Symbolic constants inside classes

Symbolic constants inside classes

[Jonas Maebe]

Hi,

Consider this C++ program:

***
class Xyz {
public:
  static const int MAGIC_NUM = 42;
private:
  char mArray[MAGIC_NUM];
};

class Xyz2 {
public:
  static const int MAGIC_NUM = 43;
private:
  char mArray2[MAGIC_NUM];
};

Xyz C;
Xyz2 C2;

int main()
{
  return 0;
}
***

g++ -gdwarf-2 generates a definition for both MAGIC_NUM constants both in the respective class scopes, and also in the global namespace:

in the class:
 <1><78>: Abbrev Number: 2 (DW_TAG_class_type)  -- Xyz declaration
     DW_AT_sibling     : <a9>   
     DW_AT_name        : Xyz    
     DW_AT_byte_size   : 42     
     DW_AT_decl_file   : 1      
     DW_AT_decl_line   : 1      
 <2><84>: Abbrev Number: 3 (DW_TAG_variable)   -- MAGIC_NUM field
     DW_AT_name        : (indirect string, offset: 0x14): MAGIC_NUM     
     DW_AT_decl_file   : 1      
     DW_AT_decl_line   : 3      
     DW_AT_MIPS_linkage_name: (indirect string, offset: 0x1e): _ZN3Xyz9MAGIC_NUME       
     DW_AT_type        : <a9>   
     DW_AT_external    : 1      
     DW_AT_declaration : 1      
     DW_AT_const_value : 42     

...

in the global namespace:
 <1><146>: Abbrev Number: 10 (DW_TAG_namespace) -- global namespace
     DW_AT_sibling     : <168>  
     DW_AT_name        : ::     
     DW_AT_decl_file   : 2      
     DW_AT_decl_line   : 0      
...
 <1><168>: Abbrev Number: 12 (DW_TAG_variable)  -- "C" global variable
     DW_AT_specification: <150> 
     DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0        (DW_OP_addr: 0)
 <1><177>: Abbrev Number: 12 (DW_TAG_variable)  -- "C2" global variable
     DW_AT_specification: <15b> 
     DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0        (DW_OP_addr: 0)
 <1><186>: Abbrev Number: 3 (DW_TAG_variable)  -- Xyz::MAGIC_NUM
     DW_AT_name        : (indirect string, offset: 0x14): MAGIC_NUM     
     DW_AT_decl_file   : 1      
     DW_AT_decl_line   : 3      
     DW_AT_MIPS_linkage_name: (indirect string, offset: 0x1e): _ZN3Xyz9MAGIC_NUME       
     DW_AT_type        : <a9>   
     DW_AT_external    : 1      
     DW_AT_declaration : 1      
     DW_AT_const_value : 42     
 <1><198>: Abbrev Number: 3 (DW_TAG_variable)  -- Xyz2::MAGIC_NUM
     DW_AT_name        : (indirect string, offset: 0x14): MAGIC_NUM     
     DW_AT_decl_file   : 1      
     DW_AT_decl_line   : 10     
     DW_AT_MIPS_linkage_name: (indirect string, offset: 0x0): _ZN4Xyz29MAGIC_NUME       
     DW_AT_type        : <a9>   
     DW_AT_external    : 1      
     DW_AT_declaration : 1      
     DW_AT_const_value : 43     


In gdb (7.0),
1) p MAGIC_NUM prints 42
2) "p Xyz::MAGIC_NUM" and "p Xyz2::MAGIC_NUM" print "static field MAGIC_NUM has been optimized out"
3) "p C.MAGIC_NUM" and "p C2.MAGIC_NUM" print "field MAGIC_NUM is nonexistent or has been optimised out"
4) ptype Xyz returns:

type = class Xyz {
  public:
    static const int MAGIC_NUM;
  private:
    char mArray[42];
}


My questions:
a) why does GDB not know the value of MAGIC_NUM inside the class scope? It is there in the debug info (see above)
b) does g++ possibly add the constants to the global scope because gdb cannot show them in the class scope (although it obviously can still only show one of them in case of a name conflict), or is this required by a DWARF rule I missed?


Jonas

Re: Symbolic constants inside classes

[Tom Tromey]

>>>>> "Jonas" == Jonas Maebe <jonas.maebe@elis.ugent.be> writes:

Jonas> Consider this C++ program:
[...]

Jonas> g++ -gdwarf-2 generates a definition for both MAGIC_NUM constants
Jonas> both in the respective class scopes, and also in the global
Jonas> namespace:

What version of g++?

Jonas> In gdb (7.0),
Jonas> 1) p MAGIC_NUM prints 42
Jonas> 2) "p Xyz::MAGIC_NUM" and "p Xyz2::MAGIC_NUM" print "static field MAGIC_NUM has been optimized out"
Jonas> 3) "p C.MAGIC_NUM" and "p C2.MAGIC_NUM" print "field MAGIC_NUM is nonexistent or has been optimised out"

I tried this example with svn trunk gcc and CVS head gdb, and it worked.
I didn't try to find out whether the fix was in gcc or gdb.

Tom

Re: Symbolic constants inside classes

[Jonas Maebe]

On 20 Dec 2010, at 22:23, Tom Tromey wrote:

>>>>>> "Jonas" == Jonas Maebe <jonas.maebe@elis.ugent.be> writes:
> 
> What version of g++?

Red Hat 4.1.2-48 (RHEL 5.5).

> I tried this example with svn trunk gcc and CVS head gdb, and it worked.
> I didn't try to find out whether the fix was in gcc or gdb.

A colleague of mine found the following, so it seems like it may have been fixed in gdb: http://permalink.gmane.org/gmane.comp.gdb.patches/59139


Jonas