static member functions access to class members and methods

[Roger Leigh]

The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
for Linux in 21 Days--I know there are better) states that "static
member functions cannot access any non-static member variables".
However, this doesn't seem entirely correct. It also doesn't mention
whether static member functions can access protected and private
member data and methods (and I couldn't spot this in the FAQ).

I have a class row<Row> which derives from row_base:

template<typena me Row>
class row : public row_base
{
public:
typedef std::auto_ptr<R ow> row_ptr;

row(): row_base() {}

row(row_state status, bool modified=false) :
row_base(status , modified) {}

virtual ~row() {}

static row_ptr create(pqxx::re sult::const_ite rator row,
pqxxobject::tra nsaction& tran)
{
row_ptr p(new Row);
p->convert_impl(r ow, tran); // protected
p->row_base::m_st ate = STATE_INITIALIS ED; // private
return p;
}

virtual void convert_impl(pq xx::result::con st_iterator row,
pqxxobject::tra nsaction& tran) = 0;

}; // class row

Here, in the static create() method, row_base::m_sta te is a private
(enum) data member of the base class, yet I can assign it directly.
However, convert_impl() is a pure virtual function which is public
here and protected in the deriving class, but I get an error when I
compile:

places.cc: In static member function `static std::auto_ptr<_ Tp1>
pqxxobject::row <Row>::create(p qxx::result::co nst_iterator,
pqxxobject::tra nsaction&) [with Row = Place]':
.../pqxx-object/table.h:172: instantiated from `std::auto_ptr< std::list<Row, std::allocator< _CharT> > > pqxxobject::tab le<Row>::find_m any(const std::string&) [with Row = Place]'
places.cc:207: instantiated from here
places.cc:175: error: `virtual void
Place::convert_ impl(pqxx::resu lt::const_itera tor, pqxxobject::tra nsaction&)'
is protected
.../pqxx-object/row.h:97: error: within this context

row.h:97 is the "p->convert_impl(r ow, tran);" line, above.

I can't see the rationale between being able to access private data
members, but not protected methods (which are actually public in the
row<> class)!


I would probably be better making create() call a specialised
protected constructor, but I'm interested in learning why the above
doesn't work.


Thanks,
Roger

--
Roger Leigh

Printing on GNU/Linux? http://gimp-print.sourceforge.net/
GPG Public Key: 0x25BFB848. Please sign and encrypt your mail.


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[Rolf Magnus]

Re: static member functions access to class members and methods

Roger Leigh wrote:
[color=blue]
> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
> for Linux in 21 Days--I know there are better) states that "static
> member functions cannot access any non-static member variables".
> However, this doesn't seem entirely correct.[/color]

You're right. If your static member function has an object of the class,
it can access member variables of that object.
[color=blue]
> It also doesn't mention whether static member functions can access
> protected and private member data and methods (and I couldn't spot
> this in the FAQ).[/color]

It doesn't matter if the member is static or not. Any member function
can access any protected and private member of the same class.
[color=blue]
> I have a class row<Row> which derives from row_base:
>
> template<typena me Row>
> class row : public row_base
> {
> public:
> typedef std::auto_ptr<R ow> row_ptr;
>
> row(): row_base() {}
>
> row(row_state status, bool modified=false) :
> row_base(status , modified) {}
>
> virtual ~row() {}
>
> static row_ptr create(pqxx::re sult::const_ite rator row,
> pqxxobject::tra nsaction& tran)
> {
> row_ptr p(new Row);
> p->convert_impl(r ow, tran); // protected
> p->row_base::m_st ate = STATE_INITIALIS ED; // private
> return p;
> }
>
> virtual void convert_impl(pq xx::result::con st_iterator row,
> pqxxobject::tra nsaction& tran) = 0;
>
> }; // class row
>
> Here, in the static create() method, row_base::m_sta te is a private
> (enum) data member of the base class, yet I can assign it directly.
> However, convert_impl() is a pure virtual function which is public
> here and protected in the deriving class, but I get an error when I
> compile:
>
> places.cc: In static member function `static std::auto_ptr<_ Tp1>
> pqxxobject::row <Row>::create(p qxx::result::co nst_iterator,
> pqxxobject::tra nsaction&) [with Row = Place]':
> ../pqxx-object/table.h:172: instantiated from
> `std::auto_ptr< std::list<Row, std::allocator< _CharT> > >
> pqxxobject::tab le<Row>::find_m any(const std::string&) [with Row =
> Place]'
> places.cc:207: instantiated from here
> places.cc:175: error: `virtual void
> Place::convert_ impl(pqxx::resu lt::const_itera tor,
> pqxxobject::tra nsaction&)' is protected
> ../pqxx-object/row.h:97: error: within this context
>
> row.h:97 is the "p->convert_impl(r ow, tran);" line, above.
>
> I can't see the rationale between being able to access private data
> members, but not protected methods (which are actually public in the
> row<> class)![/color]

They are public in the row<> class, but you are declaring your pointer
as row_ptr, which in your case is an auto_ptr<Place> . So you're
accessing the object through a pointer to Place, where, as you say, the
member is protected, i.e. only Place itself and classes derived from
that can access it.

