A thing that can be default until set. And become default after reset.

DefaultVariable.cpp

template <typename T, typename D>
class DefaultVariable
{
public:
	operator =(const T & value)
	{
		currentValue = value;
		is_default = currentValue == defaultValue;
	}
	operator const T &() const
	{
		return is_default ? defaultValue : currentValue;
	}
	bool isDefault() const
	{
		return is_default;
	}
private:
	T currentValue;
	T defaultValue = D;
	bool is_default = true;
};