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Although the compiler won't provide (default) implementations for constructors
and destructors of unrestricted unions, em(we) can (and em(must)). The task
isn't difficult, but there are some caveats.
Consider our unrestricted union's destructor. It clearly should destroy
tt(u_string)'s data if that is its currently active field; but it should do
nothing if tt(u_int) is its currently active field. But how does the
destructor know what field to destroy? It doesn't, as the unrestricted union
contains no information about what field is currently active.
This problem is tackled by embedding the unrestricted union in a larger
aggregate (like a class or a struct) where it becomes a regular data
member. We still consider the unrestricted union a data type by itself, but
its use requires caution. The surrounding class is provided with a em(d_field)
data member keeping track of the currently active union-field. The tt(d_field)
value is an enumeration value which is defined by the union. The actual use of
the unrestricted union is completely controlled by the aggregate, freeing the
aggregate's users from any administration related to the unrestricted union.
Using this design we start out with an explicit and empty implementation of
the destructor, as there's no way to tell the destructor itself what field to
destroy:
verb( Data::Union::~Union()
{};)
Nevertheless, unrestricted unions must properly destroy their class-type
fields. Since an unrestricted union itself doesn't know what its active field
is, it must be informed about that by its surrounding class. To simplify the
generalization to other types a static array of pointers to functions
destroying the current field's value is used. This array is defined in the
union's private section as
verb( static void (Union::*s_destroy[])();)
and it is initialized as:
verb( void (Union::*Union::s_destroy[])() =
{
&Union::destroyText,
&Union::destroyValue
};)
Primitive data types normally don't need any special attention when they
go out of scope, so tt(destroyValue) can be defined as an empty function:
verb( void Union::destroyValue()
{})
On the other hand, the member tt(destroyText) must explicitly call
tt(u_text's) destructor:
verb( void Union::destroyText()
{
u_text.std::string::~string();
})
Proper destruction can now be realized by a single function tt(void
destroy(Field field)) which simply calls the appropriate function:
verb( void Union::destroy(Field type)
{
(this->*s_destroy[type])();
})
Since the unrestricted union is defined as a data member of a surrounding
class, the surrounding class's destructor is responsible for the proper
destruction of its unrestricted union. As the surrounding class keeps track of
the currently active unrestricted union's field its implementation is easy:
verb( Data::~Data()
{
d_union.destroy(d_field);
})
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