Stdcinfail Method In C++

In order to figure out whether an input operation from the standard input (std::cin) has failed, use the C++ function std::cin.fail function. It is usually used to determine whether an input operation was successful after it has been performed.

(std::ios::failbit, std::ios::badbit, std::ios::eofbit) Input State Flags:

• ✅ The std::ios::failbit flag is set on the stream (std::cin in this example) when an input operation fails.
• ✅ A major issue with the stream (such as a memory allocation failure) sets the std::ios::badbit flag.
• ✅ When the input stream comes to an end, the std::ios::eofbit flag is set.

std::cin.fail Method:

• ✅ This function is a member of the std::basic_ios class template. If the failbit or badbit is set on the std::cin stream, it returns true.
• ✅ It is usually used to determine whether an input operation was successful after it has been performed.
• ✅ An input operation is considered to have failed if std::cin.fail returns true (e.g., due to invalid input).

std::cin.clear Method:

• ✅ The stream's error status flags (failbit, badbit, and eofbit) are set in the event of an input operation failure, which may stop additional input operations.
• ✅ The error status flags are cleared by std::cin.clear, enabling the execution of additional input operations.

std::cin.ignore Method:

• ✅ The method std::cin.ignore(n, delim) removes characters from the input stream until it reaches the delimiter delim, which can be up to n characters.
• ✅ It is frequently used to remove any faulty input that remains after a failed input procedure, following std::cin.clear.

Pseudocode:

• ✅ Display a prompt asking the user to enter a number.
• ✅ Read the input from the user using std::cin.
• ✅ Check if the input operation failed using std::cin.fail.
• ✅ If std::cin.fail returns true, display an error message and clear the error state of std::cin using std::cin.clear.
• ✅ Discard any remaining invalid input from std::cin using std::cin.ignore.
• ✅ Repeat steps 1-5 until a valid input is received.
• ✅ Once a valid input is received, process the input as needed.

Program 1:

Let us take an example to illustrate the std::cin.fail function in C++ .

#include <iostream>
#include <limits> // Include the header for std::numeric_limits
int main() {
    int nums;
    std::cout << "Enter a number: ";
    std::cin >> nums;
    if (std::cin.fail()) {
        std::cout << "Invalid input. Please enter a valid number." << std::endl;
        std::cin.clear(); // Clear the error flag
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // Discard invalid input
    } else {
        std::cout << "You entered: " << nums << std::endl;
    }
    return 0;
}

Output:

Program 2:

Let us take another example to illustrate the std::cin.fail function in C++.

#include <iostream>
#include <limits> // for std::numeric_limits
int main() {
    int nums2;
    bool validIn= false;
    while (!validIn) {
        std::cout << "Enter a number: ";
        std::cin >> nums2;
        if (std::cin.fail()) {
            std::cout << "Invalid input. Please enter a valid number." << std::endl;
            std::cin.clear(); // Clear the error flag
            std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // Discard invalid input
        } else {
            validIn = true;
        }
    }
    std::cout << "You entered: " << nums2<< std::endl;
    return 0;
}

Output: