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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.

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

Boolean values are not actually stored in Boolean variables as the words “true” or “false”. Instead, they are stored as integers: true becomes the integer 1, and false becomes the 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 actually store integers, they are considered an integral type.

Printing Boolean variables

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

Outputs:

1
0
0
1

If you want std::cout to print “true” or “false” instead of 0 or 1, you can use std::boolalpha. Here’s an example:

This prints:

1
0
true
false

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

Integer to Boolean conversion

You can’t initialize a Boolean with an integer using uniform initialization:

(note: some versions of g++ don’t enforce this properly)

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.

This prints:

true
false

Inputting Boolean values

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

Consider the following program:

Enter a Boolean value: true
You 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. In C++11 or newer, a failed input will also zero-out the variable, so b also gets assigned value 0. Consequently, when std::cout prints a value for b, it prints 0.

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:

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.


4.10 -- Introduction to if statements
Index
4.8 -- Floating point numbers

558 comments to 4.9 — Boolean values

  • Haseeb Ur Rehman

    Design a C++ code that has the same behavior as a XOR gate. Input variable must be
    of Bool Data type.

  • Aditya

    It seems to be that a best practice should be to **not** use cin directly for booleans unless there are performance reasons. It seems to me that the best practice should be to use cin to populate a string, and then populate a boolean after that. If this is correct, you may want to add a best practice as such. Possibly to name an alternative best practice as to ask people to put in a "1" or a "0"

  • Keshav

    Can't this be done without using boolean integrals?
    I think it can be done without boolean integrals.

  • asparagus boi

    BUT I like asparagus :(

  • Rupi

    Inputting Boolean values

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

    Consider the following program:

    you forget the #include <iostream>; its a small thing, which might have been forgotten deliberately.

  • Fosterwerks

    What exactly is std::boolalpha? From what I gather it's a function. Assuming it is, why can we use it without parentheses?

    • nascardriver

      `std::boolalpha` is both a function and a variable of type `std::ios_base::fmtflags`, which is an implementation-defined type, ie. it's unknown to us what this type is, it can vary between standard libraries. In this example, we're using the variable. When a stream, for example `std::cout`, receives a variable of this type, it changes some of its internal settings rather than printing the variable.

  • Raffaello

    brace initialization shouldn't be allowed on line 5, in fact when I tried it wouldn't compile, complaining about "narrowing conversion of '4' from 'int' to 'bool'". so I changed it to copy initialization, but left the brace initialization on line 8 and it did compile, why is that?

    • nascardriver

      Brace initialization doesn't allow conversions which can cause loss of data. If you convert 4 to a bool, you lost information, because afterwards you only know if the integer was 0 or not.

  • Mike

    Hi.In  "printing Boolean variables" section, shouldn't it be \n in the code instead of std::endl ?

  • Al

    Asparagus are great, come on! Fried (not deep fried) with salt and olive oil... :p Bacon is great with aspargus, by the way.

    • Craig

      Haha, funny. I knew someone would comment on this. Yes, Alex, if your only experience is with canned (or even frozen store-bought) asparagus... I hear ya. Fresh grilled asparagus with lots of butter is amazing!

  • Ayrton Fithiadi Sedjati

    When you wrote:

    "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. In C++11 or newer, this also sets b to 0 (which is the same value it had already been initialized with)."

    Did you mean that when a user inputs a value other than 0 and 1 for Boolean variables using std::cin, the variable would be unaffected and would retain the value it had been initialized with?

    Because I found that this program does not behave accordingly:

    When the user inputs "false" or "true", the program always prints "Boolean value: 0", inconsistent with the initial value of the variable b (true).

    Edit: My current guess is that I misunderstood you, in which you meant that such input would result in the variable b getting the value of 0 instead of retaining its initial value.

  • Scarlet Johnson

    This program on running shows 0.1 with no precision. Explain?

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