7.16 — std::cin and handling invalid input

Most programs that have a user interface of some kind need to handle user input. In the programs that you have been writing, you have been using std::cin to ask the user to enter text input. Because text input is so free-form (the user can enter anything), it’s very easy for the user to enter input that is not expected.

As you write programs, you should always consider how users will (unintentionally or otherwise) misuse your programs. A well-written program will anticipate how users will misuse it, and either handle those cases gracefully or prevent them from happening in the first place (if possible). A program that handles error cases well is said to be robust.

In this lesson, we’ll take a look specifically at ways the user can enter invalid text input via std::cin, and show you some different ways to handle those cases.

std::cin, buffers, and extraction

In order to discuss how std::cin and operator>> can fail, it first helps to know a little bit about how they work.

When we use operator>> to get user input and put it into a variable, this is called an “extraction”. The >> operator is accordingly called the extraction operator when used in this context.

When the user enters input in response to an extraction operation, that data is placed in a buffer inside of std::cin. A buffer (also called a data buffer) is simply a piece of memory set aside for storing data temporarily while it’s moved from one place to another. In this case, the buffer is used to hold user input while it’s waiting to be extracted to variables.

When the extraction operator is used, the following procedure happens:

  • If there is data already in the input buffer, that data is used for extraction.
  • If the input buffer contains no data, the user is asked to input data for extraction (this is the case most of the time). When the user hits enter, a ‘\n’ character will be placed in the input buffer.
  • operator>> extracts as much data from the input buffer as it can into the variable (ignoring any leading whitespace characters, such as spaces, tabs, or ‘\n’).
  • Any data that can not be extracted is left in the input buffer for the next extraction.

Extraction succeeds if at least one character is extracted from the input buffer. Any unextracted input is left in the input buffer for future extractions. For example:

If the user enters “5a”, 5 will be extracted, converted to an integer, and assigned to variable x. “a\n” will be left in the input stream for the next extraction.

Extraction fails if the input data does not match the type of the variable being extracted to. For example:

If the user were to enter ‘b’, extraction would fail because ‘b’ can not be extracted to an integer variable.

Validating input

The process of checking whether user input conforms to what the program is expecting is called input validation.

There are three basic ways to do input validation:

  • Inline (as the user types)
    • Prevent the user from typing invalid input in the first place.
  • Post-entry (after the user types)
    • Let the user enter whatever they want into a string, then validate whether the string is correct, and if so, convert the string to the final variable format.
    • Let the user enter whatever they want, let std::cin and operator>> try to extract it, and handle the error cases.

Some graphical user interfaces and advanced text interfaces will let you validate input as the user enters it (character by character). Generally speaking, the programmer provides a validation function that accepts the input the user has entered so far, and returns true if the input is valid, and false otherwise. This function is called every time the user presses a key. If the validation function returns true, the key the user just pressed is accepted. If the validation function returns false, the character the user just input is discarded (and not shown on the screen). Using this method, you can ensure that any input the user enters is guaranteed to be valid, because any invalid keystrokes are discovered and discarded immediately. Unfortunately, std::cin does not support this style of validation.

Since strings do not have any restrictions on what characters can be entered, extraction is guaranteed to succeed (though remember that std::cin stops extracting at the first non-leading whitespace character). Once a string is entered, the program can then parse the string to see if it is valid or not. However, parsing strings and converting string input to other types (e.g. numbers) can be challenging, so this is only done in rare cases.

Most often, we let std::cin and the extraction operator do the hard work. Under this method, we let the user enter whatever they want, have std::cin and operator>> try to extract it, and deal with the fallout if it fails. This is the easiest method, and the one we’ll talk more about below.

A sample program

Consider the following calculator program that has no error handling:

This simple program asks the user to enter two numbers and a mathematical operator.

Enter a double value: 5
Enter one of the following: +, -, *, or /: *
Enter a double value: 7
5 * 7 is 35

Now, consider where invalid user input might break this program.

First, we ask the user to enter some numbers. What if they enter something other than a number (e.g. ‘q’)? In this case, extraction will fail.

Second, we ask the user to enter one of four possible symbols. What if they enter a character other than one of the symbols we’re expecting? We’ll be able to extract the input, but we don’t currently handle what happens afterward.

Third, what if we ask the user to enter a symbol and they enter a string like “*q hello”. Although we can extract the ‘*’ character we need, there’s additional input left in the buffer that could cause problems down the road.

