C ++联合定义

Question

Is is possible to define a union like we can do for named namespace ? I mean can we declare a first type in the union and then in another file add another type to the union ?

For now, I have a based class that contains a union of bit types (unsigned int and other which correspond to each derived class). I would like to split them in the derive class and construct bit by bit this union.

Answer 1

No, such as for structs/classes , enums .

n3337 7.3/1

A namespace is an optionally-named declarative region. The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace. Unlike other declarative regions , the definition of a namespace can be split over several parts of one or more translation units.

Answer 2

No. How could it even be made to work (not that it's desirable), with separate compilation. Consider something like:

union U { int i; double d; }

void
f()
{
    U aU;
    g( &aU );
    //  ...
}

How much space is the compiler to allocated on the stack for aU . Given that a DLL not yet written contains:

union U { char buff[1000]; };

void
g( U* aU )
{
    //  Use all 1000 bytes...
}

(Note that according to the standard, the above is undefined behavior. What you are asking would require it to work.)