digitalmars.D.bugs - [Issue 12714] New: .offsetof problems in structs with alias this
- via Digitalmars-d-bugs (55/56) May 08 2014 https://issues.dlang.org/show_bug.cgi?id=12714
https://issues.dlang.org/show_bug.cgi?id=12714 Issue ID: 12714 Summary: .offsetof problems in structs with alias this Product: D Version: D2 Hardware: x86 OS: Windows Status: NEW Severity: enhancement Priority: P1 Component: DMD Assignee: nobody puremagic.com Reporter: bearophile_hugs eml.cc This issue is currently an enhancement request, but I think it's borderline a bug. This documentation page: http://dlang.org/struct.html Gives a definition of the .offsetof field: .offsetof Offset in bytes of field from beginning of struct But in presence of structs with "alias this" .offsetof has problems: A comment from Artur Skawina:if the object does not contain the requested member, but implicitly converts to another one that does have such field then the expression compiles, but yields a bogus value.<Example code: struct Foo { int f1, f2; } struct Bar { int b1, b2, b3, b4; Foo x; alias x this; } void main() { Bar b; assert(cast(void*)&b.f2 - cast(void*)&b == 20); static assert(Bar.x.offsetof == 16); static assert(Bar.f2.offsetof == 4); // *** } I think this is a little trap for D programmers. I expect "Bar.f2.offsetof" to return 20 (or not to compile). In a struct without "alias this" this situation is more clear: struct Foo { int f1, f2; } struct Bar { int b1, b2, b3, b4; Foo x; } void main() { Bar b; assert(cast(void*)&b.x.f2 - cast(void*)&b == 20); static assert(Bar.x.offsetof == 16); static assert(Bar.x.f2.offsetof == 4); } Now "Bar.f2.offsetof" doesn't compile, and "Bar.x.f2.offsetof" is 4 because it's the offset of the field f2 relative to the start of the struct instance x. --
May 08 2014