c++ convert class to boolean

Question

With all of the fundamental types of C++, one can simply query:

if(varname)

and the type is converted to a boolean for evaluation. Is there any way to replicate this functionality in a user-defined class? One of my classes is identified by an integer, although it has a number of other members, and I'd like to be able to check if the integer is set to NULL in such a manner.

Thanks.

Answer 1

Simply implement operator bool() for your class.

e.g.

class Foo
{
public:
    Foo(int x) : m_x(x) { }
    operator bool() const { return (0 != m_x); }
private:
    int m_x;
}

Foo a(1);
if (a) { // evaluates true
    // ...
}

Foo b(-1);
if (b) { // evaluates true
    // ...
}

Foo c(0);
if (c) { // evaluates false
    // ...
}

Answer 2

You can define a user-defined conversion operator. This must be a member function, e.g.:

class MyClass {
  operator int() const
  { return your_number; }
  // other fields
};

You can also implement operator bool. However, I would STRONGLY suggest against doing it because your class will become usable in arithmetic expressions which can quickly lead to a mess. IOStreams define, for example, conversion to void*. You can test void* in the same way you can test a bool, but there are no language-defined implicit conversions from void*. Another alternative is to define operator! with the desired semantics.

In short: defining conversion operator sto integer types (including booleans) is a REALLY bad idea.

Answer 3

C++ checks if the statements result is whether equal to zero nor not. So i think you can define equality operator for your class and define how your class will be different from zero in which conditions.

Answer 4

As others have stated, using operator int () or operator bool () is bad idea because of the conversions it allows. Using a pointer is better idea. The best know solution to this problem so far is to return a member (function) pointer:

class MyClass {
  void some_function () {}

  typedef void (MyClass:: * safe_bool_type) ();
  operator safe_bool_type () const
  { return cond ? &MyClass::some_function : 0; }
};