Re: pointer to a member of a member
On Jun 27, 3:00 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:
Greg Herlihy wrote:
On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@comAcast.net> wrote:
huil...@gmail.com wrote:
Say I have two classes:
class A
{
public:
int x;
};
class B
{
public:
A a;
};
Then how do I construct a member pointer to B::a.x ? What's the synta=
x
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
}
Greg
I would like to see what the OP has to say about his/her need to create
such a construct.
V
--
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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?
Thanks!