Why inherited struct members are inaccessible in union?

Question

I have following code which generates error at compile time:

struct s { int three; };

union alias {
    struct { int one; };
    struct { int two; };
    struct : s {};
};

int main()
{
    alias a;
    a.one = 1;   // OK
    a.two = 2;   // OK
    a.three = 3; // error: 'union alias' has no member named 'three'
    return 0;
}

For some reason members of inherited struct in the union are inaccessible. The main purpose of this code is to provide an aliases to members of struct s .

Why compiler doesn't see members of inherited structure? Any options to make them accessible?

Answer 1

Okay, after a bit of head-scratching, I think I can explain this correctly. Well, to start things off, this is Illegal As Heck (tm).

A struct defined within a union confuses the compiler a bit. If a struct def is followed by a name, a new variable of type of such struct would normally be created, but C++ explicitly disallows anonymous structs . If you omit that name, it could mean two things - either an anonymous struct type is created and then immediately dropped (which meets with a -Wpedantic warning), or, as seems to be happening with GCC, it considers it a creation of an anonymous member of an anonymous type , which is a non-standard extension:

union alias {
    struct { int one; };     // anonymous member of an anonymous type
    struct a { int two };    // no member, just def of "struct a"
    struct { int three; } b; // named (b) member of type anonymous struct
};

All in all, you can't really expect this to work... unless you create the union members with explicit type and data member names:

struct s { int three; };

union alias {
    struct A { int one; };
    struct B { int two; };
    struct C : s { };

    A a;
    B b;
    C c;
};

int main() {
    alias a;
    a.a.one = 1;   // OK
    a.b.two = 2;   // OK
    a.c.three = 3; // OK
}

The whole anonymous union member thing is still a bit puzzling to me, as I can't figure out anything in the standard that would make this work.