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Was bedeutet "Operator bool () const"

Zum Beispiel:

operator bool() const 
{ 
    return col != 0; 
}

col ist ein int . Wie funktioniert operator bool() const?

68
Cheng

Mitgliedsfunktionen des Formulars 

operator TypeName()

sind Konvertierungsoperatoren. Sie ermöglichen, dass Objekte des Klassentyps verwendet werden, als ob sie vom Typ TypeName wären. Wenn dies der Fall ist, werden sie mit der Konvertierungsfunktion in TypeName konvertiert. 

In diesem speziellen Fall ermöglicht operator bool(), dass ein Objekt des Klassentyps so verwendet wird, als wäre es eine bool. Wenn Sie beispielsweise ein Objekt des Klassentyps obj haben, können Sie es als verwenden

if (obj)

Dadurch wird operator bool() aufgerufen, das Ergebnis zurückgegeben und das Ergebnis als Bedingung für die if verwendet.

Es sei darauf hingewiesen, dass operator bool() eine sehr schlechte Idee ist und Sie sollten sie niemals verwenden. Eine ausführliche Erklärung, warum das Problem schlecht ist, und zur Lösung des Problems finden Sie unter "The Safe Bool Idiom".

(C++ 0x, die bevorstehende Überarbeitung des C++ - Standards, fügt Unterstützung für explizite Konvertierungsoperatoren hinzu. Mit diesen können Sie eine sichere explicit operator bool() schreiben, die ordnungsgemäß funktioniert, ohne durch die Implementierung des Safe Bool Idiom zu springen.)

112
James McNellis
operator bool() const 
{
    return col != 0;
}

Definiert, wie die Klasse in einen booleschen Wert konvertiert werden kann. Die Variable const nach () wird verwendet, um anzuzeigen, dass diese Methode nicht mutiert (ändern Sie die Mitglieder dieser Klasse).

Normalerweise würden Sie solche Operatoren wie folgt verwenden:

airplaysdk sdkInstance;
if (sdkInstance) {
    std::cout << "Instance is active" << std::endl;
} else {
    std::cout << "Instance is in-active error!" << std::endl;
}
8
langerra.com

Ich würde gerne mehr Codes geben, um es klar zu machen.

struct A
{
    operator bool() const { return true; }
};

struct B
{
    explicit operator bool() const { return true; }
};

int main()
{
    A a1;
    if (a1) cout << "true" << endl; // OK: A::operator bool()
    bool na1 = a1; // OK: copy-initialization selects A::operator bool()
    bool na2 = static_cast<bool>(a1); // OK: static_cast performs direct-initialization

    B b1;     
    if (b1) cout << "true" << endl; // OK: B::operator bool()
    // bool nb1 = b1; // error: copy-initialization does not consider B::operator bool()
    bool nb2 = static_cast<bool>(b1); // OK: static_cast performs direct-initialization
}
5
selfboot

Es ist eine benutzerdefinierte implicit-Konvertierungsfunktion zum Konvertieren Ihrer Klasse in true oder false.

//usage
bool value = yourclassinstance; //yourclassinstance is converted into bool!
3
Nawaz

Es ist eine implizite Konvertierung in bool. Das heißt Wo implizite Konvertierungen zulässig sind, kann Ihre Klasse durch Aufruf dieser Methode in bool konvertiert werden.

1
Billy ONeal

Wie die anderen schon gesagt haben, handelt es sich um eine Typumwandlung, in diesem Fall in ein bool. Zum Beispiel:

class A {
    bool isItSafe;

public:
    operator bool() const
    {
        return isItSafe;
    }

    ...
};

Jetzt kann ich ein Objekt dieser Klasse als Booleschen verwenden:

A a;
...
if (a) {
    ....
}
0
chrisaycock

Beim Schreiben meines eigenen unique_ptr habe ich diesen Fall gefunden. Vor gegebenem std::unique_ptrS operator== :

template<class T1, class D1, class T2, class D2>
bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

template <class T, class D>
bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;

template <class T, class D>
bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;

Und dieser Testfall aus libstdcxx

  std::unique_ptr<int> ptr;
  if (ptr == 0)
    { }
  if (0 == ptr)
    { }
  if (ptr != 0)
    { }
  if (0 != ptr)
    { }

Beachten Sie, dass ptr eine explicit operator bool() const noexcept; hat, so dass operator overload resolution hier gut funktioniert, beispielsweise ptr == 0 wählt 

 template <class T, class D>
 bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;`.

Wenn hier kein explicit-Schlüsselwort vorhanden ist, wird ptr in ptr == 0 in bool konvertiert. Dann wird bool in int konvertiert, da bool operator==(int, int) eingebaut ist und 0int ist. Was auf uns wartet, ist ein mehrdeutiger Überlastungsfehler. 