Types of invalid text input

We can generally separate input text errors into four types:

  • Input extraction succeeds but the input is meaningless to the program (e.g. entering ‘k’ as your mathematical operator).
  • Input extraction succeeds but the user enters additional input (e.g. entering ‘*q hello’ as your mathematical operator).
  • Input extraction fails (e.g. trying to enter ‘q’ into a numeric input).
  • Input extraction succeeds but the user overflows a numeric value.

Thus, to make our programs robust, whenever we ask the user for input, we ideally should determine whether each of the above can possibly occur, and if so, write code to handle those cases.

Let’s dig into each of these cases, and how to handle them using std::cin.

Error case 1: Extraction succeeds but input is meaningless

This is the simplest case. Consider the following execution of the above program:

Enter a double value: 5
Enter one of the following: +, -, *, or /: k
Enter a double value: 7

In this case, we asked the user to enter one of four symbols, but they entered ‘k’ instead. ‘k’ is a valid character, so std::cin happily extracts it to variable op, and this gets returned to main. But our program wasn’t expecting this to happen, so it doesn’t properly deal with this case (and thus never outputs anything).

The solution here is simple: do input validation. This usually consists of 3 steps:

1) Check whether the user’s input was what you were expecting.
2) If so, return the value to the caller.
3) If not, tell the user something went wrong and have them try again.

Here’s an updated getOperator() function that does input validation.

As you can see, we’re using a while loop to continuously loop until the user provides valid input. If they don’t, we ask them to try again until they either give us valid input, shutdown the program, or destroy their computer.

Error case 2: Extraction succeeds but with extraneous input

Consider the following execution of the above program:

Enter a double value: 5*7

What do you think happens next?

Enter a double value: 5*7
Enter one of the following: +, -, *, or /: Enter a double value: 5 * 7 is 35

The program prints the right answer, but the formatting is all messed up. Let’s take a closer look at why.

When the user enters “5*7” as input, that input goes into the buffer. Then operator>> extracts the 5 to variable x, leaving “*7\n” in the buffer. Next, the program prints “Enter one of the following: +, -, *, or /:”. However, when the extraction operator was called, it sees “*7\n” waiting in the buffer to be extracted, so it uses that instead of asking the user for more input. Consequently, it extracts the ‘*’ character, leaving “7\n” in the buffer.

After asking the user to enter another double value, the “7” in the buffer gets extracted without asking the user. Since the user never had an opportunity to enter additional data and hit enter (causing a newline), the output prompts all get run together on the same line, even though the output is correct.

Although the above problem works, the execution is messy. It would be better if any extraneous characters entered were simply ignored. Fortunately, it’s easy to ignore characters:

This call would remove up to 100 characters, but if the user entered more than 100 characters we’ll get messy output again. To ignore all characters up to the next ‘\n’, we can pass std::numeric_limits<std::streamsize>::max() to std::cin.ignore(). std::numeric_limits<std::streamsize>::max() returns the largest value that can be stored in a variable of type std::streamsize. Passing this value to std::cin.ignore() causes it to disable the count check.

To ignore everything up to and including the next ‘\n’ character, we call

Because this line is quite long for what it does, it’s handy to wrap it in a function which can be called in place of std::cin.ignore().

Since the last character the user entered must be a ‘\n’, we can tell std::cin to ignore buffered characters until it finds a newline character (which is removed as well).

Let’s update our getDouble() function to ignore any extraneous input:

Now our program will work as expected, even if we enter “5*7” for the first input -- the 5 will be extracted, and the rest of the characters will be removed from the input buffer. Since the input buffer is now empty, the user will be properly asked for input the next time an extraction operation is performed!

Author's note

Some lessons still pass 32767 to std::cin.ignore(). This is a magic number with no special meaning to std::cin.ignore() and should be avoided. If you see such an occurrence, feel free to point it out.

Error case 3: Extraction fails

Now consider the following execution of the calculator program:

Enter a double value: a

You shouldn’t be surprised that the program doesn’t perform as expected, but how it fails is interesting:

Enter a double value: a
Enter one of the following: +, -, *, or /: Enter a double value: 

and the program suddenly ends.

This looks pretty similar to the extraneous input case, but it’s a little different. Let’s take a closer look.

When the user enters ‘a’, that character is placed in the buffer. Then operator>> tries to extract ‘a’ to variable x, which is of type double. Since ‘a’ can’t be converted to a double, operator>> can’t do the extraction. Two things happen at this point: ‘a’ is left in the buffer, and std::cin goes into “failure mode”.

Once in ‘failure mode’, future requests for input extraction will silently fail. Thus in our calculator program, the output prompts still print, but any requests for further extraction are ignored. The program simply runs to the end and then terminates (without printing a result, because we never read in a valid mathematical operation).

