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# Logical operators

From cppreference.com
< cpp‎ | language

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

### [edit] Explanation

The logical operators apply logic functions (NOT, AND, and inclusive OR) to boolean arguments (or types contextually-convertible to bool), with a boolean result. Unlike the bitwise logic operators, these operators (in their built-in form) do not evaluate the second operand if the result is known after evaluating the first.

#### [edit] Builtin operators

The following built-in function signatures participate in overload resolution:

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

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`.

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. 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. If the firstoperand is true, the second operand is not evaluated.

### [edit] 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

### [edit] Example

Run this code

    #include <iostream>
    #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::cout << "Enter 'quit' to quit.\n";
        for(std::string line;    std::cout << "> "
                              && std::getline(std::cin, line)
                              && line != "quit"; )
            ;
    }

Output:

    true
    Enter 'quit' to quit.
    > test
    > quit

### [edit] 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:

operator! | applies a unary arithmetic operator to each element of the valarray (public member function of `std::valarray`)
operator&&operator|| | applies binary operators to each element of two valarrays, or a valarray and a value (function template)
operator! | checks if an error has occurred (synonym of fail()) (public member function of `std::basic_ios`) [edit]

### [edit] See also

Operator precedence

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->b a.b a->*b a.*b | a(...) a, b (type) a ? :
Special operators
`static_cast` converts one type to another compatible type `dynamic_cast` converts virtual base class to derived class `const_cast` converts type to compatible type with different cv qualifiers `reinterpret_cast` converts type to incompatible type `new` allocates memory `delete` deallocates memory `sizeof` queries the size of a type `sizeof...` queries the size of a parameter 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)

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