Re: pointer to a member of a member

Victor Bazarov <>
Fri, 27 Jun 2008 16:44:01 -0400
<g43jeh$dii$> wrote:

On Jun 27, 4:04 pm, Victor Bazarov <> wrote: wrote:

On Jun 27, 3:33 pm, wrote:

On Jun 27, 3:00 pm, Victor Bazarov <> wrote:

Greg Herlihy wrote:

On Jun 27, 10:42 am, Victor Bazarov <> wrote: wrote:

Say I have two classes:
class A
    int x;
class B
    A a;
Then how do I construct a member pointer to B::a.x ? What's the syntax
for it?

Why do you think you need it? Does this help:
     B b;
     int *ptr = &b.a.x;

The question seems to me to be asking for a member pointer - not a
pointer to a (data) member. If that is the case, then the answer would
be that it is not possible to create a single, member pointer to
b.a.x. Instead it is necessary to declare two member pointers (one for
B::a and the other for A::x) and then apply them both. For example:
    struct A
        int x;
    struct B
        A a;
    int main()
        B b;
        A B::*pa = &B::a;
        int A::*pi = &A::x;
        b.*pa.*pi = 3; // assigns 3 to b.a.x

I would like to see what the OP has to say about his/her need to create
such a construct.
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Here is an example (probably over-simplified from the actual case I'm
working on). Say I have a 2D vector class:
struct vector2d
    double x,y;
    static double vector2d::* const _v[2];
    double& operator[] (int i) { return this->*_v[i]; }
    const double& operator[] (int i) const { return this->*_v[i]; }};
double vector2d::* const vector2d::_v[] = { &vector2d::x,
&vector2d::y };
and suppose we have an object "vector2d v;" . The purpose of using
pointer to member here is to make v[0] and v.x have exactly the same
run-time efficiency, provided that the compiler is capable of
necessary optimization. (I didn't invent this technique, but I forgot
where I learned it).
Suppose now for some reason, I want to build a 5D vector class out of
this 2D vector class, say like this.
class vector5d
    vector2d v1, v2;
    double z;
and we have an object "vector5d w;"
What I want is, with as little run-time overhead as possible (maybe
using a similar method that's used by vector2d), that w[0] gives me
w.v1.x , w[1] gives w.v1.y , w[2] gives w.v2.x , w[3] gives w.v2.y ,
and w[4] gives me w.z .
Is it possible? If yes, how?

I mean, is it possible to achieve zero run-time overhead (assuming
proper optimization) in accessing members (and their members) via an
index? If we don't have a vector5d::z (in which case it's actually a
4D vector), we might want to use an array of pointers to member of a
member (I don't know how even if they do exist). Having vector5d::z
makes this even more complicated in that a pointer to vector5d::z and
a (may or may not existing) pointer to vector5d::v1.x certainly would
have different types, so they cannot be put into an array.


What you seem to be looking for is

     struct vector5d
         vector2d v1, v2;
         double z;
         double& operator[](int i) {
             switch (i) {
                 case 0: return v1[0];
                 case 1: return v1[1];
                 case 2: return v2[0];
                 case 3: return v2[1];
                 case 4: return z;
                 default: throw "bad index";

Isn't it?

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That gives the correct result, but not the best performance. A more
efficient solution would be

   case 0: return v1[i%2]; break;
   case 1: return v2[i%2]; break;
   case 2: return z; break;
   default: throw "bad index";

because v1[k] (as implemented in my earlier post) is much faster than

if ( k == 0 )
   return v1.x;
   return v1.y;

But can we achieve even better efficiency? Directing the program to
different branch based on the even- or odd-ness of an integer would
almost certainly be slower than just shifting a pointer by that
integer. That's exactly how in the vector2d class, v[0] has the same
efficiency as v.x . (again assuming proper optimization).

Are you sure about this? Has this been measured or is that your
theoretical conclusion? And if it has been measured, how much
difference on the application scale are we talking about?

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