How to make it really constant?

[shuisheng]

Dear All,

I have a question. Assume

struct A
{
int *p, *q;
};

struct B
{
A a;

const struct& GetA() const
{
return a;
}
};

I hope B::GetA() cannot change p, q pointed values, but it can, such
as

*(B::GetA().p) = 3;

How can I make it really constant? One way may be defining p and q as
follows
const int * p;
const int * q;

Whenever p, q pointed values want to change in class/struct A, do the
const_cast. But this is too cumbersome.

I remember a constant std::vector cannot change its values, even
thought it is also implemented by a inner pointer. How does
std::vector make the values constant?

Thanks and best regards,

Shuisheng

[tony_in_da_uk@yahoo.co.uk]

Re: How to make it really constant?

On Sep 19, 3:12 pm, shuisheng <shuishen...@ya hoo.comwrote:

Dear All,
>
I have a question. Assume
>
struct A
{
int *p, *q;
>
};
>
struct B
{
A a;
>
const struct& GetA() const
{
return a;
}
>
};
>
I hope B::GetA() cannot change p, q pointed values, but it can, such
as
>
*(B::GetA().p) = 3;
>
How can I make it really constant? One way may be defining p and q as
follows
const int * p;
const int * q;
>
Whenever p, q pointed values want to change in class/struct A, do the
const_cast. But this is too cumbersome.

If you want this kind of control, you should make A a class, with p
and q private. Then provide const A& get_a() const and A& get_a()
members etc..

Tony

[anon]

Re: How to make it really constant?

tony_in_da_uk@y ahoo.co.uk wrote:

On Sep 19, 3:12 pm, shuisheng <shuishen...@ya hoo.comwrote:

>Dear All,
>>
>I have a question. Assume
>>
>struct A
>{
> int *p, *q;
>>
>};
>>
>struct B
>{
> A a;
>>
> const struct& GetA() const
> {
> return a;
> }
>>
>};
>>
>I hope B::GetA() cannot change p, q pointed values, but it can, such
>as
>>
>*(B::GetA(). p) = 3;
>>
>How can I make it really constant? One way may be defining p and q as
>follows
>const int * p;
>const int * q;
>>
>Whenever p, q pointed values want to change in class/struct A, do the
>const_cast. But this is too cumbersome.

>
If you want this kind of control, you should make A a class, with p
and q private. Then provide const A& get_a() const and A& get_a()
members etc..

With this solution you can still change values. In the
const A& get_a() const
method, you couldn't change pointers, but could still change the value
they point to.

Only way is to change struct A

[Jim Z. Shi]

Re: How to make it really constant?


shuisheng wrote:

Dear All,
>
I have a question. Assume
>
struct A
{
int *p, *q;
};

add an assist method in you struct A.

i don't quite catch your program's requires, but you can try something
like this anyway.

cheers & hth,
Jim

//=============== =============== =============== =======
#include <iostream>

using namespace std;

struct A
{
int a;
int* pa;
const int* const get_pa() const
{
return pa;
}
};

struct B
{
A a;

const A& getA() const
{
return a;
}
};

int main()
{
B b;
int i = 4;
// b.getA().a = 3;
*(b.getA().get_ pa()) = i; // fail
*(b.getA().pa) = i; // OK
return 0;
}

[tony_in_da_uk@yahoo.co.uk]

Re: How to make it really constant?

On Sep 19, 3:12 pm, shuisheng <shuishen...@ya hoo.comwrote:

struct A { int *p, *q; };
>
struct B
{
A a;
const struct& GetA() const { return a; }
};
>
I hope B::GetA() cannot change p, q pointed values, but it can, such
as
>
*(B::GetA().p) = 3;

>

tony_in_da...@y ahoo.co.uk wrote:

If you want this kind of control, you should make A a class, with p
and q private. Then provide const A& get_a() const and A& get_a()
members etc..

>

On Sep 19, 4:31 pm, anon <a...@no.invali dwrote:

With this solution you can still change values. In the
const A& get_a() const
method, you couldn't change pointers, but could still change the value
they point to.
>
Only way is to change struct A [snip]

"If you want this kind of control, you should make A a class, with p
and q private."...

struct A
{
private:
int *p, *q;
};

"Then provide const A& get_a() const and A& get_a() members etc.."...

struct B
{
A a;
const struct& GetA() const
{
return a;
}

struct& GetA()
{
return a;
}
};

Clearly, there's nothing public to access in a. By implication, you
must expose p and q similarly...

struct A
{
const int* get_p() const { return p; }
int* get_p() { return p; }

const int* get_q() const { return q; }
int* get_q() { return q; }

private:
int *p, *q;
};

And there you have it.

Tony

[blargg]

Re: How to make it really constant?

In article
<8f8157c0-2671-4473-ba3a-992b92e2f73f@d1 g2000hsg.google groups.com>,
shuisheng <shuisheng75@ya hoo.comwrote:

struct A
{
int *p, *q;
};
>
struct B
{
A a;
>
const struct& GetA() const
{
return a;
}
};
>
I hope B::GetA() cannot change p, q pointed values, but it can, such
as
>
*(B::GetA().p) = 3;
>
How can I make it really constant? One way may be defining p and q as
follows
const int * p;
const int * q;
>
Whenever p, q pointed values want to change in class/struct A, do the
const_cast. But this is too cumbersome.
>
I remember a constant std::vector cannot change its values, even
thought it is also implemented by a inner pointer. How does
std::vector make the values constant?

You could write a smart pointer helper class that propagates const one
level down:

template<typena me T>
class Ptr {
T* p;
public:
Ptr& operator = ( T* rhs ) { p = rhs; return *this; }

T & operator * () { return *p; }
T const& operator * () const { return *p; }

T * operator -() { return p; }
T const* operator -() const { return p; }
// ...
};

struct A { Ptr<intx; };

int f( A& a )
{
a.x = new int; // OK
*a.x = 1234; // OK
return *a.x; // OK
}

int fc( A const& a )
{
a.x = new int; // error
*a.x = 1234; // error
return *a.x; // OK
}