[Bug c++/20547] undefined reference to "static const" fields of classes

[Bug c++/20547] undefined reference to "static const" fields of classes

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2006-09-25 21:11 -------
*** Bug 29219 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |etienne dot clement at
                   |                            |autodesk dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[lerdsuwa at gcc dot gnu dot org]

------- Additional Comments From lerdsuwa at gcc dot gnu dot org  2005-03-20 13:55 -------
Here is the relevant section of the standard (TC1, 
section 9.4.2, paragraph 4):

  If a 'static' data member is of 'const' integral or 'const' enumeral type,
  its declaration in the class definition can specify a 'constant-initializer'
  which shall be an integral constant expression (5.19).  In that case, the
  member can appear in integral constant expressions.  The member shall still
  be defined in a namespace scope if it is used in the program and the
  namespace scope definition shall not contain an 'initializer'.

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           What    |Removed                     |Added
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             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[Hu dot YuehWei at gmail dot com]

------- Additional Comments From Hu dot YuehWei at gmail dot com  2005-03-20 07:59 -------
so the "... = 3;" initialization statement below is a definition or a declaration?
According to the C++ standard, its a definition.
Should the compiler allocate a memory space for it?

struct T
{
  static char const a = 3;
};

And...
If I need to allocate memory spaces for it manually, why the standard explicitly
specify this kind of coding style?
========================
struct T
{
  static char const a = 3; /* standard explicity specify that the initialization
& definition can be put here. */
};

char const T::a; /* If I need this line of codes, then the initialization above
should be a declaration, not a definition. But a declaration can be initialized? */
========================
struct T
{
  static char const a;
};

char const T::a = 3;
========================

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           What    |Removed                     |Added
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             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-19 18:35 -------
Bacause T::A is a lvalue.

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           What    |Removed                     |Added
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             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[Hu dot YuehWei at gmail dot com]

------- Additional Comments From Hu dot YuehWei at gmail dot com  2005-03-19 18:25 -------
I see.
But why compiler doesn't also make a temporary variable for the constant defined
in a class scope?
Such as following codes:

struct T
{
  static int const a = 3;
};

void
fff(int const &a)
{
}

int
main()
{
  fff(T::a); /* compiler will not make a temporary variable for it. */
  fff(4); /* compiler will make a temporary variable for it. */

  return 0;
}


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           What    |Removed                     |Added
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             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-19 14:34 -------
(In reply to comment #5)
> I understand what you mean.
> But why the following codes works:
> 
> #include <iostream>
> #include <vector>
> 
> struct T
> {
>   static char const a = 3;
> };
> 
> std::vector<char> ddd;
> 
> int
> main()
> {
>   ddd.push_back(static_cast<char>(T::a)); // <========= here
> 
>   std::cerr << ddd.front() << std::endl;
> 
>   return 0;
> }
> 
> and if I pass an constant to a function which takes its address,
Because you changed an lvalue to a rvalue.

> then why the folloing codes doesn't produce errors?

Because constants are rvalues so there is going to be a temporary variable made for you.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[Hu dot YuehWei at gmail dot com]

------- Additional Comments From Hu dot YuehWei at gmail dot com  2005-03-19 14:15 -------
I understand what you mean.
But why the following codes works:

#include <iostream>
#include <vector>

struct T
{
  static char const a = 3;
};

std::vector<char> ddd;

int
main()
{
  ddd.push_back(static_cast<char>(T::a)); // <========= here

  std::cerr << ddd.front() << std::endl;

  return 0;
}

and if I pass an constant to a function which takes its address,
then why the folloing codes doesn't produce errors?

void
fff(char const &a)
{
}

int
main()
{
  fff(4);

  return 0;
}


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           What    |Removed                     |Added
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             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-19 14:03 -------
(In reply to comment #2)
> But according the standard, 'const static' data members of an intergral type can
> now be initialized _inside_ their class. In this case, the initialization is
> _also_ a definition, so _no_ further definitions are reuquired outside the class
> body.

No it is not a definition, a definition (or is it the other way around).  Still this is invalid.  Yes the compiler 
is allowed to "optimize" the constant which is why it worked in 2.95.3 (well because the defintion of 
push_back was wrong).

Oh, one more thing, the vector code is equivalent to (so you take the address):
struct T
{
  static char const a = 3;
};
void
fff(const char &a){ }
int
main()
{
  fff(T::a);

  return 0;
}

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[Hu dot YuehWei at gmail dot com]

------- Additional Comments From Hu dot YuehWei at gmail dot com  2005-03-19 14:01 -------
#include <vector>

struct T
{
  static char const a = 3;
};

void
fff(char a)
{
}

std::vector<char> b;

int
main()
{
  fff(T::a); /* this line of codes is fine. */

  b.push_back(T::a); /* however, this line of codes is an error. */

  return 0;
}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[Hu dot YuehWei at gmail dot com]

------- Additional Comments From Hu dot YuehWei at gmail dot com  2005-03-19 13:53 -------
But according the standard, 'const static' data members of an intergral type can
now be initialized _inside_ their class. In this case, the initialization is
_also_ a definition, so _no_ further definitions are reuquired outside the class
body.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547

[Bug c++/20547] undefined reference to "static const" fields of classes

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-19 13:45 -------
No you need to add the following to your code:
const char T::a;

Since you don't supply the memory location otherwise (which is required by the standard).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20547