Undefined / Uninitialized default values in a class

Question

Let's suppose you have this class:

class A
{
public:
  A () {}
  A (double val) : m_val(val) {}
  ~A () {}
private:
  double m_val;
};

Once I create an instance of A, how can I check if m_val has been initialized/defined? Put it in other words, is there a way to know if m_val has been initialized/defined or not? Something along the lines of the defined operator in Python, I suppose. (But correct me if I'm wrong.)

I thought of modifying the class and the c-tors the following way:

class A
{
public:
  A () : defined(false) {}
  A (double val) : m_val(val), defined(true) {}
  ~A () {}
private:
  double m_val;
  bool defined;
};

How do you rate this solution? Any suggestion?

TIA, Chris

Answer 1

IMO you should just initialize your member variables in all constructors, at least with a sensible default value:

A () : m_val(0.0) {}
A (double val) : m_val(val) {}

I don't see any benefit in retaining a garbage value in your variables (unless you plan to use them as a very crude random number generator - just kidding :-). Such garbage values and extra flags complicate the code, and would always require programmer attention to avoid bugs - and as we are humans, our attention sometimes slips...

Answer 2

You'll need to set a sensible default value in the default constructor, otherwise its value is undefined. Which basically means it will be a random value -- could be 0, NaN, or 2835.23098 -- no way to tell unless you set it explicitly.

class A
{
public:
  A () : m_val(0.0) {}
  A (double val) : m_val(val) {}
  ~A () {}
private:
  double m_val;
};