Question

I have this class which I want to pass around Windows as LPARAM parameter. Since it’s small enough to fit inside LPARAM I want to pass it by value, but in case it expands later I want it to automatically switch to passing by reference.

I would use something like

typedef boost::call_traits<CMyClass>::param_type CMyClassParam;

which should be either const CMyClass or const CMyClass&
depending on whether sizeof(CMyClass) <= sizeof(LPARAM)
but call_traits optimizes only small PODs, not small classes.

typedef is taken care of by advice of Anycorn:
typedef boost::mpl::if_c<(sizeof(CMyClass) <= sizeof(LPARAM)), CMyClass, CMyClass&>::type CMyClassParam;

And then, how do I convert between CMyClassParam and LPARAM
(in case you don’t know, LPARAM is int, long enough to store a pointer)

CMyClass::operator LPARAM()
{
    // must be either
    return *(LPARAM*)this;
    //or
    return reinterpret_cast<LPARAM>(this);
}
    //so that I may call:

void SomeWinFunc(LPARAM p);

CMyClass vi;
SomeWinFunc(vi);

// and then get it back:
void SomeWinCallback(LPARAM p)
{
    CMyClassParam vi = (?????)p;
    // which should translate into either
    CMyClass ti = *(CMyClass*)(&p); // make a bitwise copy
    // or
    CMyClass& ti = *(CMYClass*)p;
}

Answer 1

Something like that?

typedef typename mpl::if_c<(sizeof(T) <= MAX), T, T&>::type P;
P param = p;

but I’d think compiler would optimize this for you automagically