pointer to a member of a member

[huili80@gmail.com]

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 syntax
for it?
Thanks!

[Victor Bazarov]

Re: pointer to a member of a member

huili80@gmail.c om 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 syntax
for it?

Why do you think you need it? Does this help:

B b;
int *ptr = &b.a.x;

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

[n2xssvv.g02gfr12930]

Re: pointer to a member of a member

huili80@gmail.c om 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 syntax
for it?
Thanks!

Not sure what you require, but the code sample below may be of help

int A::*ptr = A::x;
A Example;
Example.*ptr = 15;

JB

[Victor Bazarov]

Re: pointer to a member of a member

n2xssvv.g02gfr1 2930 wrote:

huili80@gmail.c om 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 syntax
>for it?
>Thanks!

>
Not sure what you require, but the code sample below may be of help
>
int A::*ptr = A::x;

int A::*ptr = &A::x;

(without the ampersand it's not legal).

A Example;
Example.*ptr = 15;

Now do that for a 'B'... :-)

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

[n2xssvv.g02gfr12930]

Re: pointer to a member of a member

Victor Bazarov wrote:

n2xssvv.g02gfr1 2930 wrote:

>huili80@gmail.c om 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 syntax
>>for it?
>>Thanks!

>>
>Not sure what you require, but the code sample below may be of help
>>
>int A::*ptr = A::x;

>
int A::*ptr = &A::x;
>
(without the ampersand it's not legal).
>

>A Example;
>Example.*ptr = 15;

>
Now do that for a 'B'... :-)
>
V

A careless mistake, cheers Victor

JB

[Greg Herlihy]

Re: pointer to a member of a member

On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@com Acast.netwrote:

huil...@gmail.c om 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 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
}

Greg

[Victor Bazarov]

Re: pointer to a member of a member

Greg Herlihy wrote:

On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@com Acast.netwrote:

>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 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
}
>
Greg

I would like to see what the OP has to say about his/her need to create
such a construct.

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

[huili80@gmail.com]

Re: pointer to a member of a member

On Jun 27, 3:00 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

Greg Herlihy wrote:

On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@com Acast.netwrote:

huil...@gmail.c om 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 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
    }

>

Greg

>
I would like to see what the OP has to say about his/her need to create
such a construct.
>
V
--
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?

Thanks!

[huili80@gmail.com]

Re: pointer to a member of a member

On Jun 27, 3:33 pm, huil...@gmail.c om wrote:

On Jun 27, 3:00 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
>
>
>

Greg Herlihy wrote:

On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@com Acast.netwrote:
>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 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
    }

>

Greg

>

I would like to see what the OP has to say about his/her need to create
such a construct.

>

V
--
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.

>
Thanks!

[Victor Bazarov]

Re: pointer to a member of a member

huili80@gmail.c om wrote:

On Jun 27, 3:33 pm, huil...@gmail.c om wrote:

>On Jun 27, 3:00 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
>>
>>
>>

>>Greg Herlihy wrote:
>>>On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@com Acast.netwrote:
>>>>huil...@gma il.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 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
>>> }
>>>Greg
>>I would like to see what the OP has to say about his/her need to create
>>such a construct.
>>V
>>--
>>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.

>Thanks!

>

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?

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

[huili80@gmail.com]

Re: pointer to a member of a member

On Jun 27, 4:04 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

huil...@gmail.c om wrote:

On Jun 27, 3:33 pm, huil...@gmail.c om wrote:

On Jun 27, 3:00 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

>

>Greg Herlihy wrote:
>>On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@com Acast.netwrote:
>>>huil...@gmai l.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 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
>>    }
>>Greg
>I would like to see what the OP has to say about his/her need to create
>such a construct.
>V
>--
>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.

Thanks!

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

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

switch(i/2)
{
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;
else
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).

