What is the meaning of operator bool const

Question

For example   operator bool   const         return col    0       col is an int  How does operator bool   const work

User · Answer

When writing my own unique ptr  I found this case  Given std  unique ptr s operator     template lt class T1  class D1  class T2  class D2 gt  bool operator   const unique ptr lt T1  D1 gt  amp  x  const unique ptr lt T2  D2 gt  amp  y    template  lt class T  class D gt  bool operator   const unique ptr lt T  D gt  amp  x  nullptr t  noexcept   template  lt class T  class D gt  bool operator   nullptr t  const unique ptr lt T  D gt  amp  x  noexcept    And this test case from libstdcxx      std  unique ptr lt int gt  ptr    if  ptr    0            if  0    ptr            if  ptr    0            if  0    ptr            Note because that ptr has a explicit operator bool   const noexcept   so operator overload resolution works fine here  e g   ptr    0 chooses    template  lt class T  class D gt   bool operator   const unique ptr lt T  D gt  amp  x  nullptr t  noexcept      If it has no explicit keyword here  ptr in ptr    0 will be converted into bool  then bool will be converted into int  because bool operator   int  int  is built-in and 0 is int  What is waiting for us is ambiguous overload resolution error    Here is a Minimal  Complete  and Verifiable example    include  lt cstddef gt  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    lt built-in gt     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

User · Answer

I d like to give more codes to make it clear   struct A       operator bool   const   return true        struct B       explicit operator bool   const   return true        int main         A a1      if  a1  cout  lt  lt   true   lt  lt  endl     OK  A  operator bool       bool na1   a1     OK  copy-initialization selects A  operator bool       bool na2   static cast lt bool gt  a1      OK  static cast performs direct-initialization      B b1           if  b1  cout  lt  lt   true   lt  lt  endl     OK  B  operator bool          bool nb1   b1     error  copy-initialization does not consider B  operator bool       bool nb2   static cast lt bool gt  b1      OK  static cast performs direct-initialization

User · Answer

It s an implicit conversion to bool  I e  wherever implicit conversions are allowed  your class can be converted to bool by calling that method

User · Answer

As the others have said  it s for type conversion  in this case to a bool  For example   class A       bool isItSafe   public      operator bool   const               return isItSafe                      Now I can use an object of this class as if it s a boolean   A a      if  a

User · Answer

It s user-defined implicit conversion function to convert your class into either true or false     usage bool value   yourclassinstance    yourclassinstance is converted into bool

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Another common use is for std containers to do equality comparison on key values inside custom objects  class Foo       public  int val      class Comparer   public  bool operator     Foo amp  a  Foo amp b  const   return a val    b val       class Blah   std  set lt  Foo  Comparer  gt   mySet

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operator bool   const        return col    0      Defines how the class is convertable to a boolean value  the const after the    is used to indicate this method does not mutate  change the members of this class    You would usually use such operators as follows   airplaysdk sdkInstance  if  sdkInstance        std  cout  lt  lt   Instance is active   lt  lt  std  endl    else       std  cout  lt  lt   Instance is in-active error    lt  lt  std  endl

User · Answer

Member functions of the form   operator TypeName     are conversion operators   They allow objects of the class type to be used as if they were of type TypeName and when they are  they are converted to TypeName using the conversion function     In this particular case  operator bool   allows an object of the class type to be used as if it were a bool   For example  if you have an object of the class type named obj  you can use it as  if  obj    This will call the operator bool    return the result  and use the result as the condition of the if   It should be noted that operator bool   is A Very Bad Idea and you should really never use it   For a detailed explanation as to why it is bad and for the solution to the problem  see  The Safe Bool Idiom     C  0x  the forthcoming revision of the C   Standard  adds support for explicit conversion operators   These will allow you to write a safe explicit operator bool   that works correctly without having to jump through the hoops of implementing the Safe Bool Idiom

[c++] What is the meaning of "operator bool() const"

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