[Martijn Lievaart]

Re: static member functions access to class members and methods

On Tue, 20 Jan 2004 12:48:55 +0100, Rolf Magnus wrote:
[color=blue]
> Roger Leigh wrote:
>[color=green]
>> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
>> for Linux in 21 Days--I know there are better) states that "static
>> member functions cannot access any non-static member variables".
>> However, this doesn't seem entirely correct.[/color]
>
> You're right. If your static member function has an object of the class,
> it can access member variables of that object.[/color]

I guess it's a clumsy way of saying you have no this-pointer.

M4

[Rolf Magnus]

Re: static member functions access to class members and methods

Martijn Lievaart wrote:
[color=blue]
> On Tue, 20 Jan 2004 12:48:55 +0100, Rolf Magnus wrote:
>[color=green]
>> Roger Leigh wrote:
>>[color=darkred]
>>> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
>>> for Linux in 21 Days--I know there are better) states that "static
>>> member functions cannot access any non-static member variables".
>>> However, this doesn't seem entirely correct.[/color]
>>
>> You're right. If your static member function has an object of the
>> class, it can access member variables of that object.[/color]
>
> I guess it's a clumsy way of saying you have no this-pointer.[/color]

It's not clumsy, it's just more explicit. Anyway, saying that static
members cannot access non-static members is just plain wrong.

[Martijn Lievaart]

Re: static member functions access to class members and methods

On Tue, 20 Jan 2004 19:14:13 +0100, Rolf Magnus wrote:
[color=blue]
> Martijn Lievaart wrote:
>[color=green]
>> On Tue, 20 Jan 2004 12:48:55 +0100, Rolf Magnus wrote:
>>[color=darkred]
>>> Roger Leigh wrote:
>>>
>>>> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
>>>> for Linux in 21 Days--I know there are better) states that "static
>>>> member functions cannot access any non-static member variables".
>>>> However, this doesn't seem entirely correct.
>>>
>>> You're right. If your static member function has an object of the
>>> class, it can access member variables of that object.[/color]
>>
>> I guess it's a clumsy way of saying you have no this-pointer.[/color]
>
> It's not clumsy, it's just more explicit. Anyway, saying that static
> members cannot access non-static members is just plain wrong.[/color]

My bad, I was referring to the original statement, not your explanation. I
should have made that clearer.

M4

[c++novice]

Re: static member functions access to class members and methods

Roger Leigh <${roger}@inval id.whinlatter.u klinux.net.inva lid> wrote in message news:<87oesz3pp r.fsf@wrynose.w hinlatter.uklin ux.net>...[color=blue]
> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
> for Linux in 21 Days--I know there are better) states that "static
> member functions cannot access any non-static member variables".[/color]

The static functions can address only the static data of a class;
non-static data are unavailable to these functions. If non-static data
could be addressed, to which object would they belong? Any attempt
to access a normal class member will generate a compile time
error. Similarly, static functions cannot call non-static functions of
the class. All this is caused by the fact that static functions have
no this pointer.


