Logical operators

Returns the result of a boolean operation.

Operator name	Syntax	Overloadable	Prototype examples (for `class T`)
Operator name	Syntax	Overloadable	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 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.

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.

Results

`a`	`true`	`false`
`!a`	`false`	`true`

`and`		`a`
`and`		`true`	`false`
`b`	`true`	`true`	`false`
`b`	`false`	`false`	`false`

`or`		`a`
`or`		`true`	`false`
`b`	`true`	`true`	`true`
`b`	`false`	`true`	`false`

Example

#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

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`)

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 ? :`
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` 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).

Links:

http://en.cppreference.com/w/cpp/language/operator_logical

doc_CPP

2016-10-11 09:59:07

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