Re: pointer to a member of a member

Fri, 27 Jun 2008 12:45:38 -0700 (PDT)
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 syn=


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 woul=


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 fo=


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.

Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

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.


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