[color=blue]
> However, this doesn't seem entirely correct. It also doesn't mention
> whether static member functions can access protected and private
> member data and methods (and I couldn't spot this in the FAQ).
>
> I have a class row<Row> which derives from row_base:
>
> template<typena me Row>
> class row : public row_base
> {
> public:
> typedef std::auto_ptr<R ow> row_ptr;
>
> row(): row_base() {}
>
> row(row_state status, bool modified=false) :
> row_base(status , modified) {}
>
> virtual ~row() {}
>
> static row_ptr create(pqxx::re sult::const_ite rator row,
> pqxxobject::tra nsaction& tran)
> {
> row_ptr p(new Row);
> p->convert_impl(r ow, tran); // protected
> p->row_base::m_st ate = STATE_INITIALIS ED; // private
> return p;
> }
>
> virtual void convert_impl(pq xx::result::con st_iterator row,
> pqxxobject::tra nsaction& tran) = 0;
>
> }; // class row
>
> Here, in the static create() method, row_base::m_sta te is a private
> (enum) data member of the base class, yet I can assign it directly.
> However, convert_impl() is a pure virtual function which is public
> here and protected in the deriving class, but I get an error when I
> compile:
>
> places.cc: In static member function `static std::auto_ptr<_ Tp1>
> pqxxobject::row <Row>::create(p qxx::result::co nst_iterator,
> pqxxobject::tra nsaction&) [with Row = Place]':
> ../pqxx-object/table.h:172: instantiated from `std::auto_ptr< std::list<Row, std::allocator< _CharT> > > pqxxobject::tab le<Row>::find_m any(const std::string&) [with Row = Place]'
> places.cc:207: instantiated from here
> places.cc:175: error: `virtual void
> Place::convert_ impl(pqxx::resu lt::const_itera tor, pqxxobject::tra nsaction&)'
> is protected
> ../pqxx-object/row.h:97: error: within this context
>
> row.h:97 is the "p->convert_impl(r ow, tran);" line, above.
>
> I can't see the rationale between being able to access private data
> members, but not protected methods (which are actually public in the
> row<> class)!
>
>
> I would probably be better making create() call a specialised
> protected constructor, but I'm interested in learning why the above
> doesn't work.
>
>
> Thanks,
> Roger
>
> --
> Roger Leigh
>
> Printing on GNU/Linux? http://gimp-print.sourceforge.net/
> GPG Public Key: 0x25BFB848. Please sign and encrypt your mail.
>
>
> -----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
> http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
> -----== Over 100,000 Newsgroups - 19 Different Servers! =-----[/color]

[Nick Hounsome]

Re: static member functions access to class members and methods


"c++novice" <belthezarcus20 04@yahoo.com> wrote in message
news:3471bdd3.0 401201220.2c933 de4@posting.goo gle.com...[color=blue]
> Roger Leigh <${roger}@inval id.whinlatter.u klinux.net.inva lid> wrote in[/color]
message news:<87oesz3pp r.fsf@wrynose.w hinlatter.uklin ux.net>...[color=blue][color=green]
> > The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
> > for Linux in 21 Days--I know there are better) states that "static
> > member functions cannot access any non-static member variables".[/color]
>
> The static functions can address only the static data of a class;
> non-static data are unavailable to these functions. If non-static data
> could be addressed, to which object would they belong? Any attempt
> to access a normal class member will generate a compile time
> error. Similarly, static functions cannot call non-static functions of
> the class. All this is caused by the fact that static functions have
> no this pointer.
>[/color]

No they can access any data and methods of the class - they just don't have
a 'this'.
[color=blue]
>
>[color=green]
> > However, this doesn't seem entirely correct. It also doesn't mention
> > whether static member functions can access protected and private
> > member data and methods (and I couldn't spot this in the FAQ).
> >
> > I have a class row<Row> which derives from row_base:
> >
> > template<typena me Row>
> > class row : public row_base
> > {
> > public:
> > typedef std::auto_ptr<R ow> row_ptr;
> >
> > row(): row_base() {}
> >
> > row(row_state status, bool modified=false) :
> > row_base(status , modified) {}
> >
> > virtual ~row() {}
> >
> > static row_ptr create(pqxx::re sult::const_ite rator row,
> > pqxxobject::tra nsaction& tran)
> > {
> > row_ptr p(new Row);
> > p->convert_impl(r ow, tran); // protected
> > p->row_base::m_st ate = STATE_INITIALIS ED; // private
> > return p;
> > }
> >
> > virtual void convert_impl(pq xx::result::con st_iterator row,
> > pqxxobject::tra nsaction& tran) = 0;
> >
> > }; // class row
> >
> > Here, in the static create() method, row_base::m_sta te is a private
> > (enum) data member of the base class, yet I can assign it directly.
> > However, convert_impl() is a pure virtual function which is public
> > here and protected in the deriving class, but I get an error when I
> > compile:
> >
> > places.cc: In static member function `static std::auto_ptr<_ Tp1>
> > pqxxobject::row <Row>::create(p qxx::result::co nst_iterator,
> > pqxxobject::tra nsaction&) [with Row = Place]':
> > ../pqxx-object/table.h:172: instantiated from[/color][/color]
`std::auto_ptr< std::list<Row, std::allocator< _CharT> > >
pqxxobject::tab le<Row>::find_m any(const std::string&) [with Row = Place]'[color=blue][color=green]
> > places.cc:207: instantiated from here
> > places.cc:175: error: `virtual void
> > Place::convert_ impl(pqxx::resu lt::const_itera tor,[/color][/color]
pqxxobject::tra nsaction&)'[color=blue][color=green]
> > is protected
> > ../pqxx-object/row.h:97: error: within this context
> >
> > row.h:97 is the "p->convert_impl(r ow, tran);" line, above.[/color][/color]

