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 falsebool 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:
1001
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:
10truefalse
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 falsebool bTrue { 1 }; // okay: initialized to truebool bNo { 2 }; // error: narrowing conversions disallowedstd::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 falsebool b1 = 4 ; // copy initialization allows implicit conversion from int to boolstd::cout << b1 << '\n';bool b2 = 0 ; // copy initialization allows implicit conversion from int to boolstd::cout << b2 << '\n';return 0;}
This prints:
truefalse
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 falsestd::cout << "Enter a boolean value: ";std::cin >> b;std::cout << "You entered: " << b << '\n';return 0;}
Enter a Boolean value: trueYou entered: 0
Wait, what?
It turns out that std::cin
only accepts two inputs for Boolean variables: 0
and 1
(not true
or false
). Any other inputs will cause std::cin
to silently fail. In this case, because we entered true
, std::cin
silently failed. A failed input will also zero-out the variable, so b
is assigned the value false
. Consequently, when std::cout
prints a value for b
, it prints 0
.
To allow std::cin
to accept 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 workstd::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, 0
and 1
will no longer be interpreted as Booleans inputs (they both resolve to false
as does any non-true input).
Warning
Enabling std::boolalpha
for input will only allow lower-cased false
or true
to be accepted. Variations with capital letters will not be accepted. 0
and 1
will also no longer be accepted.
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 otherwisebool 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: 5Enter another integer: 55 and 5 are equal? true
Enter an integer: 6Enter another integer: 46 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.