Elfutils library forward declaration error

I'm currently working in C++ with the elfutils library and I'm struggling with something that should be obvious.

Here is a MCVE that shows my problem:

file.cpp

struct Elf64_Sym;

#include <elf.h>

Of course in the complete code, the forward declaration is done in a .hpp file and the <elf.h> include is done in a .cpp file but this small example reproduce the exact same issue. You can try to compile it using the following command line g++ file.cpp and you will receive the following error:

In file included from /usr/include/libelf.h:36,
                 from file.cpp:3:
/usr/include/elf.h:538:3: error: conflicting declaration 'typedef struct Elf64_Sym Elf64_Sym'
  538 | } Elf64_Sym;
      |   ^~~~~~~~~
file.cpp:1:8: note: previous declaration as 'struct Elf64_Sym'
    1 | struct Elf64_Sym;
      |        ^~~~~~~~~

This compile fine with gcc (so in C) but not with g++ (so in C++).

Do you have a solution that would allow a forward declaration of the Elf64_Sym structure in C++?

And for completeness sake here is the definition of this struct in <elf.h>:

typedef struct
{
  Elf64_Word    st_name;        /* Symbol name (string tbl index) */
  unsigned char st_info;        /* Symbol type and binding */
  unsigned char st_other;       /* Symbol visibility */
  Elf64_Section st_shndx;       /* Section index */
  Elf64_Addr    st_value;       /* Symbol value */
  Elf64_Xword   st_size;        /* Symbol size */
} Elf64_Sym;

Updated MCVE

Here is a MCVE that shows my problem (and thanks to @user12002570 for pointing out that my previous MCVE wasn't correct):

file.hpp

#ifndef FILE_HPP
#define FILE_HPP

#include <stdint.h>

struct Elf64_Sym;

uint64_t getSize(Elf64_Sym * ptr);

#endif

file.cpp

#include "file.hpp"
#include <elf.h>

uint64_t getSize(Elf64_Sym * ptr)
{
    return ptr->st_size;
}

You can try to compile it using the following command line g++ -c file.cpp and you will receive the following error:

/usr/include/elf.h:538:3: error: conflicting declaration 'typedef struct Elf64_Sym Elf64_Sym'
  538 | } Elf64_Sym;
      |   ^~~~~~~~~
In file included from file.cpp:1:
file.hpp:6:8: note: previous declaration as 'struct Elf64_Sym'
    6 | struct Elf64_Sym;
      |        ^~~~~~~~~

Do you have a solution that would allow a forward declaration of the Elf64_Sym structure in C++?

And for completeness sake here is the definition of this struct in <elf.h>:

typedef struct
{
  Elf64_Word    st_name;        /* Symbol name (string tbl index) */
  unsigned char st_info;        /* Symbol type and binding */
  unsigned char st_other;       /* Symbol visibility */
  Elf64_Section st_shndx;       /* Section index */
  Elf64_Addr    st_value;       /* Symbol value */
  Elf64_Xword   st_size;        /* Symbol size */
} Elf64_Sym;

Solution

If you need a forward declaration of a typedef struct, you can declare an empty struct and then typedef it eg

struct _Elf64Sym;
typedef _Elf64Sym Elf64_Sym;

So long as the second typedef is defined identically, a second typedef of the same struct will compile.

If the typedef is of an anonymous struct, you need to give the struct a name. If you can't rename the struct in the other header file, you're out of options.