This is expected. The inheritance chain ultimately ends up a B in both cases,
so the reference to j is ambiguous, and the compiler can’t resolve which B is
meant.
This works.
class B
{
protected:
int i;
union { int j; };
};
class C
{
protected:
int i;
union { int j; };
};
class X : public B { };
class Y : public C { };
class Z : public X, public Y
{
int a() { return X::i; }
int b() { return Y::i; }
int c() { return X::j; }
int d() { return Y::j; }
};
If “B” is a metaphor for a common base class in the “real” case, you are going
to run into one of the pitfalls of C++ multiple inheritance, the “diamond”
inheritance problem (Z inherits from X and Y, which both inherit from B, forms
an inheritance “diamond” if you graph it out). That is why C++ has virtual
inheritance. However, with anonymous unions, there is still an ambiguity as to
which union member to access (as far as clang++ is concerned). Giving it a
name or encasing it in another class instance resolves that ambigiuity.
Jonathan
> On Dec 16, 2017, at 9:08 AM, David Zarzycki via swift-dev
> <swift-dev@swift.org> wrote:
>
> Hello,
>
> I’m trying to improve SILNode memory layout density by adopting the AST
> bitfield macros. Unfortunately, multiple inheritance doesn’t seem to get
> along with anonymous/unnamed unions. Here is a distillation of the problem:
>
> class B {
> protected:
> int i;
> union { int j; };
> };
>
> class X : public B { };
> class Y : public B { };
>
> class Z : public X, public Y {
> int a() { return X::i; } // works
> int b() { return X::j; } // fails
> };
>
> Is this expected C++ behavior? I can certainly workaround this by naming the
> unnamed union, but before I do, I thought that I should check here first.
>
> Thanks,
> Dave
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