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Logical operators

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Bedingte Ausführung Aussagen
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Inline Montage
 

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'
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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 alternative Darstellungen)
  • Alle eingebauten Operatoren Rückkehr bool, und die meisten benutzerdefinierten Überlastungen auch bool so dass die benutzerdefinierten Operatoren in der gleichen Weise wie die built-ins verwendet werden zurückkehren. Jedoch in einer benutzerdefinierten Operator Überlast kann jeder Typ als Rückkehr-Typ (einschließlich void) verwendet werden .
    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

Inhaltsverzeichnis

[Bearbeiten] Erklärung

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.

[Bearbeiten] 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.

[Bearbeiten] 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

[Bearbeiten] Beispiel

#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

[Bearbeiten] Standard-Bibliothek

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:

gilt eine unäre arithmetische Operator für jedes Element des valarray
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applies a unary arithmetic operator to each element of the valarray
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(öffentliche Elementfunktion of std::valarray)
gilt Binäroperatoren auf jedes Element von zwei valarrays oder ein valarray und einem Wert
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applies binary operators to each element of two valarrays, or a valarray and a value
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(Funktions-Template)
prüft, ob ein Fehler aufgetreten ist (Synonym fail())
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checks if an error has occurred (synonym of fail())
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(öffentliche Elementfunktion of std::basic_ios) [edit]

[Bearbeiten] Siehe auch

Operator Vorrang

Common operators
Zuweisungen incrementNJdecrement Arithmetik logisch Vergleich memberNJaccess andererseits

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 wandelt einem Typ in einen anderen kompatiblen Typ
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static_cast converts one type to another compatible type
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dynamic_cast wandelt virtuellen Basisklasse abgeleitet class
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dynamic_cast converts virtual base class to derived class
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const_cast wandelt Typ kompatiblen Typ mit unterschiedlichen cv qualifiers
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const_cast converts type to compatible type with different cv qualifiers
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reinterpret_cast wandelt Typ inkompatibel type
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reinterpret_cast converts type to incompatible type
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new ordnet memory
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new allocates memory
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delete freigibt memory
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delete deallocates memory
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sizeof fragt die Größe eines type
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sizeof queries the size of a type
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sizeof... fragt die Größe eines Parameter Pack (seit C++11)
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sizeof... queries the size of a Parameter Pack (seit C++11)
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typeid fragt die Typinformationen eines type
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typeid queries the type information of a type
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noexcept prüft, ob ein Ausdruck eine Ausnahme (seit C++11)
werfen kann
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noexcept checks if an expression can throw an exception (seit C++11)
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alignof Abfragen Ausrichtungsanforderungen eines Typs (seit C++11)
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alignof queries alignment requirements of a type (seit C++11)
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