Logical operators

 

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User-defined conversion

 

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.

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:

operator!	applies a unary arithmetic operator to each element of the valarray (public member function of std::valarray<T>)
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<CharT,Traits>)

See also

Operator precedence

Operator overloading

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 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)