class variables: scoping

[Neil Zanella]

Hello,

Suppose I have some method:

Foo::foo() {
static int x;
int y;
/* ... */
}

Here variables x is shared by all instances of the class, but has
function scope. On the other hand, variable y, also has function
scope, but is reinitialized each time the method is called.

Now, I would like to have a variable which has function scope,
that is, is not visible by other class functions. However, every
time I instantiate an instance of Foo a new such variable is created.
Essentially, I would like to have something like the equivalent of:

class Foo {
/* ... */
private:
/* ... */
int z;
/* ... */
};

However, I do not want to have all methods inside Foo see z. I only
want method foo to see it, just like I did for x which can only be
seen by foo().

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.

It would be nice if C++ were extended to include this nice idea.
It could for example allow something like:

Foo::foo() {
static int x;
int y;
semistatic z;
/* ... */
}

Where semistatic means: shared by all invocations within the same
instance, and only visible within foo().

I think this feature is missing from C++ and is quite useful. Because
if some class variable is used only in one method, then why should I
have to place it in the class definition. It's like having to make
things global just because they are shared by instances.
Comments welcome.

Regards,

Neil

[Leor Zolman]

On 3 Apr 2004 13:23:11 -0800, nz******@cs.mun .ca (Neil Zanella) wrote:

Hello,

Suppose I have some method: How about calling them "functions" now? You've been around for a while...
;-)

Foo::foo() {
static int x;
int y;
/* ... */
}

Here variables x is shared by all instances of the class,
Not /that/ variable x, if you're referring to the one in Foo:foo(). That's
a block-scope static, which means it can only be referred to within that
function (and its value persists across multiple calls to it. By the way,
it gets initialized to zero the first time the function is called, if you
don't give it an initializer.)

but has
function scope. On the other hand, variable y, also has function
scope, but is reinitialized each time the method is called.
Not by the code you've shown. Automatic locals without an initializer are
left uninitialized.

Now, I would like to have a variable which has function scope,
that is, is not visible by other class functions. However, every
time I instantiate an instance of Foo a new such variable is created.
So far you've described a simple local automatic variable.
Essentially, I would like to have something like the equivalent of:

class Foo {
/* ... */
private:
/* ... */
int z;
/* ... */
};
Oh boy.

However, I do not want to have all methods inside Foo see z. I only
want method foo to see it, just like I did for x which can only be
seen by foo().
The x you actually defined up there in foo() satisfies this (but not what
you go on to specify below.)

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.
I /think/ I see what you want, but there's no primitive way to declare that
in C++: You want an instance variable (one copy associated with each
instance of an object) that is only in scope within a particular function,
right? No can do. You'll just have to exercise some self-discipline by
declaring a private data member and being careful to only access it from
within the one function.

It would be nice if C++ were extended to include this nice idea.
It could for example allow something like:

Foo::foo() {
static int x;
int y;
semistatic z;
/* ... */
}

Where semistatic means: shared by all invocations within the same
instance, and only visible within foo().
Lots of things would be nice. This one isn't going to happen, though.
I think this feature is missing from C++ and is quite useful. Because
if some class variable is used only in one method, then why should I
have to place it in the class definition.
To indicate to the compiler that it has to make room for it in each object
instantiated. At least the way C++ is currently defined ;-)
It's like having to make
things global just because they are shared by instances.
Comments welcome.
You got 'em.
-leor

Regards,

Neil

--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html

[Leor Zolman]

On 3 Apr 2004 13:23:11 -0800, nz******@cs.mun .ca (Neil Zanella) wrote:

Hello,

Suppose I have some method: How about calling them "functions" now? You've been around for a while...
;-)

Foo::foo() {
static int x;
int y;
/* ... */
}

Here variables x is shared by all instances of the class,
Not /that/ variable x, if you're referring to the one in Foo:foo(). That's
a block-scope static, which means it can only be referred to within that
function (and its value persists across multiple calls to it. By the way,
it gets initialized to zero the first time the function is called, if you
don't give it an initializer.)

but has
function scope. On the other hand, variable y, also has function
scope, but is reinitialized each time the method is called.
Not by the code you've shown. Automatic locals without an initializer are
left uninitialized.

Now, I would like to have a variable which has function scope,
that is, is not visible by other class functions. However, every
time I instantiate an instance of Foo a new such variable is created.
So far you've described a simple local automatic variable.
Essentially, I would like to have something like the equivalent of:

class Foo {
/* ... */
private:
/* ... */
int z;
/* ... */
};
Oh boy.

However, I do not want to have all methods inside Foo see z. I only
want method foo to see it, just like I did for x which can only be
seen by foo().
The x you actually defined up there in foo() satisfies this (but not what
you go on to specify below.)

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.
I /think/ I see what you want, but there's no primitive way to declare that
in C++: You want an instance variable (one copy associated with each
instance of an object) that is only in scope within a particular function,
right? No can do. You'll just have to exercise some self-discipline by
declaring a private data member and being careful to only access it from
within the one function.

