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Fix Bugzilla 14945 - unions are missing from the ABI page #3915

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Fix Bugzilla 14945 - unions are missing from the ABI page #3915

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9 changes: 7 additions & 2 deletions spec/abi.dd
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Original file line number Diff line number Diff line change
Expand Up @@ -68,9 +68,14 @@ $(H2 $(LNAME2 delegates, Delegates))
function's stack frame (nested functions).
)

$(H2 $(LNAME2 structs, Structs))
$(H2 $(LNAME2 structs, Structs and Unions))

$(P Conforms to the target's C ABI struct layout.)
$(P These conform to the target's C ABI struct layout, except:)

* An `extern(D)` struct or union with no fields has size 1, not 0.
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Also extern(C++) structs seem to have non-zero size judging by running a simple example. The C++ Standard (all versions) agrees with that.

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* An `extern(D)` struct or union with no fields has size 1, not 0.
* An `extern(D)` or `extern(C++)` struct or union with no fields has size 1, not 0.

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This paragraph is about differences from C. The link that follows is for further information.

See $(DDSUBLINK spec/struct, struct_layout, Struct Layout).
* Unions are $(RELATIVE_LINK2 name_mangling, name-mangled) as if they were structs.
* The rest of this document treats unions extrinsically the same as structs.

$(H2 $(LNAME2 classes, Classes))

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13 changes: 12 additions & 1 deletion spec/struct.dd
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Expand Up @@ -176,7 +176,18 @@ $(H2 $(LNAME2 struct_layout, Struct Layout))
the first field and the start of the object.
)

$(P Structs with no fields of non-zero size (aka $(I Empty Structs)) have a size of one byte.)
$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields of non-zero
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@Bolpat Bolpat Sep 25, 2024 •
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Avoid the double negation “no … with non-…” and instead use “all … with …”.

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$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields of non-zero
$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with all fields of zero

Mathematically speaking, any all statement about things is vacuously true if there are none. Most people who read this part of the spec will understand it, but if you’re in doubt, you could do this instead:

Suggested change
$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields of non-zero
$(P $(DDSUBLINK spec/attribute, linkage, `extern(D)`) structs with no fields or all fields of zero

size (aka $(I Empty Structs)) have a size of one byte.)

$(SPEC_RUNNABLE_EXAMPLE_COMPILE
---
extern(C) struct C {}
struct D {}

static assert(C.sizeof == 0);
static assert(D.sizeof == 1);
Comment on lines +184 to +188
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Might be worth adding an extern(C++) example.

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extern(C) struct C {}
struct D {}
static assert(C.sizeof == 0);
static assert(D.sizeof == 1);
extern(C) struct C {}
extern(C++) struct Cpp {}
struct D {}
static assert(C.sizeof == 0);
static assert(Cpp.sizeof == 1);
static assert(D.sizeof == 1);

---
)

$(P Non-static $(RELATIVE_LINK2 nested, function-nested D structs), which access the context of
their enclosing scope, have an extra field.
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