Re: [Boost-users] sizeof(boost::fusion:map)

Subject: Re: [Boost-users] sizeof(boost::fusion:map)
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2010-11-05 19:56:23

Christopher Schmidt schrieb:
> Robert Lupton the Good schrieb:
>> I was very excited to find the boost::fusion library, expecting that I could use it to implement structs with introspection.  I still think I can, but I was disappointed to see that I'm paying a size penalty. I'd have expected all the book-keeping to be handled at compile time.
>>
>> The following code prints
>> 	sizeof(Foo) = 12 sizeof(FooS) = 4
>> with g++ 4.2 or 4.4 on an os/x 10.6 or Centos box.
>>
>> Here Foo is
>> 	boost::fusion::map<boost::fusion::pair<Name, int> > 
>>
> [snip]
> 
> I have no idea why there is that space overhead. This problem is not
> fusion specific, though.
> 
> #include <iostream>
> struct X{};
> struct A:X{int i;};
> struct B:X{A a;};
> int main()
> {std::cout << sizeof(A) << " " << sizeof(B) << std::endl;}
> 
> When compiled with gcc 4.5.1 i686-pc-mingw32 and -O2, the output is "4
> 8". When compiled with MSVC 2010 (x86) and /O2, "4 4" is printed.
> 
> -Christopher

I think gcc is correct. There are two distinct instances of X in B, and
those may not have the same address. That's why there is that 4 byte
displacement.
It is pretty much the same in Fusion. All inbuilt sequences and the
vector data storage share a common base class - fusion::sequence_root -
that's where the 4 resp. 8 bytes come from.

-Christopher