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The IceUtil::Optional Template

The C++ mapping uses a template to hold the values of optional data members and parameters:

{zcode:cpp}
namespace IceUtil {
template<typename T>
class Optional {
public:
    typedef T element_type;

    Optional();
    Optional(NoneType);
    Optional(const T&);
    
    template<typename Y>
    Optional(const Optional<Y>&);

    Optional(const Optional& r);
    
    Optional& operator=(NoneType);
    Optional& operator=(const T&);

    template<typename Y>
    Optional& operator=(const Optional<Y>&);
    Optional& operator=(const Optional&);

    const T& get() const;
    T& get();
    const T* operator->() const;
    T* operator->();
    const T& operator*() const;
    T& operator*();

    operator bool() const;
    bool operator!() const;

    void swap(Optional& other);

    ...
};
}
{zcode}

The IceUtil::Optional template provides constructors and assignment operators that allow you to initialize an instance using the element type or an existing optional value. The default constructor initializes an instance to an unset condition. The get method and dereference operators retrieve the current value held by the instance, or throw IceUtil::OptionalNotSetException if no value is currently set. Use the bool or ! operators to test whether the instance has a value prior to dereferencing it. Finally, the swap method exchanges the state of two instances.

The IceUtil::None Value

The template includes a constructor and assignment operator that accept NoneType. Ice defines an instance of this type, IceUtil::None, that you can use to initialize (or reset) an Optional instance to an unset condition:

{zcode:cpp}
IceUtil::Optional<int> i = 5;
i = IceUtil::None;
assert(!i); // true
{zcode}

You can pass IceUtil::None anywhere an IceUtil::Optional value is expected.


See Also