Tail pad optimization, cache friendlyness and C++ interrop

[Remo via Digitalmars-d]

On Wednesday, 11 June 2014 at 07:22:51 UTC, deadalnix wrote:
> On Wednesday, 11 June 2014 at 06:30:26 UTC, Walter Bright wrote:
>> On 6/10/2014 11:11 PM, deadalnix wrote:
>>> On Wednesday, 11 June 2014 at 04:11:53 UTC, Walter Bright 
>>> wrote:
>>>> Hmm, this could have serious problems with the following:
>>>>
>>>> S1 s1;
>>>> S2 s2;
>>>> s2.c = 3;
>>>> memcpy(&s2.s1, &s1, sizeof(S1)); // Oops! stomped on s2.c
>>>
>>> Yes, that is why they do it only in specific condition (ie 
>>> when the thing use
>>> C++ specific feature and not C one).
>>
>> I don't understand - didn't you say this was expressible as C 
>> structs? Aren't those supposed to be compatible with C?
>>
>>
>
> struct S1 {
>     int a;
> private:
>     char b;
> };
>
> struct S2 : S1 {
>     char c;
> };
>
> S2 is 8 bytes. If you remove private in S1, then S2 becomes 12 
> bytes. S1 is not a "C" struct anymore and do not need to follow 
> the standard layout.

This is pretty strange behavior.
At least on Windows I can not confirm this.
Visual Studio 2013, Intel Compiler and Clang for windows have the 
same consistent behavior here.

private  do NOT affect struct size.

But there is a parameter in Visual Studio that will affect it 
called "Struct Member Alignment"
http://msdn.microsoft.com/en-us/library/xh3e3fd0.aspx

1 Byte (/Zp1)  sizeof(S1)=5,  sizeof(S2)=6
2 Byte (/Zp2)  sizeof(S1)=6,  sizeof(S2)=8
4 Byte (/Zp4)  sizeof(S1)=8,  sizeof(S2)=12
8 Byte (/Zp8)  sizeof(S1)=8,  sizeof(S2)=12   this is the default.

Of course the same can be done with #pragma pack(?) .

There is also __declspec(align(?)) and in C++11  alignas(?) but 
its behavior is different from #pragma pack(?) .