Dev Notes

Various Cheat Sheets and Resources by David Egan/Carawebs.

User Input cin in C++


cpp
David Egan

If an input variable is declared, and the value collected by std::cin does not match, std::cin returns false.

For example:

int input = 0;

std::cin >> input;

// Entering an integer at this point works.
// Entering a value which is not an integer and not numeric, std::cin returns false.
// Entering a real number (with a decimal point): fractional part is discarded and integer is saved.

It’s better to take input within a loop, with some input validation:


#include <iostream>
#include <typeinfo>
#include <limits>

int main()
{
	int input = 0;
	while (1) {
		std::cout << "Enter an int:\n";
	
		// Enter this block if taking input from cin has failed.
		if (!(std::cin >> input)) {
	
			// The error flag is set on std::cin - future attempts to
			// get input will fail unless the error is cleared.
			std::cin.clear();

			// The failed input is in the input buffer. The default for `ignore` is
			// to skip a single character. To be sure, remove the max streamsize
			// number of chars up until a newline is encountered
			std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
			continue;
		}
		break;
	}
	std::cout << "You entered: " << input << '\n';
	return 0;
}
  • std::cin returns false if the input can’t match the expected type (in this case, int).
  • std::cin.ignore() extracts and discards unwanted values.
  • std::cin.clear() changes the internal state of the stream - unsets the error flag.

For a working example, see here

References


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