sizeof of two template types, when both are derived from one base

Question

Sample code

#include <iostream>

struct base {};

template<typename Type>
struct left : base {

   Type value;
};

template<typename Type>
struct right : base {

   Type value;
}; 

int main() {
   std::cout << "sizeof left<base> = " << sizeof(left<base>) << std::endl;
   std::cout << "sizeof left<right<base>>    = " << sizeof(left<right<base>>) << std::endl;
   std::cout << "sizeof left<right<left<right<left<base>>>>> = " << sizeof(left<right<left<right<left<base>>>>>) << std::endl;
}

Output

With GCC 4.6 is

sizeof left<base> = 2  
sizeof left<right<base>>    = 3  
sizeof left<right<left<right<left<base>>>>> = 6

With clang 3.1

sizeof left<base> = 2  
sizeof left<right<base>>    = 3  
sizeof left<right<left<right<left<base>>>>> = 6

With MSVC 2012

sizeof left<base> = 1
sizeof left<right<base>>    = 1
sizeof left<right<left<right<left<base>>>>> = 1

So, question is, is it bug in GCC/clang, or is it implementation defined, or is it right output (quotes from standard, or explanations of such behaviour will be nice)

Answer 1

The relevant quote is 1.8 [intro.object] paragraph 6:

Unless an object is a bit-field or a base class subobject of zero size, the address of that object is the address of the first byte it occupies. Two objects that are not bit-fields may have the same address if one is a subobject of the other, or if at least one is a base class subobject of zero size and they are of different types; otherwise, they shall have distinct addresses.

In your right<T> and left<T> object (why have to different class templates? One should have been enough) you each have a member value (of type T). Each one needs to get its own unique address. Thus,

sizeof(left<right<left<right<left<base>>>>>) == 1

is definitely wrong! There are 6 distinct objects:

• 5 values
• one left<right<left<right<left<base>>>>>

and only the left<right<left<right<left<base>>>>> and one of its subjects the (the first value if I recall other rules) can share an address. That is, the size of the object needs to be at least 5. Since objects work best when aligned it seems to get padded to 6 bytes (which is odd; I'd expect it to be padded to a multiple of 4).

Even size of left<base> could't be 1: There are two base objects involved already! One in the form of the base class of lef<base> and one in form of a member of this class. These twobase` objects need distinct addresses and, thus, the size needs to be at least 2.

In any case, object sizes only have requirements how big they are at least. They don't have any requirement that they shall not be bigger than something. This is considered a quality of implementation issue. Based on this, the only compiler being wrong (assuming the sizes quotes are, indeed, correct) is MSVC++. The other sizes may sometimes be slightly bigger than desired but this isn't an error.