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The unrestricted union becomes a data member of the surrounding aggregate
(e.g., tt(class Data)). The tt(class Data) is provided with a data member
tt(Union::Field d_field) and tt(Data)'s users might query the currently active
field from, e.g., an accessor tt(field):
verb( class Data
{
Union::Field d_field;
Union d_union;
public:
Data(int value = 0);
Data(Data const &other);
Data(Data &&tmp);
Data(std::string const &text);
~Data(); // empty body
Union::Field field() const;
...
};)
tt(Data)'s constructors receive tt(int) or tt(string) values. To pass these
values to tt(d_union), we need tt(Union) constructors for the various union
fields.
The unrestricted union itself starts out like this:
verb( union Union
{
enum Field
{
TEXT,
VALUE
};
private:
std::string u_text;
int u_value;
public:
Union(Union const &other) = delete;
~Union(); // empty
Union(int value);
Union(std::string const &text);
Union(Union const &other, Field type);
Union(Union &&tmp, Field type);
...
};)
The last two tt(Union) constructors are comparable to the standard copy-
and move constructors. With unrestricted unions, however, the existing union's
actual type needs to be specified so that the correct field is initialized.
To simplify the generalization to other types we apply a procedure that is
comparable to the procedure we followed for destroying an understricted union:
we define a static array of pointers to copy-functions. This array is declared
in the union's private section as
verb( static void (Union::*s_copy[])(Union const &other);)
and it is defined as:
verb( void (Union::*Union::s_copy[])(Union const &other) =
{
&Union::copyText,
&Union::copyValue
};)
The tt(copyText) and tt(copyValue) private members are responsible for
copying tt(other's) data fields. However, there is a little snag. Although
basic types can directly be assigned, class-type fields cannot. Destination
fields cannot be initialized using member initializers as the field to
initialize depends on the tt(Field) type that's passed to the
constructor. Because of that the initialization must be performed inside the
constructors' bodies. At that point the data fields are
merely a series of uninitialized bytes, and so placement new is used to
copy-construct class-type fields. Here are the implementations of the copy
functions:
verb( void Union::copyValue(Union const &other)
{
u_value = other.u_value;
}
void Union::copyText(Union const &other)
{
new(&u_text) string{ other.u_text };
})
When implementing the union's move constructor other considerations must be
taken into account. Since we're free to do whatever we want with the move
constructor's tt(Union &&tmp) object, we can simply grab its current field,
and store a tt(VALUE) type of value into tt(tmp). For that we use the
tt(Union's) tt(swap) facility, the current object's field, another tt(Union)
object, and the other tt(Union's) field type (swapping is discussed in the
next section). Of course, if there isn't any primitive typed field this
doesn't work. In that case field-specific move functions must be used,
comparable to the ones used when copy-constructing a tt(Union) object.
Now we're ready for the constructors' implementations:
verb( Union::Union(std::string const &text)
:
u_text(text)
{}
Union::Union(int value)
:
u_value(value)
{}
Union::Union(Union &&tmp, Field type)
{
swap(VALUE, tmp, type);
}
Union::Union(Union const &other, Field type)
{
(this->*s_copy[type])(other);
}
Data::Data(Data const &other)
:
d_field(other.d_field),
d_union(other.d_union, d_field)
{}
Data::Data(int value)
:
d_field(Union::VALUE),
d_union(value)
{}
Data::Data(Data const &other)
:
d_field(other.d_field),
d_union(other.d_union, d_field)
{})
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