bool - Boolean Data Type in Solidity

Like other programming languages, Solidity has a boolean data type. It is used for values that are binary, a boolean is either true or false. This makes booleans very efficient to store.

To declare a boolean in Solidity, the bool keyword is used:

bool my_boolean;

default value of boolean
The default value of a boolean in Solidity is false. We have to declare a boolean as true, otherwise, it will be false.


// boolean data type has a default value of false
bool test1; // defaults to false
bool test2 = true; // true


size of boolean
The boolean data type is a very efficient way of storing information. The outcome is always binary: true or false. Therefore a boolean takes only 1 bytes of storage in Solidity.

bool to uint
We can use the ternary operator to convert booleans to integers: 0 for false and 1 for true.


// convert bool to uint
bool myBool = true;
uint myFlag = myBool ? uint(1) : uint(0);


Booleans can also be used as return values of functions. 


// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

contract booleanReturnExample {

    bool public isChecked;

    function set_checked() public returns (bool)
    {
        isChecked = true;

        return isChecked; // return boolean value true
    }
}


In this contract the boolean isChecked is first declared as false. After the function is called the boolean variable is changed to true.

boolean operators

logical negation !
logical conjunction &&
logical disjunction ||
equality ==
inequality !=


The following code shows the use of boolean operators and related output:


// SPDX-License-Identifier: MIT
pragma solidity ^0.8.1;

contract BooleanOperators {
    bool public bool1 = true;
    bool public bool2 = false;
    bool public conjunction = bool1 && bool2; // output false
    bool public disjunction = bool1 || bool2; // output true
    bool public equality = bool1 == bool2; // output false
    bool public inequality = bool1 != bool2; // output true
    bool public negation = !bool1 == bool2; // output true
}