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Logical operators

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noexcept specificatore (C++11)
Eccezioni
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Montaggio in linea
 

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);
'Nota'
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Notes
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  • The keyword-like forms (and,or,not) and the symbol-like forms (&&,||,!) can be used interchangeably (See rappresentazioni alternative)
  • Tutti incorporato ritorno bool operatori, e la maggior parte definiti dall'utente sovraccarichi anche restituire bool modo definito dall'utente operatori possono essere utilizzati nello stesso modo come il built-in. Tuttavia, in una definita dall'utente overload dell'operatore, qualsiasi tipo può essere utilizzato come tipo di ritorno (compresi void).
    Original:
    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).
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  • 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

Indice

[modifica] Spiegazione

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.

[modifica] 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.

[modifica] 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

[modifica] Esempio

#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

[modifica] Libreria standard

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:

applica un operatore unario aritmetica per ogni elemento del valarray
Original:
applies a unary arithmetic operator to each element of the valarray
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(membro pubblico of std::valarray funzione)
vale operatori binari a ciascun elemento di due valarrays, o un valarray e un valore
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applies binary operators to each element of two valarrays, or a valarray and a value
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(funzione di modello)
controlla se è verificato un errore (sinonimo di fail())
Original:
checks if an error has occurred (synonym of fail())
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(membro pubblico of std::basic_ios funzione) [edit]

[modifica] Vedi anche

Precedenza degli operatori

Common operators
assegnazione incrementNJdecrement aritmetica logico confronto memberNJaccess altra

a = b
a = rvalue
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 converte un tipo a un altro
tipo compatibile
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static_cast converts one type to another compatible type
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dynamic_cast converte classe virtuale di base per class
derivato
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dynamic_cast converts virtual base class to derived class
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const_cast converte il tipo di tipo compatibile con diversi cv qualifiers
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const_cast converts type to compatible type with different cv qualifiers
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reinterpret_cast converte tipo type
incompatibile
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reinterpret_cast converts type to incompatible type
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new alloca memory
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new allocates memory
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delete dealloca memory
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delete deallocates memory
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sizeof interroga la dimensione di un type
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sizeof queries the size of a type
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sizeof... interroga le dimensioni di un parametro confezione (dal C++11)
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sizeof... queries the size of a parametro confezione (dal C++11)
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typeid interroga le informazioni sul tipo di una type
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typeid queries the type information of a type
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noexcept controlla se un'espressione può lanciare una (dal C++11)
un'eccezione
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noexcept checks if an expression can throw an exception (dal C++11)
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alignof query requisiti di allineamento di un (dal C++11) tipo
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alignof queries alignment requirements of a type (dal C++11)
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