Question

How do I make a class in C++, when initialized, return a Boolean value when its name is invoked, but no explicit function call make, like ifstream. I want to be able to do this:

objdef anobj();
if(anobj){
  //initialize check is true
}else{
  //cannot use object right now
}

not just for initialization, but a check for its ability to be used.

Answer 1

The way istream does it is by providing an implicit conversion to void*

• http://www.cplusplus.com/reference/iostream/ios/operator_voidpt/
• stream output and implicit void* cast operator function invocation

Update In reaction to the comments, the Safe Bool Idiom would be a far better solution to this: (code directly taken from that page)

  class Testable {
    bool ok_;
    typedef void (Testable::*bool_type)() const;
    void this_type_does_not_support_comparisons() const {}
  public:
    explicit Testable(bool b=true):ok_(b) {}

    operator bool_type() const {
      return ok_==true ? 
        &Testable::this_type_does_not_support_comparisons : 0;
    }
  };

  template <typename T> 
    bool operator!=(const Testable& lhs,const T& rhs) {
    lhs.this_type_does_not_support_comparisons();   
      return false; 
    } 
  template <typename T>
    bool operator==(const Testable& lhs,const T& rhs) {
    lhs.this_type_does_not_support_comparisons();
      return false;     
    }

The article by Bjorn Karlsson contains a reusable implementation for the Safe Bool Idiom

---

Old sample:

For enjoyment, I still show the straight forward implementation with operator void* overloading, for clarity and also to show the problem with that:

#include <iostream>

struct myclass
{
     bool m_isOk;

     myclass() : m_isOk(true) { }
     operator void* () const { return (void*) (m_isOk? 0x1 : 0x0); }
};

myclass instance;

int main()
{
    if (instance)
        std::cout << "Ok" << std::endl;

    // the trouble with this:
    delete instance; // no compile error !
    return 0;
}