Cin.Ignore(Numeric_Limits<Streamsize>::Max(), '\N')

cin.ignore(numeric_limits streamsize ::max(), '\n')

This line ignores the rest of the current line, up to '\n' or EOF - whichever comes first:

• '\n' sets the delimiter, i.e. the character after which cin stops ignoring
• numeric_limits<streamsize>::max() sets the maximum number of characters to ignore. Since this is the upper limit on the size of a stream, you are effectively telling cin that there is no limit to the number of characters to ignore.

cin.ignore(std::numeric_limits std::streamsize ::max(), '\n'); Gives me an error

You have to add

#include <limits>

to use std::numeric_limits, so add that to the top of your code.

std::cin.ignore(std::numeric_limits std::streamsize ::max(), '\n') error when using #include Windows.h

The <windows.h> header has had the min() and max() macros since time immemorial, and they frequently cause problems with C++. Fortunately, you can disable them by adding #define NOMINMAX before including <windows.h>.

Value of numeric_limits streamsize ::max() in c++

isn't this a very large value exceeding my HDD space.

That's exactly the purpose of this value. You want to skip as many char as possible. In fact, this value does indicate infinite, since the count test is disabled for this value:

count characters were extracted. This test is disabled in the special
case when count equals std::numeric_limits< std::streamsize >::max()

Does cin.ignore with a limit continue discarding new input?

Your code is clearing any error flag and then discarding additional characters until the hard limit of numeric_limits<streamsize>::max() or an EOF character (you can usually enter one with Ctrl+Z) is hit. Therefore it is, as you suspected, continuously discarding your input.

A fixed version of your code could just discard characters until a newline is found

#include <iostream>
#include <limits>

using namespace std;

int main()
{
  int num;

  // Get valid input
  do
  { 
    if (cin.fail())
    {
      // Clear error state and flush any garbage input
      cin.clear();
      cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    cout << "Please enter a number: ";
    cin >> num;

  } while (cin.fail());

  // Echo back what we heard
  cout << "You entered: " << num << "\n";

  cin.get();
}

Live Example

And possibly a more idiomatic and compact version

#include <iostream>
#include <limits>
using namespace std;

int main()
{
  int num;

  // Get valid input
  while( (cout << "Please enter a number: ") && !(cin >> num) )
  {
    cout << "Invalid number entered" << endl;
    // Clear error state and flush any garbage input
    cin.clear();
    cin.ignore(cin.rdbuf()->in_avail());
  }

  // Echo back what we heard
  cout << "You entered: " << num << "\n";

  cin.get();
}

Live Example

As you noted cin.rdbuf()->in_avail() might be an easier solution since it will discard exactly the number of characters available to read in the stream.

cin ws vs cin.ignore(numeric_limits streamsize ::max(), '\n')?

First of all, ignore discards all characters until the delimiter, not only white-space.

Second of all, ignore discards until the specified delimiter character, which could be any character (not only newline) while ws ignore leading space, until there's any non-space character.

Third of all, newline '\n' is a white-space character so would be discarded by the ws manipulator.

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