problem passing a static const int member by reference

[Amadeus W. M.]

I have a member static const int x defined in class Foo, and I'm passing
it by reference, and by value elsewhere (see the code below). Passing it
by value works, but by reference it doesn't: it thinks x is undefined.
Could someone explain what's going on here? Why can't I pass a static
const member by reference?

This is how I compile it:

g++ -g -Wall sample_main.C # g++ -v 4.0.1
/tmp/ccUJx59K.o(.gnu .linkonce.t._ZN 3Foo12bad_funct ionER3Bar[Foo::bad_functi on(Bar&)]+0xa): In function `Foo::bad_funct ion(Bar&)':
/tmp/STATICCONST/sample_main.C:1 8: undefined reference to `Foo::x'
collect2: ld returned 1 exit status
#include <cstdlib>
using namespace std;

class Bar
{
public:
void good(const int a) {}
void bad(const int & a) {}
};
class Foo
{
public:
static const int x=0;

void good_function(c lass Bar & B){ B.good(x); }
void bad_function(cl ass Bar & B) { B.bad(x); }
};

int main(int argc, char * argv[])
{
Foo F;
Bar B;

F.good_function (B);
F.bad_function( B);

return 0;
}

[Gianni Mariani]

Amadeus W. M. wrote:
....

class Foo
{
public:
static const int x=0;

void good_function(c lass Bar & B){ B.good(x); }
void bad_function(cl ass Bar & B) { B.bad(x); }
};

Add this:

const int Foo::x;

[John Harrison]

Amadeus W. M. wrote:

I have a member static const int x defined in class Foo, and I'm passing
it by reference, and by value elsewhere (see the code below). Passing it
by value works, but by reference it doesn't: it thinks x is undefined.
Could someone explain what's going on here? Why can't I pass a static
const member by reference?

You can but the difference is that when you pass by value the compiler
substitutes the value 0 so storage for the actual variable is not
needed. But when you pass by reference you need an actual variable which
you have failed to declare (what is in the class is a definition not a
declaration).

You need to add the declaration to sample_main.C as Gianni showed.

john

[John Harrison]

John Harrison wrote:

Amadeus W. M. wrote:

I have a member static const int x defined in class Foo, and I'm passing
it by reference, and by value elsewhere (see the code below). Passing it
by value works, but by reference it doesn't: it thinks x is undefined.
Could someone explain what's going on here? Why can't I pass a static
const member by reference?

You can but the difference is that when you pass by value the compiler
substitutes the value 0 so storage for the actual variable is not
needed. But when you pass by reference you need an actual variable which
you have failed to declare (what is in the class is a definition not a
declaration).

You need to add the declaration to sample_main.C as Gianni showed.

john

Sorry, got the words definition and declaration completely the wrong way
round in the above explanation.

john

[Amadeus W. M.]

On Sat, 17 Sep 2005 11:14:56 -0700, Gianni Mariani wrote:

Amadeus W. M. wrote:
...

class Foo
{
public:
static const int x=0;

void good_function(c lass Bar & B){ B.good(x); }
void bad_function(cl ass Bar & B) { B.bad(x); }
};

Add this:

const int Foo::x;

Of course, this works, but I thought only non-integer static consts
require this. I thought static const int can be defined inside the class.
And it can be defined inside the class, as long I don't pass it by
reference.

[Amadeus W. M.]

On Sat, 17 Sep 2005 18:22:51 +0000, John Harrison wrote:

Amadeus W. M. wrote:

I have a member static const int x defined in class Foo, and I'm passing
it by reference, and by value elsewhere (see the code below). Passing it
by value works, but by reference it doesn't: it thinks x is undefined.
Could someone explain what's going on here? Why can't I pass a static
const member by reference?
You can but the difference is that when you pass by value the compiler
substitutes the value 0 so storage for the actual variable is not
needed. But when you pass by reference you need an actual variable which
you have failed to declare (what is in the class is a definition not a
declaration).

But isn't x supposed to be _defined_ (hence allocated and initialized)
when I say

static const int x=0; // inside class Foo;

?? Is it that the compiler is trying to be smart, or am I not complying
with the standard?
You need to add the declaration to sample_main.C as Gianni showed.

john

[Jon Slaughter]

"Amadeus W. M." <am*******@cabl espeed.com> wrote in message
news:pa******** *************** *****@cablespee d.com...
<snip>

#include <cstdlib>
using namespace std;

class Bar
{
public:
void good(const int a) {}
void bad(const int & a) {}
};
class Foo
{
public:
static const int x=0;

void good_function(c lass Bar & B){ B.good(x); }
void bad_function(cl ass Bar & B) { B.bad(x); }
};

int main(int argc, char * argv[])
{
Foo F;
Bar B;

F.good_function (B);
F.bad_function( B);

return 0;
}

This code works fine under VS8 ;/

[Amadeus W. M.]