Are you certain because it seems to me that you shouldn't even be able to
create a row since you have
declared convert_impl pure virtual.
[color=blue][color=green]
> >
> > I can't see the rationale between being able to access private data
> > members, but not protected methods (which are actually public in the
> > row<> class)!
> >
> >
> > I would probably be better making create() call a specialised
> > protected constructor, but I'm interested in learning why the above
> > doesn't work.
> >
> >
> > Thanks,
> > Roger
> >
> > --
> > Roger Leigh
> >
> > Printing on GNU/Linux?[/color][/color]
http://gimp-print.sourceforge.net/[color=blue][color=green]
> > GPG Public Key: 0x25BFB848. Please sign and encrypt[/color][/color]
your mail.[color=blue][color=green]
> >
> >
> > -----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
> > http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
> > -----== Over 100,000 Newsgroups - 19 Different Servers! =-----[/color][/color]

[Rolf Magnus]

Re: static member functions access to class members and methods

c++novice wrote:
[color=blue]
> Roger Leigh <${roger}@inval id.whinlatter.u klinux.net.inva lid> wrote in
> message news:<87oesz3pp r.fsf@wrynose.w hinlatter.uklin ux.net>...[color=green]
>> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
>> for Linux in 21 Days--I know there are better) states that "static
>> member functions cannot access any non-static member variables".[/color]
>
> The static functions can address only the static data of a class;
> non-static data are unavailable to these functions. If non-static data
> could be addressed, to which object would they belong?[/color]

Any object that is available to the function.
[color=blue]
> Any attempt
> to access a normal class member will generate a compile time
> error. Similarly, static functions cannot call non-static functions of
> the class.[/color]

Just like a non-member-function can access non-static data of a class, a
static member function can. If you're still not convinced, explain
this:

#include <iostream>

class X
{
public:
static void foo(const X& x)
{
std::cout << x.text;
}

const char* text;
};

int main()
{
X x;
x.text = "Hello world!\n";
X::foo(x);
}

X::text is a non-static member of X, and still the static member
function does access it.
[color=blue]
> All this is caused by the fact that static functions have
> no this pointer.[/color]

Right, they have no this pointer, but they can still access non-static
members of the class if they get an instance of that class. They just
are not called for a specific instance.

[c++novice]

Re: static member functions access to class members and methods

Rolf Magnus <ramagnus@t-online.de> wrote in message news:<buk9ms$m4 e$05$1@news.t-online.com>...[color=blue]
> c++novice wrote:
>[color=green]
> > Roger Leigh <${roger}@inval id.whinlatter.u klinux.net.inva lid> wrote in
> > message news:<87oesz3pp r.fsf@wrynose.w hinlatter.uklin ux.net>...[color=darkred]
> >> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
> >> for Linux in 21 Days--I know there are better) states that "static
> >> member functions cannot access any non-static member variables".[/color]
> >
> > The static functions can address only the static data of a class;
> > non-static data are unavailable to these functions. If non-static data
> > could be addressed, to which object would they belong?[/color]
>
> Any object that is available to the function.
>[color=green]
> > Any attempt
> > to access a normal class member will generate a compile time
> > error. Similarly, static functions cannot call non-static functions of
> > the class.[/color]
>
> Just like a non-member-function can access non-static data of a class, a
> static member function can. If you're still not convinced, explain
> this:
>
> #include <iostream>
>
> class X
> {
> public:
> static void foo(const X& x)
> {
> std::cout << x.text;
> }
>
> const char* text;
> };
>
> int main()
> {
> X x;
> x.text = "Hello world!\n";
> X::foo(x);
> }
>
> X::text is a non-static member of X, and still the static member
> function does access it.[/color]

U r very much right because here an object of type X is passed in
static member function foo()as a parameter,
But foo() is not directly accessing the member like
std::cout << text;

[color=blue]
>[color=green]
> > All this is caused by the fact that static functions have
> > no this pointer.[/color]
>
> Right, they have no this pointer, but they can still access non-static
> members of the class if they get an instance of that class. They just
> are not called for a specific instance.[/color]

[Roger Leigh]