Fortunately, we can detect whether an extraction has failed and fix it:

That’s it!

Let’s integrate that into our getDouble() function:

Note: Prior to C++11, a failed extraction would not modify the variable being extracted to. This means that if a variable was uninitialized, it would stay uninitialized in the failed extraction case. However, as of C++11, a failed extraction due to invalid input will cause the variable to be zero-initialized. Zero initialization means the variable is set to 0, 0.0, “”, or whatever value 0 converts to for that type.

Error case 4: Extraction succeeds but the user overflows a numeric value

Consider the following simple example:

What happens if the user enters a number that is too large (e.g. 40000)?

Enter a number between -32768 and 32767: 40000
Enter another number between -32768 and 32767: The sum is: 32767

In the above case, std::cin goes immediately into “failure mode”, but also assigns the closest in-range value to the variable. Consequently, x is left with the assigned value of 32767. Additional inputs are skipped, leaving y with the initialized value of 0. We can handle this kind of error in the same way as a failed extraction.

Note: Prior to C++11, a failed extraction would not modify the variable being extracted to. This means that if a variable was uninitialized, it would stay uninitialized in the failed extraction case. However, as of C++11, an out-of-range failed extraction will cause the variable to be set to the closest in-range value.

Putting it all together

Here’s our example calculator with full error checking:


As you write your programs, consider how users will misuse your program, especially around text input. For each point of text input, consider:

  • Could extraction fail?
  • Could the user enter more input than expected?
  • Could the user enter meaningless input?
  • Could the user overflow an input?

You can use if statements and boolean logic to test whether input is expected and meaningful.

The following code will clear any extraneous input:

The following code will test for and fix failed extractions or overflow:

Finally, use loops to ask the user to re-enter input if the original input was invalid.

Author's note

Input validation is important and useful, but it also tends to make examples more complicated and harder to follow. Accordingly, in future lessons, we will generally not do any kind of input validation unless it’s relevant to something we’re trying to teach.

7.17 -- Assert and static_assert
7.15 -- Detecting and handling errors

260 comments to 7.16 — std::cin and handling invalid input

  • Forhad Rahman

    Can anyone kindly elaborate a bit how the 'while' loop is working here? while(true), repeat the loop, ok. But what exactly going on here? What's while doing? And how it's doing?

    As far I know, while(true) loop will be terminated when 'true' evaluates to 'false' at some point, inside the loop. But here, if the user enter wrong input, it's false right? Then the while loop should stop... I guess?

    What the heck is going on here??? Please explain.

    • nascardriver

      The loop stops when the loop condition evaluates to `false`. `true` is never `false`, so the loop never stops. Unless you use other means of stopping the loop, ie. `break` or `return`. The loop stops when `return operation;` is executed.

  • abcdaf

    How can i handle invalid input in the EnterNames function? for example if the user decides to enter an integer or an operator instead of their actual name or if they enter names with very few letters?

  • kensus

    Alex and NASCARDRIVER, referring to your conclusion "chapter 7.16 — std::cin and handling invalid input". is a perfect example that one would wish to get in order to feel like learning, you shouldn't hesitate in doing so in other lessons as it doesn't scare at all instead it motivates having known how to. Thanks a lot for the tutorial.

  • EternalSkid

    Hey Alex and nascardriver, first of all happy Chinese new year!

    I have a question that has been boggling me for hours,

    In this lesson, it's mentioned that

    operator>> extracts as much data from the input buffer as it can into the variable (ignoring any leading whitespace characters, such as spaces, tabs, or ‘\n’).

    However, in lesson 8.1 -- An introduction to std::strings specifically here on line 8, it is mentioned that std::cin breaks on whitespace.

    I would like to clarify, what's the difference here between ignore and break? Thanks so much for your time.

    • nascardriver


      Leading whitespace is ignored.
      Then extraction starts.
      Extraction stops (breaks) on whitespace, or if the next character can't be extracted into the given type.

  • EternalSkid

    Hey alex and nascardriver, a suggestion i'd make to improve the Putting All Together code, line 50 i'd suggest also calling the ignoreLine() function, as input such as *7 will cause 7 to be still in the buffer, skipping second input for double. If you disagree, please let me know why as well.

  • Roshani

    error is " no opertaor '>>' matches these operands operand types are std::istream >> void(int)"

    • nascardriver

      Please post a minimal reproducible example with the exact error message (including line number). Chances are you'll figure it out yourself while doing this.

