ما معنى "المشغل bool () const"

Question

علي سبيل المثال:

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

col هو int. كيف operator bool() const الشغل؟

Answer 1

وظائف الأعضاء في النموذج

operator TypeName()

هم مشغلي التحويل. يسمحون باستخدام كائنات من نوع الفصل كما لو كانت من النوع TypeName وعندما يتم تحويلهم إلى TypeName باستخدام وظيفة التحويل.

في هذه الحالة بالذات ، operator bool() يسمح لاستخدام كائن من نوع الفئة كما لو كان أ bool. على سبيل المثال ، إذا كان لديك كائن من نوع الفئة المسماة obj, ، يمكنك استخدامه ك

if (obj)

هذا سوف يسمي operator bool(), ، إرجاع النتيجة ، واستخدم النتيجة كشرط if.

تجدر الإشارة إلى ذلك operator bool() هي فكرة سيئة للغاية ويجب ألا تستخدمها أبدًا. للحصول على شرح مفصل حول سبب كونه سيئًا وللحل للمشكلة ، انظر "المصطلح الآمن Bool."

(C ++ 0x ، المراجعة القادمة لمعيار C ++ ، تضيف دعمًا لمشغلي التحويل الصريح. explicit operator bool() يعمل بشكل صحيح دون الحاجة إلى القفز عبر أطواق تنفيذ تعبير Bool الآمن.)

Answer 2

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

يحدد كيف يكون الفصل قابلاً للتحويل إلى قيمة منطقية ، const بعد () يستخدم للإشارة إلى أن هذه الطريقة لا تتحول (تغيير أعضاء هذه الفئة).

عادة ما تستخدم هؤلاء المشغلين على النحو التالي:

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

Answer 3

أود أن أعطي المزيد من الرموز لتوضيح الأمر.

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
}

Answer 4

إنه محدد المستخدم implicit وظيفة التحويل لتحويل فصلك إلى أي منهما true أو false.

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

Answer 5

إنه تحويل ضمني إلى bool. أي أينما يُسمح بالتحويلات الضمنية ، يمكن تحويل فصلك إلى bool عن طريق استدعاء هذه الطريقة.

Answer 6

كما قال الآخرون ، إنه لتحويل النوع ، في هذه الحالة إلى أ bool. علي سبيل المثال:

class A {
    bool isItSafe;

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

    ...
};

الآن يمكنني استخدام كائن من هذه الفئة كما لو كان منطقيًا:

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

Answer 7

عند كتابة فريدة من نوعها ، وجدت هذه الحالة. منح std::unique_ptr'س 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;

وهذه حالة الاختبار من libstdcxx:

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

لاحظ ذلك ptr لديه explicit operator bool() const noexcept;, ، لذا operator overload resolution يعمل بشكل جيد هنا ، على سبيل المثال ، ptr == 0 يختار

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

إذا لم يكن لديه explicit الكلمة الرئيسية هنا ، ptr في ptr == 0 سيتم تحويله إلى bool, ، من ثم bool سيتم تحويله إلى int, ، لان bool operator==(int, int) مدمج و 0 هو int. ما ينتظرنا هو خطأ في حل الحمل الزائد الغامض.

هنا الحد الأدنى والكامل والتحقق:

#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;
}

مجلس التعاون الخليجي:

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.

Answer 8

استخدام شائع آخر هو لحاويات الأمراض المنقولة جنسيا لإجراء مقارنة المساواة على القيم الرئيسية داخل الكائنات المخصصة

class Foo
{
    public: int val;
};

class Comparer { public:
bool operator () (Foo& a, Foo&b) const {
return a.val == b.val; 
};

class Blah
{
std::set< Foo, Comparer > _mySet;
};