Class object as condition?

**TriKri** · 10-30-2009

Hi!

What happens if you have an object my_obj of class my_class, and you write:

Code:

if (my_obj) {...} else {...}

How do you know which code block that is executed? The first or the second? Can you make my_class decide what value an object of that type should have when it is looked at as a boolean variable (here true can be any value other than 0 so it doesn't have to be of the type bool)?

For example, I have a class which is basically a kind of wrapper class for a variable v (it contains some other stuff too). When I write if(obj) I want it to be the same as if I would have written if(obj.v).

Thanks in advance!

**grib** · 10-30-2009

Your best bet is to go ahead and give a meaningful name to the test, even if it's just is_valid(). This makes things easier to read and maintain. If you really want to have that magic test, write a conversion operator from bool.

Code:

operator bool() const {return v;}

**Ronix** · 10-30-2009

The most typical way to handle it is with operator void*:

Code:

class Myclass {
public:
   operator void* () const
      {
      return (isThisObjectValid)? (void*)this : 0;
      }
};

Using operator void* prevents nonsensical operations that may arise when you use operator bool (Like doing Myclass() + Myclass()).

This has a few corner cases where such an approach may cause problems (IE: deleting an instance of Myclass), in which case you may want to read on the safe bool idiom. Using operator void* should suffice in most situations, though.

**King Mir** · 10-30-2009

You can also return a member pointer. That cannot be deleted.

Code:

class MyClass {
private:
   bool isThisObjectValid;
   void nop();
public:
   typedef  void (MyClass::*MyClassFn)(); 
   operator MyClassFn () const
      {
      return (isThisObjectValid)? &MyClass::nop : 0;
      }
};

**laserlight** · 10-30-2009

Originally Posted by King Mir

You can also return a member pointer. That cannot be deleted.

That comes one step closer towards the safe bool idiom. The next step would be to disable comparison operators for MyClass if they do not make sense.

Anyway, I am still waiting for explicit conversion functions to finally arrive

**TriKri** · 10-31-2009

Originally Posted by King Mir

You can also return a member pointer. That cannot be deleted.

Code:

class MyClass {
private:
   bool isThisObjectValid;
   void nop();
public:
   typedef  void (MyClass::*MyClassFn)(); 
   operator MyClassFn () const
      {
      return (isThisObjectValid)? &MyClass::nop : 0;
      }
};

King Mir, can you please explain the code? First you typedef MyClassFn as a pointer to a void function taking no arguments (I guess?). Is it MyClassFn that is returned when you use an object of MyClass in an if-statement, since it's a void function? Or does it return a pointer to a void function (I'm confused)? And why do you return nop, what's it good for? Is all this code really necessary?

**King Mir** · 10-31-2009

MyClassFn is a pointer to member of MyClass that points to a method that takes void function taking no arguments, except the implicit this pointer. What matters it that it is the type of a pointer to nop.

MyClassFn used the type of the conversion operator. It is the same as void* in Ronix's example or bool in grib's. The actual value returned is NULL in the case of invalid, or a pointer to the method nop in the case of valid. Any non static method can be used in place of nop, provided that the type is made to match the typedef.

I tried to find a way to do this without the typedef, but could not get it to compile.

Every line serves a necessary function.

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