09 - std::cin

std::cin is an input stream (a globally accessible object of type std::istream). It can be used to extract data from operating system's character input stream (called stdin). For most systems, this simply means that if you run the executable in a terminal shell it will accept input from the keyboard.

Unlike std::cout, std::cin uses >> (it reads to variables, << is used for output). Similarly to output, it supports many data types.

Example:

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#include <iostream>

int main()
{
	std::cout << "Enter a number: ";
	int x;
	std::cin >> x;
	std::cout << "You have entered: " << x << "\n";
}

You can chain reads too. Just like with <<, operands are processed from left to right.

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#include <iostream>

int main()
{
	std::cout << "Enter 2 numbers\n";
	int x;
	int y;
	std::cin >> x >> y; // x is read first, then y
	std::cout << "x = " << x << "\ny = " << y << "\n";
}

Input operations may fail. Reasons may vary:

• strictly technical such as failure to allocate memory (generally absent)

• invalid data (e.g. letters when digits for numeric input are expected)

• too long data (e.g. too many digits for given numeric type - value not representable within supported range)

Upon erroneous operation, C++ streams go into failure state, ignoring any further input unless the state is explicitly reset (all input operations will finish immediately). Because error handling requires more knowledge, for now programs will assume that the input is always valid.

For each input request, std::cin will first discard any whitespace characters (space, tab, line break, etc.) and then consume as much valid characters as possible. For an input like space-123-enter-456-enter (technically :cch:` 123n456n$$$str`), it will:

• first input:

  • read and ignore the space character

  • read all digits

  • stop on line break (the character will not be consumed)

• second input:

  • read and ignore line break

  • read all digits

  • stop on line break

Note that what is seen on the terminal may not be exactly the same thing that the program is receiving (though in most cases it is). In addition, various command-line terminals may offer specific behavior under specific key combinations (e.g. tab for autocompletion) - such key strokes are not sent to the executed program but intercepted by the terminal program. I can't really list any recommendation here - different terminals differ significantly, the only convention widely agreed upon is tab for autocompletion and enter for any confirmation.