Input validation for integer - c++

Question

I am having trouble with some basic input validation. I have done a lot of searching but cant seem to find the answer that works with my code. I am trying to validate integer input. I am able to test whether a char was entered or not but I need another parameter to test if the number is actually an int and not a double or float.
do
{
cout << "\nHow many numbers would you like to enter?" << endl;
cin >> size;
if (size < 1 || cin.fail())
{
cout << "\nInvalid input" << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
}while(size < 1 || cin.fail());

Answer 1

Too late. By the time operator>> does its job, the horse has left the barn already. You have to figure it out before you actually parse the value.
If you tell operator>> to extract a value into an integer, operator>> will, first, skip any spaces, parse an optional negative sign, then as many digits as it sees. And that's what you will get.
If the last digit is followed by a ".", operator>> doesn't care. It stops, because you told it to parse an integer, and calls it a day.
Similarly, if you tell operator>> to extract a value into one of the floating point types (float, double, long double), operator>> will parse anything that looks like a floating point number, and put it into the floating point type instance. Since an integer looks like a perfectly valid floating point number, either an integer or a floating point number will parse equally, and stored.
You have no indication of what was actually parsed, except the value itself.
So, there are two ways of doing this. One way is to always parse the value into a floating point number, then check if the parsed number is a whole number, or contains any fractional part, then proceed accordingly.
Or, you could parse this yourself. Use get() to parse one character at a time. Ignore any leading whitespace, then collect everything that follows: an optional minus sign, zero or more digits, an optional period followed by zero or more digits, and an optional "e" followed by an exponent.
If you collected a period or an "e", as part of this process, you know that you just parsed a floating point number, otherwise it's going to be an integer. Then, take everything that's been collected, and then parse it, with std::istringstream's assistance, into your selected value.
(Technically, "e" does not necessarily indicate a floating point value; for example 1e3 will happily live inside an int, it's up to you to decide how you wish to handle this case).

Answer 2

I think you could read the input first then do the validation afterward.
If you can use a c11 compiler and the application only wants positive number and do not care symbols e.g "+", "-", you could try something like the following code:
#include <regex>
...
std::regex integers(R"(\d+)");
enum{INPUT_SIZE=256};
char input[INPUT_SIZE];
do
{
memset(input, 0, INPUT_SIZE);
cin >> input;
if(std::regex_match(input, input+strlen(input), integers))
{
cout << input << " is an integer\n";
}
else
{
cout << "input is not an integer";
}
}while(1);