C++ implementation of Option Design Pattern

[Weddingcrasher]

Hi everybody,

I am trying to realize the Option design pattern (from http://www.codeproject.com/Articles/17607/The-Option-Pattern) in C++. I am trying it with usage of C++ templates, but somehow got stuck, because I can't check against null if the template parameter T is a primitive datatype. My current source code is shown below.

Any suggestions? Many thanks in advance.

Option.h:

#ifndef OPTION_H_
#define OPTION_H_

	template <typename T>
	class Option
	{
	private:
		bool _valueIsPresent;
		T _value;

	public:

                 /**
		 * Create a new option without any value.
		 */
		Option()
		{
			_valueIsPresent = false;
		}

		/**
		 * Create a new option containing the given value.
		 * @param The value to store in the option.
		 */
		Option(T value)
		{
			// VALUE MUST BE PROVIDED
			if (value == 0)
			{
				// THROW EXCEPTION HERE
			}

			_value = value;
			_valueIsPresent = true;
		}

		/**
		 * Gets information whether there is a value present.
		 * @return Returns true if value is present, false otherwise.
		 */
		inline bool isValuePresent() { return _valueIsPresent; }

		/**
		 * Gets the value.
		 * Note: If no value is present and this method is called, an InvalidOperationException exception is thrown.
		 */
		T getValue()
		{
			// Prevent access to non existing value
			if (!_valueIsPresent)
			{
				throw InvalidOperationException("Tried to get a non existing value.");
			}

			return _value;
		}
	};

#endif /* OPTION_H_ */

[coder777]

because I can't check against null if the template parameter T is a primitive datatype.

What? For primitive type (aka POD) you can always check against 0. When you have complex types you cannot.

What you can do is:

		Option() : _value() // This initializes _value with a default value (in case of POD 0)
		{
...

_value = T(); // This initializes _value with a default value (in case of POD 0)
if(_value == T()) // This compares POD's with 0 and other types with the default object 

[aquaz]

If you want to enforce a pointer to be bot null(but why the user would not be able to pass a null pointer? I think an option on a null pointer makes sense) you can specialize the class for pointer types:

template <typename T>
class Option<T*>
{
...
}

[morando]

http://en.cppreference.com/w/cpp/types/is_pod