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Logical operators

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Returns the result of a boolean operation.

Operator name Syntax Over​load​able Prototype examples (for class T )
Inside class definition Outside class definition
negation not a

! a

Yes bool T :: operator ! ( ) const ; bool operator ! ( const T & a ) ;
AND a and b

a && b

Yes bool T :: operator && ( const T2 & b ) const ; bool operator && ( const T & a, const T2 & b ) ;
inclusive OR a or b

a || b

Yes bool T :: operator || ( const T2 & b ) const ; bool operator || ( const T & a, const T2 & b ) ;
Notes
  • The keyword-like forms ( and , or , not ) and the symbol-like forms ( && , || , ! ) can be used interchangeably (see alternative representations ).
  • All built-in operators return bool , and most user-defined overloads also return bool so that the user-defined operators can be used in the same manner as the built-ins. However, in a user-defined operator overload, any type can be used as return type (including void ).
  • Builtin operators && and || perform short-circuit evaluation (do not evaluate the second operand if the result is known after evaluating the first), but overloaded operators behave like regular function calls and always evaluate both operands.

Contents

Explanation

The logic operator expressions have the form

! rhs (1)
lhs && rhs (2)
lhs || rhs (3)
1) Logical NOT
2) Logical AND
3) Logical inclusive OR

If the operand is not bool , it is converted to bool using contextual conversion to bool : it is only well-formed if the declaration bool t(arg) is well-formed, for some invented temporary t .

The result is a bool prvalue.

For the built-in logical NOT operator, the result is true if the operand is false . Otherwise, the result is false .

For the built-in logical AND operator, the result is true if both operands are true . Otherwise, the result is false . This operator is short-circuiting : if the first operand is false , the second operand is not evaluated.

For the built-in logical OR operator, the result is true if either the first or the second operand (or both) is true . This operator is short-circuiting: if the first operand is true , the second operand is not evaluated.

Note that bitwise logic operators do not perform short-circuiting.

Results

a true false
! a false true
and a
true false
b true true false
false false false
or a
true false
b true true true
false true false

In overload resolution against user-defined operators , the following built-in function signatures participate in overload resolution:

bool operator ! ( bool )
bool operator && ( bool , bool )
bool operator || ( bool , bool )

Example

Run this code 
#include <iostream>
#include <sstream>
#include <string>
int main()
{
    int n = 2;
    int* p = &n;
    // pointers are convertible to bool
    if (    p && *p == 2  // "*p" is safe to use after "p &&"
        || !p &&  n != 2) // || has lower precedence than &&
        std::cout << "true\n";
    // streams are also convertible to bool
    std::stringstream cin;
    cin << "3...\n" << "2...\n" << "1...\n" << "quit";
    std::cout << "Enter 'quit' to quit.\n";
    for (std::string line;    std::cout << "> "
                           && std::getline(cin, line)
                           && line != "quit";)
        std::cout << line << '\n';
}

Output: 

true
Enter 'quit' to quit.
> 3...
> 2...
> 1...
>

Standard library

Because the short-circuiting properties of operator&& and operator|| do not apply to overloads, and because types with boolean semantics are uncommon, only two standard library classes overload these operators:

applies a unary arithmetic operator to each element of the valarray
(public member function of std::valarray<T> )
applies binary operators to each element of two valarrays, or a valarray and a value
(function template)
checks if an error has occurred (synonym of fail() )
(public member function of std::basic_ios<CharT,Traits> )

See also

Operator precedence

Operator overloading

function object implementing x && y
(class template)
function object implementing x || y
(class template)
function object implementing ! x
(class template)
Common operators
assignment increment
decrement 		arithmetic logical comparison member
access 		other

a = b
a + = b
a - = b
a * = b
a / = b
a % = b
a & = b
a | = b
a ^ = b
a <<= b
a >>= b

++ a
-- a
a ++
a --

+ a
- a
a + b
a - b
a * b
a / b
a % b
~a
a & b
a | b
a ^ b
a << b
a >> b

! a
a && b
a || b

a == b
a ! = b
a < b
a > b
a <= b
a >= b
a <=> b

a [ ... ]
* a
& a
a - > b
a. b
a - > * b
a. * b

function call

a ( ... )
comma

a, b
conditional

a ? b : c
Special operators

static_cast converts one type to another related type
dynamic_cast converts within inheritance hierarchies
const_cast adds or removes cv -qualifiers
reinterpret_cast converts type to unrelated type
C-style cast converts one type to another by a mix of static_cast , const_cast , and reinterpret_cast
new creates objects with dynamic storage duration
delete destructs objects previously created by the new expression and releases obtained memory area
sizeof queries the size of a type
sizeof... queries the size of a pack (since C++11)
typeid queries the type information of a type
noexcept checks if an expression can throw an exception (since C++11)
alignof queries alignment requirements of a type (since C++11)

C documentation for Logical operators