It would be nice if C++ were extended to include this nice idea.
It could for example allow something like:

Foo::foo() {
static int x;
int y;
semistatic z;
/* ... */
}

Where semistatic means: shared by all invocations within the same
instance, and only visible within foo().
Lots of things would be nice. This one isn't going to happen, though.
I think this feature is missing from C++ and is quite useful. Because
if some class variable is used only in one method, then why should I
have to place it in the class definition.
To indicate to the compiler that it has to make room for it in each object
instantiated. At least the way C++ is currently defined ;-)
It's like having to make
things global just because they are shared by instances.
Comments welcome.
You got 'em.
-leor

Regards,

Neil

--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html

[@(none)]

Neil Zanella wrote:

[...]However, I do not want to have all methods inside Foo see z. I only
want method foo to see it, just like I did for x which can only be
seen by foo().

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.

It would be nice if C++ were extended to include this nice idea.
It could for example allow something like:

Foo::foo() {
static int x;
int y;
semistatic z;
/* ... */
}
If I understood correctly, you want to associate a variable (state) to a
member function (behavior)... I would say you need a new object for that
(and a class, of course). Your little object would have just one method,
so I guess it's safe to call it a functor, maybe overwrite the "()"
opertor to look more like a function...
Where semistatic means: shared by all invocations within the same
instance, and only visible within foo().

I think this feature is missing from C++ and is quite useful. Because
if some class variable is used only in one method, then why should I
have to place it in the class definition. It's like having to make
things global just because they are shared by instances.
Comments welcome.

I'm not sure it is that common to justify inclusion in the language..
and it's not that difficult to get the same effect using existing
language constructs.

regards,
iuli

[@(none)]

Neil Zanella wrote:

[...]However, I do not want to have all methods inside Foo see z. I only
want method foo to see it, just like I did for x which can only be
seen by foo().

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.

It would be nice if C++ were extended to include this nice idea.
It could for example allow something like:

Foo::foo() {
static int x;
int y;
semistatic z;
/* ... */
}
If I understood correctly, you want to associate a variable (state) to a
member function (behavior)... I would say you need a new object for that
(and a class, of course). Your little object would have just one method,
so I guess it's safe to call it a functor, maybe overwrite the "()"
opertor to look more like a function...
Where semistatic means: shared by all invocations within the same
instance, and only visible within foo().

I think this feature is missing from C++ and is quite useful. Because
if some class variable is used only in one method, then why should I
have to place it in the class definition. It's like having to make
things global just because they are shared by instances.
Comments welcome.

I'm not sure it is that common to justify inclusion in the language..
and it's not that difficult to get the same effect using existing
language constructs.

regards,
iuli

[Siemel Naran]

"Neil Zanella" <nz******@cs.mu n.ca> wrote in message

Now, I would like to have a variable which has function scope,
that is, is not visible by other class functions. However, every
time I instantiate an instance of Foo a new such variable is created.
Essentially, I would like to have something like the equivalent of:

Basically, a private data member accessible to one or more member functions.

I tried the following once but it fails compile. But maybe we can tweak it
a bit, and then it will compile? Maybe with the pointer to implementation
concept it would work?

class Foo;

class Foo_f_Data
{
int x;
friend void Foo::f(); // this line fails compile because class Foo not
defined
};

class Foo : private Foo_f_Data
{
public:
void f();
void g();
};

void Foo::f() {
x = 3; // ok
}

void Foo::g() {
x = 5; // fails compile, c
}

Of course, comments are a nice idea too!

[Siemel Naran]

"Neil Zanella" <nz******@cs.mu n.ca> wrote in message

Now, I would like to have a variable which has function scope,
that is, is not visible by other class functions. However, every
time I instantiate an instance of Foo a new such variable is created.
Essentially, I would like to have something like the equivalent of:

Basically, a private data member accessible to one or more member functions.

I tried the following once but it fails compile. But maybe we can tweak it
a bit, and then it will compile? Maybe with the pointer to implementation
concept it would work?

class Foo;

class Foo_f_Data
{
int x;
friend void Foo::f(); // this line fails compile because class Foo not
defined
};

class Foo : private Foo_f_Data
{
public:
void f();
void g();
};

void Foo::f() {
x = 3; // ok
}

void Foo::g() {
x = 5; // fails compile, c
}

Of course, comments are a nice idea too!

[Alf P. Steinbach]

* nz******@cs.mun .ca (Neil Zanella) schriebt:

Essentially, I would like to have something like the equivalent of:

class Foo {
/* ... */
private:
/* ... */
int z;
/* ... */
};

However, I do not want to have all methods inside Foo see z. I only
want method foo to see it, just like I did for x which can only be
seen by foo().

