Empty base member optimisation with multiple inheritance

[David Welch]

Hi,

I have a bit of code where I am relying on empty base member
optimisation. The bit of code is below:

template<typena me Enum>
struct EncodePrefix
{
template<Enum e>
struct Apply
{
struct Type
{
const unsigned char Value;
Type() : Value((unsigned char)e) {}
};
};

template<>
struct Apply<(Enum)ENU M_NONE>
{
//typedef Apply Type;
struct Type {};
};
};
template<EnumRe pLockPrefix RepLockPrefix, EnumSegPrefix SegPrefix
,EnumOperandSiz ePrefix OperandSizePref ix
,EnumAddressSiz ePrefix AddressSizePref ix
,EnumSimdPrefix SimdPrefix>
struct EncodePrefixes
: EncodePrefix<En umRepLockPrefix >::Apply<RepLoc kPrefix>::Type
, EncodePrefix<En umSegPrefix>::A pply<SegPrefix> ::Type
, EncodePrefix<En umOperandSizePr efix>
::Apply<Operand SizePrefix>::Ty pe
, EncodePrefix<En umAddressSizePr efix>
::Apply<Address SizePrefix>::Ty pe
, EncodePrefix<En umSimdPrefix>:: Apply<SimdPrefi x>::Type
{
};

So when I define EncodePrefixes like this:

EncodePrefixes< NONE,NONE,NONE, NONE,NONE> ep;

I expect:
std::assert(siz eof(ep) == 1);

Unfortunately this is not the case because the msvc doesn't do empty
base member optimisation for multiple inheritance, in-case you are
wondering, sizeof(ep) == 4.

Does anyone have a work-around for which doesn't involve explicit
specialisation of the EncodePrefix class 5*2 times.

---
David

[David Welch]

David Welch wrote:

Hi,

I have a bit of code where I am relying on empty base member
optimisation.

Ahhhhhh, I solved the problem myself ;-)

template<unsign ed char Prefix_, typename Next = void>
struct EncodePrefix
{
struct Type : Next::Type
{ const unsigned char Prefix; Type() : Prefix(Prefix_) {} };
};

template<typena me Next>
struct EncodePrefix<EN UM_NONE, Next>
{
struct Type : Next::Type { };
};

template<unsign ed char Prefix_>
struct EncodePrefix<Pr efix_, void>
{
struct Type
{ const unsigned char Prefix; Type() : Prefix(Prefix_) {} };
};

template<>
struct EncodePrefix<EN UM_NONE, void>
{
struct Type { };
};

template<EnumRe pLockPrefix RepLockPrefix
,EnumSegPrefix SegPrefix
,EnumOperandSiz ePrefix OperandSizePref ix
,EnumAddressSiz ePrefix AddressSizePref ix
,EnumSimdPrefix SimdPrefix>
struct EncodePrefixes
: EncodePrefix<Re pLockPrefix,
EncodePrefix<Se gPrefix,
EncodePrefix<Op erandSizePrefix ,
EncodePrefix<Ad dressSizePrefix ,
EncodePrefix<Si mdPrefix> > > > >
{
};