On Sat, 17 Sep 2005 14:23:44 -0500, Jon Slaughter wrote:

"Amadeus W. M." <am*******@cabl espeed.com> wrote in message
news:pa******** *************** *****@cablespee d.com...
<snip>

#include <cstdlib>
using namespace std;

class Bar
{
public:
void good(const int a) {}
void bad(const int & a) {}
};
class Foo
{
public:
static const int x=0;

void good_function(c lass Bar & B){ B.good(x); }
void bad_function(cl ass Bar & B) { B.bad(x); }
};

int main(int argc, char * argv[])
{
Foo F;
Bar B;

F.good_function (B);
F.bad_function( B);

return 0;
}

This code works fine under VS8 ;/

Thank you, maybe I should ask on a g++ newsgroup then.

[Amadeus W. M.]

In fact, here's what I want to do: I have 1 header file Foo.H included in
two source files sample_main.C and foobar.C. The definition of Foo::x
should really be in Foo.H.
// file Foo.H:
#ifndef _FOO_H
#define _FOO_H

#include <cstdlib>
using namespace std;

class Bar
{
public:
void good(const int a) const {}
void bad(const int & a) const {}
};
class Foo
{
public:
static const int x=0;

void good_function(c onst Bar & B) const { B.good(x); }
void bad_function(co nst Bar & B) const { B.bad(x); }
};
const int Foo::x;

#endif

// file foobar.C:
#include <cstdlib>
#include "Foo.H"

using namespace std;

void foobar()
{
Foo F;
Bar B;

F.good_function (B);
F.bad_function( B);
}

// file sample_main.C
#include <cstdlib>
#include "Foo.H"

using namespace std;

extern void foobar();

int main(int argc, char * argv[])
{
Foo F;
Bar B;

F.good_function (B);
F.bad_function( B);

foobar();

return 0;
}

g++ sample_main.C foobar.C

gives (understandably ) multiple definition of Foo::x. Of course, if I put
const int Foo::x;
in sample_main.C, it works, but I it's more natural that the definition of
Foo::x should be in Foo.H.

If I don't put const int Foo::x; anywhere (neither in main nor in Foo.H),
it works, as long as I don't call Foo::bad_functi on() i.e. as long as I
don't access Foo::x by reference.


If, on the other hand, I make Foo a template class and define

template <class DummyType>
const int Foo<DummyType>: :x;

outside Foo in Foo.H, it works (no multiple definition of
Foo<DummyType>: :x, no undefined references). I would rather not make Foo
a template class though, if I don't have to.

Is there another way? Thanks!

[John Harrison]

Amadeus W. M. wrote:

In fact, here's what I want to do: I have 1 header file Foo.H included in
two source files sample_main.C and foobar.C. The definition of Foo::x
should really be in Foo.H.
// file Foo.H:
#ifndef _FOO_H
#define _FOO_H

#include <cstdlib>
using namespace std;

class Bar
{
public:
void good(const int a) const {}
void bad(const int & a) const {}
};
class Foo
{
public:
static const int x=0;

void good_function(c onst Bar & B) const { B.good(x); }
void bad_function(co nst Bar & B) const { B.bad(x); }
};
const int Foo::x;

#endif

// file foobar.C:
#include <cstdlib>
#include "Foo.H"

using namespace std;

void foobar()
{
Foo F;
Bar B;

F.good_function (B);
F.bad_function( B);
}

// file sample_main.C
#include <cstdlib>
#include "Foo.H"

using namespace std;

extern void foobar();

int main(int argc, char * argv[])
{
Foo F;
Bar B;

F.good_function (B);
F.bad_function( B);

foobar();

return 0;
}

g++ sample_main.C foobar.C

gives (understandably ) multiple definition of Foo::x. Of course, if I put
const int Foo::x;
in sample_main.C, it works, but I it's more natural that the definition of
Foo::x should be in Foo.H.
Well that is tough, C++ says it should be in sample_main.C (or foobar.C).

If I don't put const int Foo::x; anywhere (neither in main nor in Foo.H),
it works, as long as I don't call Foo::bad_functi on() i.e. as long as I
don't access Foo::x by reference.

That is true, although I'm not sure the C++ standard guarantees that.

If, on the other hand, I make Foo a template class and define

template <class DummyType>
const int Foo<DummyType>: :x;

outside Foo in Foo.H, it works (no multiple definition of
Foo<DummyType>: :x, no undefined references). I would rather not make Foo
a template class though, if I don't have to.

Well that is because the rules for templates are different. Template
code *always* goes in header files.
Is there another way? Thanks!

No, unless an enum would suffice.

class Foo
{
enum { x = 1 };
};

In the old days you would have been required to define and initialise
Foo::x in a .C file. Since standardisation you can now initialise Foo::x
in the header file, but you are still required to define it in a .C file.

john