4.9 — Boolean values

In real-life, it’s common to ask or be asked questions that can be answered with “yes” or “no”. “Is an apple a fruit?” Yes. “Do you like asparagus?” No.

Now consider a similar statement that can be answered with a “true” or “false”: “Apples are a fruit”. It’s clearly true. Or how about, “I like asparagus”. Absolutely false (yuck!).

These kinds of sentences that have only two possible outcomes: yes/true, or no/false are so common, that many programming languages include a special type for dealing with them. That type is called a Boolean type (note: Boolean is properly capitalized in the English language because it’s named after its inventor, George Boole).

Boolean variables

Boolean variables are variables that can have only two possible values: true, and false.

To declare a Boolean variable, we use the keyword bool.

bool b;

To initialize or assign a true or false value to a Boolean variable, we use the keywords true and false.

bool b1 { true };
bool b2 { false };
b1 = false;
bool b3 {}; // default initialize to false

Just as the unary minus operator (-) can be used to make an integer negative, the logical NOT operator (!) can be used to flip a Boolean value from true to false, or false to true:

bool b1 { !true }; // b1 will be initialized with the value false
bool b2 { !false }; // b2 will be initialized with the value true

Boolean values are not actually stored in Boolean variables as the words “true” or “false”. Instead, they are stored as integral values: true is stored as integer 1, and false is stored as integer 0. Similarly, when Boolean values are evaluated, they don’t actually evaluate to “true” or “false”. They evaluate to the integers 0 (false) or 1 (true). Because Booleans store integral values, they are considered to be an integral type.

Printing Boolean values

When we print Boolean values, std::cout prints 0 for false, and 1 for true:

#include <iostream>

int main()
{
    std::cout << true << '\n'; // true evaluates to 1
    std::cout << !true << '\n'; // !true evaluates to 0

    bool b {false};
    std::cout << b << '\n'; // b is false, which evaluates to 0
    std::cout << !b << '\n'; // !b is true, which evaluates to 1
    return 0;
}

Outputs:

1
0
0
1

If you want std::cout to print true or false instead of 0 or 1, you can output std::boolalpha. This doesn’t output anything, but manipulates the way std::cout outputs bool values.

Here’s an example:

#include <iostream>

int main()
{
    std::cout << true << '\n';
    std::cout << false << '\n';

    std::cout << std::boolalpha; // print bools as true or false

    std::cout << true << '\n';
    std::cout << false << '\n';
    return 0;
}

This prints:

1
0
true
false

You can use std::noboolalpha to turn it back off.

Integer to Boolean conversion

When using uniform initialization, you can initialize a variable using integer literals 0 (for false) and 1 (for true) (but you really should be using false and true instead). Other integer literals cause compilation errors:

#include <iostream>

int main()
{
	bool bFalse { 0 }; // okay: initialized to false
	bool bTrue  { 1 }; // okay: initialized to true
	bool bNo    { 2 }; // error: narrowing conversions disallowed

	std::cout << bFalse << bTrue << bNo << '\n';
	
	return 0;
}

However, in any context where an integer can be converted to a Boolean, the integer 0 is converted to false, and any other integer is converted to true.

#include <iostream>

int main()
{
	std::cout << std::boolalpha; // print bools as true or false

	bool b1 = 4 ; // copy initialization allows implicit conversion from int to bool
	std::cout << b1 << '\n';

	bool b2 = 0 ; // copy initialization allows implicit conversion from int to bool
	std::cout << b2 << '\n';

	return 0;
}

This prints:

true
false

Note: bool b1 = 4; may generate a warning. If so you’ll have to disable treating warnings as errors to compile the example.

Inputting Boolean values

Inputting Boolean values using std::cin sometimes trips new programmers up.

Consider the following program:

#include <iostream>

int main()
{
	bool b{}; // default initialize to false
	std::cout << "Enter a boolean value: ";
	std::cin >> b;
	std::cout << "You entered: " << b << '\n';

	return 0;
}

Enter a Boolean value: true
You entered: 0

Wait, what?

By default, std::cin only accepts numeric input for Boolean variables: 0 is false, and 1 is true. Any other numeric value will be interpreted as true, and will cause std::cin to enter failure mode. Any non-numeric value will be interpreted as false and will cause std::cin to enter failure mode.

In this case, because we entered true, std::cin silently failed and set b to false. Consequently, when std::cout prints a value for b, it prints 0.

To allow std::cin to accept the words false and true as inputs, you must first input to std::boolalpha:

#include <iostream>

int main()
{
	bool b{};
	std::cout << "Enter a boolean value: ";

	// Allow the user to input 'true' or 'false' for boolean values
	// This is case-sensitive, so True or TRUE will not work
	std::cin >> std::boolalpha;
	std::cin >> b;

	// Let's also output bool values as `true` or `false`
	std::cout << std::boolalpha;
	std::cout << "You entered: " << b << '\n';

	return 0;
}

However, when std::boolalpha is enabled for input, numeric values will no longer be accepted (they evaluate to false and cause std::cin to enter failure mode).

Note that we use std::cin >> std::boolalpha; to input bool values as true or false, and std::cout << std::boolalpha; to output bool values as true or false. These are independent controls that can be turned on (using std::boolalpha) or off (using std::noboolalpha) separately.

Boolean return values

Boolean values are often used as the return values for functions that check whether something is true or not. Such functions are typically named starting with the word is (e.g. isEqual) or has (e.g. hasCommonDivisor).

Consider the following example, which is quite similar to the above:

#include <iostream>

// returns true if x and y are equal, false otherwise
bool isEqual(int x, int y)
{
    return x == y; // operator== returns true if x equals y, and false otherwise
}

int main()
{
    std::cout << "Enter an integer: ";
    int x{};
    std::cin >> x;

    std::cout << "Enter another integer: ";
    int y{};
    std::cin >> y;

    std::cout << std::boolalpha; // print bools as true or false
    
    std::cout << x << " and " << y << " are equal? ";
    std::cout << isEqual(x, y) << '\n'; // will return true or false

    return 0;
}

Here’s output from two runs of this program:

Enter an integer: 5
Enter another integer: 5
5 and 5 are equal? true

Enter an integer: 6
Enter another integer: 4
6 and 4 are equal? false

How does this work? First we read in integer values for x and y. Next, the expression isEqual(x, y) is evaluated. In the first run, this results in a function call to isEqual(5, 5). Inside that function, 5 == 5 is evaluated, producing the value true. The value true is returned back to the caller to be printed by std::cout. In the second run, the call to isEqual(6, 4) returns the value false.

Boolean values take a little bit of getting used to, but once you get your mind wrapped around them, they’re quite refreshing in their simplicity! Boolean values are also a huge part of the language -- you’ll end up using them more than all the other fundamental types put together!

We’ll continue our exploration of Boolean values in the next lesson.