Hier ist ein Minimales, Vollständiges und Überprüfbares Beispiel :

#include <cstddef>
struct A
{
    constexpr A(std::nullptr_t) {}
    operator bool() 
    {
        return true;
    }
};

constexpr bool operator ==(A, A) noexcept
{
    return true;
}

constexpr bool operator ==(A, std::nullptr_t) noexcept
{
    return true;
}

constexpr bool operator ==(std::nullptr_t, A) noexcept
{
    return true;
}

int main()
{
    A a1(nullptr);
    A a2(0);
    a1 == 0;
}

gcc

prog.cc: In function 'int main()':
prog.cc:30:8: error: ambiguous overload for 'operator==' (operand types are 'A' and 'int')
   30 |     a1 == 0;
      |     ~~ ^~ ~
      |     |     |
      |     A     int
prog.cc:30:8: note: candidate: 'operator==(int, int)' <built-in>
   30 |     a1 == 0;
      |     ~~~^~~~
prog.cc:11:16: note: candidate: 'constexpr bool operator==(A, A)'
   11 | constexpr bool operator ==(A, A) noexcept
      |                ^~~~~~~~
prog.cc:16:16: note: candidate: 'constexpr bool operator==(A, std::nullptr_t)'
   16 | constexpr bool operator ==(A, std::nullptr_t) noexcept
      |                ^~~~~~~~

clang

prog.cc:30:8: error: use of overloaded operator '==' is ambiguous (with operand types 'A' and 'int')
    a1 == 0;
    ~~ ^  ~
prog.cc:16:16: note: candidate function
constexpr bool operator ==(A, std::nullptr_t) noexcept
               ^
prog.cc:11:16: note: candidate function
constexpr bool operator ==(A, A) noexcept
               ^
prog.cc:30:8: note: built-in candidate operator==(int, int)
    a1 == 0;
       ^
prog.cc:30:8: note: built-in candidate operator==(float, int)
prog.cc:30:8: note: built-in candidate operator==(double, int)
prog.cc:30:8: note: built-in candidate operator==(long double, int)
prog.cc:30:8: note: built-in candidate operator==(__float128, int)
prog.cc:30:8: note: built-in candidate operator==(int, float)
prog.cc:30:8: note: built-in candidate operator==(int, double)
prog.cc:30:8: note: built-in candidate operator==(int, long double)
prog.cc:30:8: note: built-in candidate operator==(int, __float128)
prog.cc:30:8: note: built-in candidate operator==(int, long)
prog.cc:30:8: note: built-in candidate operator==(int, long long)
prog.cc:30:8: note: built-in candidate operator==(int, __int128)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(int, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long, int)
prog.cc:30:8: note: built-in candidate operator==(long long, int)
prog.cc:30:8: note: built-in candidate operator==(__int128, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, int)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, int)
prog.cc:30:8: note: built-in candidate operator==(float, float)
prog.cc:30:8: note: built-in candidate operator==(float, double)
prog.cc:30:8: note: built-in candidate operator==(float, long double)
prog.cc:30:8: note: built-in candidate operator==(float, __float128)
prog.cc:30:8: note: built-in candidate operator==(float, long)
prog.cc:30:8: note: built-in candidate operator==(float, long long)
prog.cc:30:8: note: built-in candidate operator==(float, __int128)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(float, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(double, float)
prog.cc:30:8: note: built-in candidate operator==(double, double)
prog.cc:30:8: note: built-in candidate operator==(double, long double)
prog.cc:30:8: note: built-in candidate operator==(double, __float128)
prog.cc:30:8: note: built-in candidate operator==(double, long)
prog.cc:30:8: note: built-in candidate operator==(double, long long)
prog.cc:30:8: note: built-in candidate operator==(double, __int128)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(double, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long double, float)
prog.cc:30:8: note: built-in candidate operator==(long double, double)
prog.cc:30:8: note: built-in candidate operator==(long double, long double)
prog.cc:30:8: note: built-in candidate operator==(long double, __float128)
prog.cc:30:8: note: built-in candidate operator==(long double, long)
prog.cc:30:8: note: built-in candidate operator==(long double, long long)
prog.cc:30:8: note: built-in candidate operator==(long double, __int128)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(long double, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(__float128, float)
prog.cc:30:8: note: built-in candidate operator==(__float128, double)
prog.cc:30:8: note: built-in candidate operator==(__float128, long double)
prog.cc:30:8: note: built-in candidate operator==(__float128, __float128)
prog.cc:30:8: note: built-in candidate operator==(__float128, long)
prog.cc:30:8: note: built-in candidate operator==(__float128, long long)
prog.cc:30:8: note: built-in candidate operator==(__float128, __int128)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(__float128, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long, float)
prog.cc:30:8: note: built-in candidate operator==(long, double)
prog.cc:30:8: note: built-in candidate operator==(long, long double)
prog.cc:30:8: note: built-in candidate operator==(long, __float128)
prog.cc:30:8: note: built-in candidate operator==(long, long)
prog.cc:30:8: note: built-in candidate operator==(long, long long)
prog.cc:30:8: note: built-in candidate operator==(long, __int128)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(long long, float)
prog.cc:30:8: note: built-in candidate operator==(long long, double)
prog.cc:30:8: note: built-in candidate operator==(long long, long double)
prog.cc:30:8: note: built-in candidate operator==(long long, __float128)
prog.cc:30:8: note: built-in candidate operator==(long long, long)
prog.cc:30:8: note: built-in candidate operator==(long long, long long)
prog.cc:30:8: note: built-in candidate operator==(long long, __int128)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(long long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(__int128, float)
prog.cc:30:8: note: built-in candidate operator==(__int128, double)
prog.cc:30:8: note: built-in candidate operator==(__int128, long double)
prog.cc:30:8: note: built-in candidate operator==(__int128, __float128)
prog.cc:30:8: note: built-in candidate operator==(__int128, long)
prog.cc:30:8: note: built-in candidate operator==(__int128, long long)
prog.cc:30:8: note: built-in candidate operator==(__int128, __int128)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(__int128, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned int, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned long long, unsigned __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, float)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, double)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, long double)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, __float128)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, __int128)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned int)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned long long)
prog.cc:30:8: note: built-in candidate operator==(unsigned __int128, unsigned __int128)
1 error generated.
0
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