union
The union
keyword appears in functions that relate to discriminated unions.
MIDL supports
two types of discriminated unions: encapsulated unions and nonencapsulated
unions. The encapsulated union is compatible with previous implementations of
RPC (NCA version 1). The nonencapsulated union eliminates some of the
restrictions of the encapsulated union and provides a more visible discriminant
than the encapsulated union.
The
encapsulated union is identified by the switch keyword and the absence
of other union-related keywords.
The
nonencapsulated union, also known as a union, is identified by the presence of
the switch_is
and switch_type
keywords, which identify the discriminant and its type.
When you use in,
out unions, be aware that changing the value of the union switch during
the call can make the remote call behave differently from a local call. On
return, the stubs copy the in, out parameter into memory that is
already present on the client. When the remote procedure modifies the value of
the union switch and consequently changes the data object s size, the stubs can
overwrite valid memory with the out value. When the union switch changes
the data object from a base type to a pointer type, the stubs can overwrite
valid memory when they copy the pointer referent into the memory location
indicated by the in value of a base type.
The shape of
unions must be identical across platforms to ensure interconnectivity.
See Also