Re: static member functions access to class members and methods

Rolf Magnus <ramagnus@t-online.de> writes:
[color=blue]
> Martijn Lievaart wrote:
>[color=green]
>> On Tue, 20 Jan 2004 12:48:55 +0100, Rolf Magnus wrote:
>>[color=darkred]
>>> Roger Leigh wrote:>
>>>
>>>> The C++ book I have to hand (Liberty and Horvath, Teach yourself C++
>>>> for Linux in 21 Days--I know there are better) states that "static
>>>> member functions cannot access any non-static member variables".
>>>> However, this doesn't seem entirely correct.
>>>
>>> You're right. If your static member function has an object of the
>>> class, it can access member variables of that object.[/color]
>>
>> I guess it's a clumsy way of saying you have no this-pointer.[/color]
>
> It's not clumsy, it's just more explicit. Anyway, saying that static
> members cannot access non-static members is just plain wrong.[/color]

Much of the book is like that. I found it fairly readable for the
basics of the language, but it leaves a lot of questions unanswered,
and the devil is always in the details.

I did get a copy of Stroustrup's TCPL from the library at the same
time, but as a C programmer with no OO knowledge, I was out of my
depth several pages into the introduction!! It would probably be OK
now I have a decent grasp of the language, but the 21 days books was
far more understandable at the time.


--
Roger Leigh

Printing on GNU/Linux? http://gimp-print.sourceforge.net/
GPG Public Key: 0x25BFB848. Please sign and encrypt your mail.


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[Roger Leigh]

Re: static member functions access to class members and methods

Rolf Magnus <ramagnus@t-online.de> writes:
[color=blue]
> Roger Leigh wrote:
>[color=green]
>> I have a class row<Row> which derives from row_base:
>>
>> template<typena me Row>
>> class row : public row_base
>> {
>> public:
>> typedef std::auto_ptr<R ow> row_ptr;
>>
>> row(): row_base() {}
>>
>> row(row_state status, bool modified=false) :
>> row_base(status , modified) {}
>>
>> virtual ~row() {}
>>
>> static row_ptr create(pqxx::re sult::const_ite rator row,
>> pqxxobject::tra nsaction& tran)
>> {
>> row_ptr p(new Row);
>> p->convert_impl(r ow, tran); // protected
>> p->row_base::m_st ate = STATE_INITIALIS ED; // private
>> return p;
>> }
>>
>> virtual void convert_impl(pq xx::result::con st_iterator row,
>> pqxxobject::tra nsaction& tran) = 0;
>>
>> }; // class row
>>
>> Here, in the static create() method, row_base::m_sta te is a private
>> (enum) data member of the base class, yet I can assign it directly.
>> However, convert_impl() is a pure virtual function which is public
>> here and protected in the deriving class, but I get an error when I
>> compile:[/color][/color]
[...][color=blue][color=green]
>> I can't see the rationale between being able to access private data
>> members, but not protected methods (which are actually public in the
>> row<> class)![/color]
>
> They are public in the row<> class, but you are declaring your pointer
> as row_ptr, which in your case is an auto_ptr<Place> . So you're
> accessing the object through a pointer to Place, where, as you say, the
> member is protected, i.e. only Place itself and classes derived from
> that can access it.[/color]

Ah, that makes sense!

I've now got it working, but only after discovering a wart in the
language: the base class is a template and if I declare a pure virtual
(or virtual) function in the template, when I call the function it
does not call the derived version. It appears that virtual does not
work with templates at all :-\.

I've come up with two solutions: declare the function in the deriving
class public, so the template can call it directly (not desirable), or
keep it protected/private and make the base class a friend, so it can
call them even though they are protected (better, but still not
clean enough for my liking).

Is there a better alternative to these approaches? Basically, I'd
like to be able to fake virtual to achieve this effect:

template<typena me T>
class foo {
public:
void func() { func_impl(); }
private:
virtual void func_impl();
}

class bar : public foo<int> {
private:
virtual void func_impl();
}

i.e. when I do this:
bar c;
c.func()
c.func_impl() gets called by foo<int>::func( ).

Thanks,
Roger

--
Roger Leigh

Printing on GNU/Linux? http://gimp-print.sourceforge.net/
GPG Public Key: 0x25BFB848. Please sign and encrypt your mail.


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[Jeff Schwab]

Re: static member functions access to class members and methods

Roger Leigh wrote:
[color=blue]
> I did get a copy of Stroustrup's TCPL from the library at the same
> time, but as a C programmer with no OO knowledge, I was out of my
> depth several pages into the introduction!! It would probably be OK
> now I have a decent grasp of the language, but the 21 days books was
> far more understandable at the time.[/color]

Try skipping the intro. If you already have some familiarity with C,
the first few chapters of TC++PL should be mostly review for you, and
get you up to speed quickly on C++ as a "better C."