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.
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.
This seems like a bug; anonymous unions are just supposed to inject their member names into the containing scope as if there was an ordinary member there, and the explicit scope-qualification should resolve which subobject is meant for such injected names the same it resolves them for ordinary fields. But if it's a bug in all existing clangs, it's a bug we're going to have to work around.
John.
···
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.
On Dec 16, 2017, at 10:31 AM, John McCall via swift-dev <swift-dev@swift.org> wrote:
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.
This seems like a bug; anonymous unions are just supposed to inject their member names into the containing scope as if there was an ordinary member there, and the explicit scope-qualification should resolve which subobject is meant for such injected names the same it resolves them for ordinary fields. But if it's a bug in all existing clangs, it's a bug we're going to have to work around.
Feel free to file a bug if you’re motivated. Personally speaking, I don’t expect this bug to be on anybody’s priority list. The workaround of using named structs/unions has no downside for most projects, and Swift is no exception.
On Dec 16, 2017, at 10:31 AM, John McCall via swift-dev <swift-dev@swift.org> wrote:
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.
This seems like a bug; anonymous unions are just supposed to inject their member names into the containing scope as if there was an ordinary member there, and the explicit scope-qualification should resolve which subobject is meant for such injected names the same it resolves them for ordinary fields. But if it's a bug in all existing clangs, it's a bug we're going to have to work around.
Feel free to file a bug if you’re motivated. Personally speaking, I don’t expect this bug to be on anybody’s priority list. The workaround of using named structs/unions has no downside for most projects, and Swift is no exception.
On Dec 16, 2017, at 10:31 AM, John McCall via swift-dev <swift-dev@swift.org> wrote:
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.
This seems like a bug; anonymous unions are just supposed to inject their member names into the containing scope as if there was an ordinary member there, and the explicit scope-qualification should resolve which subobject is meant for such injected names the same it resolves them for ordinary fields. But if it's a bug in all existing clangs, it's a bug we're going to have to work around.
