C++ library系列 -- STL实现中的ODR “one-definition-rule” for types
Linking issue
- While different modules (.obj) using istreambuf_iterator/ostreambuf_iterator, compiled with different options on HID/no-HID and SCL/no-SCL, these modules could not be linked successfully;
The error comes directly from the CLR when a type has multiple definitions that are not consistent based upon the ODR, one-definition-rule for types. And, the linker itself isn't involved.
For example, with one module compile with /D_SECURE_SCL=0, while another is compiled with _SECURE_SCL=1;
At first, it's found that with _SECURE_SCL, the only thing that could be different as following,
#if _SECURE_SCL
typedef _Range_checked_iterator_tag _Checked_iterator_category;
#endif
But, actually, it's not the typedef that changed the layout the these iterators (istreambuf_iterator/ostreambuf_iterator), and further they don't really use the extra pointer that _SECURE_SCL adds.
Finally, it's found the root cause is that, these iterators, istreambuf_iterator/ostreambuf_iterator had been moved from <xutility> to <streambuf>, and their ultimate base class had been changed from _Iterator_base_secure to _Iterator_base. And, the layout of _Iterator_base would be different between HID and no-HID, and between SCL and no-SCL. It is the cause where the issue comes from.
What we can learn from such issue,
These iterators really shouldn't derive from either _Iterator_base_secure or _Iterator_base, because these classes contain data members (pointers) which are entirely unused. It would result in unnecessary bloat and extra work being performed in ctor/dtor etc.
Introduce a new class, _Iterator_base_universal, which is defined identically regardless of HID/no-HID and SCL/no-SCL. It would contains the three internal typedefs that all standard iterators need to have, and nothing else. And _Iterator_base (in all of its variants) and _Iterator_base_secure now should derive from _Iterator_base_universal to get these typedefs.
Now, when an iterator wants these typedefs, but not the data members of _Iterator_base and _Iterator_base_secure, it could derive from _Iterator_base_universal. And istreambuf_iterator and ostreambuf_iterator are now as small as possible, and keep identical representations or layout across HID/no-HID, SCL/no-SCL.