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.

class Foo;

class FooFunctor
{
private:
Foo& myFoo;
int myZ;
public:
FooFunctor( Foo& aFoo ): myFoo( aFoo ), myZ( 0 ) {}
int operator()();
};

class Foo
{
private:
int x;
FooFunctor foo;
public:
Foo(): x(0), foo(*this) {}

void bar()
{
x = foo();
}
};

int FooFunctor::ope rator()(){ ... }

It would be nice if C++ were extended to include this nice idea.
It could for example allow something like:

Foo::foo() {
static int x;
int y;
semistatic z;
/* ... */
}

Where semistatic means: shared by all invocations within the same
instance, and only visible within foo().

As I recall there is a proposal to add members with automatic back-
pointers, like Java inner classes, but whether it will be adopted...

--
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Alf P. Steinbach]

* nz******@cs.mun .ca (Neil Zanella) schriebt:

Essentially, I would like to have something like the equivalent of:

class Foo {
/* ... */
private:
/* ... */
int z;
/* ... */
};

However, I do not want to have all methods inside Foo see z. I only
want method foo to see it, just like I did for x which can only be
seen by foo().

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.

class Foo;

class FooFunctor
{
private:
Foo& myFoo;
int myZ;
public:
FooFunctor( Foo& aFoo ): myFoo( aFoo ), myZ( 0 ) {}
int operator()();
};

class Foo
{
private:
int x;
FooFunctor foo;
public:
Foo(): x(0), foo(*this) {}

void bar()
{
x = foo();
}
};

int FooFunctor::ope rator()(){ ... }

It would be nice if C++ were extended to include this nice idea.
It could for example allow something like:

Foo::foo() {
static int x;
int y;
semistatic z;
/* ... */
}

Where semistatic means: shared by all invocations within the same
instance, and only visible within foo().

As I recall there is a proposal to add members with automatic back-
pointers, like Java inner classes, but whether it will be adopted...

--
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Neil Zanella]

Leor Zolman <le**@bdsoft.co m> wrote in message:

Foo::foo() {
static int x;
int y;
/* ... */
}

Here variable x is shared by all instances of the class,
Not /that/ variable x, if you're referring to the one in Foo:foo(). That's
a block-scope static, which means it can only be referred to within that
function (and its value persists across multiple calls to it. By the way,
it gets initialized to zero the first time the function is called, if you
don't give it an initializer.)

Perhaps my explanation was somewhat inadequate. What you have stated in the
above paragraph is correct. Indeed, x can only be referred to from within
member function Foo::foo(). And indeed since it is static the compiler will
automatically arrange for it to be initialized to zero when no initializer
is present. What I meant to express by saying that x is shared by all
instances of Foo is exemplified in the following code:

#include <iostream>

class Foo {
public:
void foo() {
static int x;
std::cout << x++ << std::endl;
}
};

int main() {
Foo foo;
foo.foo();
foo.foo();
Foo bar;
bar.foo();
Foo foobar;
foobar.foo();
}

Output:

0
1
2
3

The goal is to achieve the output:

0
1
0
0

without changing the contents of main and without changing the contents
of the class definition (which seems impossible in C++, since it does not
support the feature I describe).

The value of z should survive different calls from within the same
instance, but should be initialized for each new instance, like above.
But it should have method scope, not class scope.

I /think/ I see what you want, but there's no primitive way to declare that
in C++: You want an instance variable (one copy associated with each
instance of an object) that is only in scope within a particular function,

Correct. That is indeed the feature I was describing. Furthermore I was
poining out that C++ supports combinations:

(one copy per class, class scope) (static data members in class body)
(one copy per class, function scope) (static data members in function body)
(one copy per instance, class scope) (automatic data in class body)

but not the combination:

(one copy per instance, function scope)

I think this is poor design of the C++ language, because several functions
need have boolean variables called initialize which act as follows. Class
Foo has methods foo(), bar(), and foobar(). We don't want the constructor
to call these because they consume too much precious time, sometimes only
to initialize data members that are not needed later on. So we have the
functions foo(), bar(), and foobar() do the respective data member
initializations they need when they are called. Given that the
sets of data members they initialize are disjoint in the
given scenario, and that foo() may initialize more than
one data member, it makes sense to have a variable
which satisfies the missing C++ feature in order
to perform the initialization when foo() is called.
Alas, I need put such initializer variables inside
the function body.

I wonder if any languages have what I describe. Smalltalk?
right? No can do. You'll just have to exercise some self-discipline by
declaring a private data member and being careful to only access it from
within the one function.

I think this feature is missing from C++ and is quite useful. Because
if some class variable is used only in one method, then why should I
have to place it in the class definition.

To indicate to the compiler that it has to make room for it in each object
instantiated. At least the way C++ is currently defined ;-)

Hmmm... but the compiler went and fetched those static variables for
the BSS segment. I gues they belong to another data segment altogether
hence that is what makes it possible???

Regards,

Neil