is exactly what it sounds like.

Instead of "throwing away" stuff you don't need, it ignores the number of characters you specify when you call it, up to the breakpoint character you provide.

It supports both input and output buffers.

Essentially, you use ignore before a getline call for std::cin statements because when a user enters anything with std::cin, they click enter and a 'n' char enters the cin buffer.

Then, when you use getline, it returns the newline character rather than the string you desire.

So you execute std::cin.ignore(1000,'n'), which should empty the buffer up to the desired text.

(The 1000 is used to skip a set number of characters before the given break point, which in this case is the n newline character.)