On Tue, 6 Dec 2011, Jonathan Wakely wrote:
On 6 December 2011 20:59, Bruce Fraser wrote:
The following code was compiled with g++ 4.1.2 and again with g++
4.6.2 on 64-bit Linux (more details at the end). Note that S1 and S3
have identical data members. S1 has a constructor defined, S3 does
not. S2 inherits from S1 and S4 inherits from S2. The size and
layout SHOULD be the same, but it is not as you can see from the
output.
For C++98 / C++03 there are no guarantees about class layout except
for POD types, and the presence of base classes and user-defined
constructors prevents your types being PODs.
C++11 provides the more fine-grained concept of a standard-layout
struct, but even then I don't think there's any guarantee your S2 and
S4 will have the same layout.
But g++ doesn't use a random layout, it (mostly) follows:
http://sourcery.mentor.com/public/cxx-abi/abi.html
so it makes sense to look at whether the layouts should be the same.
It seems to define the layout differently depending on whether a type is
POD (in the C++03 sense).
In particular, I note: "the C++ standard requires that compilers not
overlay the tail padding in a POD", but the ABI doesn't seem to impose any
such restriction on non-PODs.
It does look peculiar that this incites you to artificially make your
types non-POD for a more compact layout...
--
Marc Glisse
