How to validate an integer properly in C++?

Question

I want to validate the input by user to make sure it's an integer (fully). I've tried a few methods, like !cin, but none of them work properly..

Most of methods fail to validate input like this:

32tgf

When there is a number first and then letters, it doesn't fail, but it takes it as valid entry..

Note: It's a project for college and it's specified that the variable should be of type int.

You say you've tried methods... where is the code? How were you unable to find any of the dozens of good existing answers here on StackOverflow? — Ben Voigt, Jan 03 '14 at 17:57
What syntax? Leading '+|-' sign? Does it handle hexadecimal, etc? — Brett Hale, Jan 03 '14 at 18:40

score 3 · Accepted Answer · answered Jan 03 '14 at 17:50

3

Read into a string, then use e.g. std::stoi or std::strtol to both convert to an integer and validate the input.

Or read into a string, put that string in a std::istringstream which you use to extract the integer. Then check if there's anything more in the istringstream.

I'd recommend the first method though.

answered Jan 03 '14 at 17:50

Some programmer dude

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This still won't work for a case of 32tgf. The functions you mention will stop at the first non-digit character. – Thomas Matthews Jan 03 '14 at 17:52
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@ThomasMatthews Yes, but they do have arguments that return the position where it stops. If it's not at the end then you don't have a valid input. – Some programmer dude Jan 03 '14 at 17:53
@Thomas: see http://stackoverflow.com/a/16575025/103167 – Ben Voigt Jan 03 '14 at 17:53
@JoachimPileborg According to the documentation for std::stoi, it takes "31337 with words" as "31337" which is invalid entry in my case. – user2699298 Jan 03 '14 at 17:59
@BenVoigt: see http://en.cppreference.com/w/c/string/byte/strtol, unless you can quote from the C++ standard, any algorithm that stops at the first non-digit character satisfies these functions, but still fails with the test case. – Thomas Matthews Jan 03 '14 at 18:00
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@ThomasMatthews: Look at the CODE I linked you to. It handles the test case perfectly fine. – Ben Voigt Jan 03 '14 at 18:01
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Demo: http://ideone.com/wZ1fuF – Ben Voigt Jan 03 '14 at 18:03
std::strtol is a C++11 function and I can't use C++11 for this exercise.. any alternative options? – user2699298 Jan 03 '14 at 18:10
@user2699298 `std::stoi` is a C++11 function. `std::strtol` is way older. – Some programmer dude Jan 03 '14 at 18:15
@JoachimPileborg That's weird, because when I go to http://www.cplusplus.com/reference/cstdlib/strtoll/ it has a C++11 icon on it. – user2699298 Jan 03 '14 at 18:17
@user2699298 `strtoll` compared to `strtol`. One for `long long`, the other (old one) for `long`. – Some programmer dude Jan 03 '14 at 18:19

How to validate an integer properly in C++?

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