[Victor Bazarov]

Re: pointer to a member of a member

huili80@gmail.c om wrote:

On Jun 27, 4:04 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:

>huil...@gmail. com wrote:

>>On Jun 27, 3:33 pm, huil...@gmail.c om wrote:
>>>On Jun 27, 3:00 pm, Victor Bazarov <v.Abaza...@com Acast.netwrote:
>>>>Greg Herlihy wrote:
>>>>>On Jun 27, 10:42 am, Victor Bazarov <v.Abaza...@com Acast.netwrote:
>>>>>>huil...@g mail.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 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
>>>>> }
>>>>>Greg
>>>>I would like to see what the OP has to say about his/her need to create
>>>>such a construct.
>>>>V
>>>>--
>>>>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.
>>>Thanks!

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

>
>
That gives the correct result, but not the best performance. A more
efficient solution would be
>
switch(i/2)
{
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;
else
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?

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

[Frank Birbacher]

Re: pointer to a member of a member

Hi!

Apart from what Victor says:

huili80@gmail.c om schrieb:

That gives the correct result, but not the best performance. A more
efficient solution would be
>
switch(i/2)
{
case 0: return v1[i%2]; break;
case 1: return v2[i%2]; break;
case 2: return z; break;
default: throw "bad index";
}

This is probably slower due to extra calculations

because v1[k] (as implemented in my earlier post) is much faster than
>
if ( k == 0 )
return v1.x;
else
return v1.y;

Are you aware of the fact that the "switch" statement is supposed to be
optimized by table lookup. That means it cannot be compare to a series
of if-else statements. The "switch" can have a performance of O(1).
Which is exactly the same as your array. And I guess the table-lookup
optimization for the "switch" statement is more likely implemented than
a combination of function inlining and constant array lookup.

This optimization is why a switch will only accept integral values.

Anyway, unless you have measured runtime performance of an array of
member pointers compared to a chained if-else compared to a switch,
there is no point in discussing which one could be faster. It just depends.

Frank

[huili80@gmail.com]

Re: pointer to a member of a member

On Jun 27, 6:32 pm, Frank Birbacher <bloodymir.c... @gmx.netwrote:

Hi!
>
Apart from what Victor says:
>
huil...@gmail.c om schrieb:
>

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

>

switch(i/2)
{
   case 0: return v1[i%2]; break;
   case 1: return v2[i%2]; break;
   case 2: return z; break;
   default: throw "bad index";
}

>
This is probably slower due to extra calculations
>

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

>

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

>
Are you aware of the fact that the "switch" statement is supposed to be
optimized by table lookup. That means it cannot be compare to a series
of if-else statements. The "switch" can have a performance of O(1).
Which is exactly the same as your array. And I guess the table-lookup
optimization for the "switch" statement is more likely implemented than
a combination of function inlining and constant array lookup.
>
This optimization is why a switch will only accept integral values.
>
Anyway, unless you have measured runtime performance of an array of
member pointers compared to a chained if-else compared to a switch,
there is no point in discussing which one could be faster. It just depends.
>
Frank

I agree, without measurement it's meaningless to talk about
performance. But I wasn't asking about the performance in the first
place anyway (it appeared in a later example that was aksed for). Even
if the example was completely meansingless performance-wise, it
doesn't mean my question about "pointer to a member of a member" is
necessary meaningless.

I was trying to get an answer on how to get a "pointer to a member of
a member" if there exists something like that.
Maybe such a thing can be used in a good way, who knows.

[Andrey Tarasevich]

Re: pointer to a member of a member

huili80@gmail.c om wrote:

>
I was trying to get an answer on how to get a "pointer to a member of
a member" if there exists something like that.
Maybe such a thing can be used in a good way, who knows.

Of course, it can be. There's no real technical or conceptual difference
between 'pointer-to-data-member' and
'pointer-to-data-member-of-data-member'. They would be used in exactly
the same way.

Technically, the implementation of such a pointer would be exactly the
same as that of the existing 'pointer-to-data-member', meaning that the
pointer _type_ itself is already in the language. The only thing that's
really missing is the syntax that would let us to assign the proper
value to such a pointer.

Because of that latter part, the answer is no, you can't use such a
pointer in standard C++.

--
Best regards,
Andrey Tarasevich