  • abcdaf

    When i type 50*44 as input, it only takes 50 as input, my question here is when does std::cin stop extracting input?

    • nascardriver

      At the first character that can't be stored in the variable you're trying to extract into. '*' cannot be converted to a `double`, so `std::cin` stop extraction.

  • SuperNoob

    I have a question. Consider the programs below.

    Program 1:

    Sample Input:


    What's happening here is: 'a' doesnt match with type of variable x. So only 5 is stored in x and 'a' is left in the input buffer for later extraction.

    My question is: How can i check if there is any extraneus value in the input buffer (after the successful extraction of 5)? For the previous example, the extraneus value is 'a'.

    I want to do something like this:

    Program 2:

    Sample Output:

    Enter an integer: 5a
    Extraneus value left in the input buffer is a

    I know I can extract the value left in the input buffer by using another std::cin function right after the 6th line of Program 1. Like say,

    But in that case, if the user input is completely valid and doesn't have any extraneus value, then the program will halt and want another input from the user, which is not my purpose. So, is there any solution for what I am trying to do?

    I intend to do some task depending on the existence of extraneus value in the input buffer.

    • nascardriver

      `std::cin.peek()` returns the next character in the input. If you've used an extraction operation that leaves the line feed in the input buffer, you can check if `std::cin.peek()` returns a line feed.
      If you call `std::cin.peek()` when the buffer is empty (eg. after calling `std::getline()`), the user will be prompted for input.

  • nav

  • TimT

    The completed calculator program needs another ignoreLine(); statement just after line 49 in getOperator(), in case the user gives something like '+andsomenonsense'. Otherwise, it gives a getDouble() error message instead of an operator error.

  • Tomek

    Putting it all together, line 27. Typo! - "ingore"

    • Tomek

      2. In the getDouble function I added something like this:

      and then i added ++count on the succesful input. Is that a good approach?

      • nascardriver

        `static` local variables make functions non-reusable. Only use `static` local variables if they never need to be reset (eg. a random number generator). Lets the caller print the prompt or make `count` a parameter.

  • Waldo Lemmer

    Wow, I've learnt a lot in this lesson. You managed to make error handling, which used to look like a daunting task, easy to comprehend and remember :D
    A few corrections and questions:

    1. Why do you use

    in section "Error case 1: Extraction succeeds but input is meaningless", but

    everywhere else?

    2. Section "Error case 2: Extraction succeeds but with extraneous input":
    > Since the user never had an opportunity to enter additional data and hit enter (causing a newline),
    This is ambiguous, I recommend changing "causing a newline" to "which would cause a newline".

    3. Section "Error case 3: Extraction fails":
    > Let’s integrate that into our getDouble() function:
    You should ask the user to try again (after line 14), like you did in section "Putting it all together", line 17.

    4. When a line in a code snippet on this website wraps, the wrapped parts are still counted as lines. You can test this by shrinking your browser window horizontally. I noticed it on a 1080p monitor with my browser docked on the right.

    5. Is there a way to make this more readable?

    That's an awful amount of clutter for such a short output string.

    I can't wait to learn about exceptions!

    • nascardriver

      1. Alex uses the magic number, which is wrong. Lesson updated.

      2. I think it's valid, and identical to your suggestion, as written.

      3. Fixed

      4. @Alex

      5. No easy way around it. C++20 has a new formatting library that might be interesting to you.

      • Waldo Lemmer

        5. Found it, thanks! (

        Edit: I can't #include <format> yet, so I guess I'll have to do without it for now (

  • kio

    Hi Alex and Nascardriver, for the programs we are going to make following this tutorial. Which kind of error handling do you suggested?
    I assume during the development we can use expectation and assertion, but for the "release" we should implement the methods that you wrote in this chapter, right?
    Or is there a better way inside the STL?


    • nascardriver

      Hi kio!

      The error handling shown here is unrelated to exceptions or assertions. You have to do it, unless you don't care about errors.

      Assertions are for developers. If a developer does something wrong, an assertion should catch it. An assertion should never be triggered by a user.

      Exceptions are for unlikely errors that can occur in finished software, eg. when a file doesn't exist.

      Non-exception error reporting (eg. returning `std::optional`, or ``) is for likely errors that can occur in finished software, eg. wrong user input.

      In the "release", what you need for sure are assertions, they're there to help you, the developer. What other kinds of error handling you use depends on which libraries you use or how you want to handle errors. If you use `std::cin`, you have to use its error handling method, as shown in this lesson. If you use a library that throws, you have to handle exceptions.

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