Help fixing C++ invalid conversion from const char* to char

primerib · 03-04-2010, 03:27 PM

Hi. I'm getting the following error when trying to compile:

Quote:

$ make
g++ -march=athlon64 -O2 -pipe -fomit-frame-pointer -msse3 -c -D_GNU_SOURCE -DPLUGIN_NAME_I18N='"undelete"' -DHAVE_SVDRP -I../../../include undelete.c
undelete.c: In member function ‘virtual cString cPluginUndelete::SVDRPCommand(const char*, const char*, int&)’:
undelete.c:945: error: invalid conversion from ‘const char*’ to ‘char*’
make: *** [undelete.o] Error 1

The offending line undelete.c:945 is:

Quote:

944: cString NewName = recording->FileName();
945: char *ext = strrchr(NewName, '.');

The entire undelete.c can be found at: http://pastebin.com/aTUFB6vE

To say I'm not a C++ coder would be giving me too much credit so anyone willing to help, please know that this is very new to me. Thanks!

Sergei Steshenko · 03-04-2010, 03:29 PM

Quote:

Originally Posted by primerib

Hi. I'm getting the following error when trying to compile:

The offending line undelete.c:945 is:

The entire undelete.c can be found at: http://pastebin.com/aTUFB6vE

To say I'm not a C++ coder would be giving me too much credit so anyone willing to help, please know that this is very new to me. Thanks!

What is the return type of 'strrchr' ?

primerib · 03-04-2010, 03:52 PM

I forgot to mention I'm using cpp (Debian 4.4.2-9) 4.4.3 20100108 (prerelease) in debian testing linux.

Not sure if either of these is what you're looking for but I found,

in /usr/include/c++/4.4/cstring

Code:

  inline char*
  strrchr(char* __s1, int __n)
  { return __builtin_strrchr(const_cast<const char*>(__s1), __n); }

and in /usr/include/string.h

Code:

#ifdef __CORRECT_ISO_CPP_STRING_H_PROTO
extern "C++"
{
extern char *strrchr (char *__s, int __c)
     __THROW __asm ("strrchr") __attribute_pure__ __nonnull ((1));
extern __const char *strrchr (__const char *__s, int __c)
     __THROW __asm ("strrchr") __attribute_pure__ __nonnull ((1));

# ifdef __OPTIMIZE__
__extern_always_inline char *
strrchr (char *__s, int __c) __THROW
{
  return __builtin_strrchr (__s, __c);
}

__extern_always_inline __const char *
strrchr (__const char *__s, int __c) __THROW
{
  return __builtin_strrchr (__s, __c);
}
# endif
}
#else
extern char *strrchr (__const char *__s, int __c)
     __THROW __attribute_pure__ __nonnull ((1));
#endif
__END_NAMESPACE_STD

Thanks for your reply!

Btw, in c++/4.4/cstring I also see:

Code:

#define _GLIBCXX_CSTRING 1

// Get rid of those macros defined in <string.h> in lieu of real functions.
#undef strrchr

and

Code:

_GLIBCXX_BEGIN_NAMESPACE(std)
using ::strrchr;

Sergei Steshenko · 03-04-2010, 04:09 PM

Quote:

Originally Posted by primerib

I forgot to mention I'm using cpp (Debian 4.4.2-9) 4.4.3 20100108 (prerelease) in debian testing linux.

Not sure if either of these is what you're looking for but I found,

in /usr/include/c++/4.4/cstring

Code:

  inline char*
  strrchr(char* __s1, int __n)
  { return __builtin_strrchr(const_cast<const char*>(__s1), __n); }

and in /usr/include/string.h

Code:

#ifdef __CORRECT_ISO_CPP_STRING_H_PROTO
extern "C++"
{
extern char *strrchr (char *__s, int __c)
     __THROW __asm ("strrchr") __attribute_pure__ __nonnull ((1));
extern __const char *strrchr (__const char *__s, int __c)
     __THROW __asm ("strrchr") __attribute_pure__ __nonnull ((1));

# ifdef __OPTIMIZE__
__extern_always_inline char *
strrchr (char *__s, int __c) __THROW
{
  return __builtin_strrchr (__s, __c);
}

__extern_always_inline __const char *
strrchr (__const char *__s, int __c) __THROW
{
  return __builtin_strrchr (__s, __c);
}
# endif
}
#else
extern char *strrchr (__const char *__s, int __c)
     __THROW __attribute_pure__ __nonnull ((1));
#endif
__END_NAMESPACE_STD

Thanks for your reply!

Btw, in c++/4.4/cstring I also see:

Code:

#define _GLIBCXX_CSTRING 1

// Get rid of those macros defined in <string.h> in lieu of real functions.
#undef strrchr

and

Code:

_GLIBCXX_BEGIN_NAMESPACE(std)
using ::strrchr;

I suggest to first obtain preprocessed input file in order to better understand which of the implementations 'strrchr' is used.

The issue is moot in a sense there is a number of places where both 'const char *' and 'char *' are used.

So, you can also/first try to rewrite the code this way:

Code:

char *ext =
strrchr
  (
  NewName, 
  '.'
  );

- in such a manner hopefully the compiler will point you to the exact place where the problem is, i.e. whether it dislikes conversion at 'NewName' or at 'ext = strrchr', i.e. whether it's the argument or the result.

primerib · 03-04-2010, 04:20 PM

Quote:

Originally Posted by Sergei Steshenko

I suggest to first obtain preprocessed input file in order to better understand which of the implementations 'strrchr' is used.

I just search the entire source tree and found a reference to "#include <string.h>" in several files, no references to cstring.h anywhere so I guess string.h it is.

Quote:

So, you can also/first try to rewrite the code this way:

Code:

char *ext =
strrchr
  (
  NewName, 
  '.'
  );

- in such a manner hopefully the compiler will point you to the exact place where the problem is, i.e. whether it dislikes conversion at 'NewName' or at 'ext = strrchr', i.e. whether it's the argument or the result.

I tried this and it fails with "undelete.c:951: error: invalid conversion from ‘const char*’ to ‘char*’" on the " );" line.

Sergei Steshenko · 03-04-2010, 04:35 PM

Quote:

Originally Posted by primerib

I just search the entire source tree and found a reference to "#include <string.h>" in several files, no references to cstring.h anywhere so I guess string.h it is.

I tried this and it fails with "undelete.c:951: error: invalid conversion from ‘const char*’ to ‘char*’" on the " );" line.

Well, try brute force (i.e. type cast) then:

Code:

(char *)strrchr

- even if it works, it doesn't look clean. I.e. it's better to understand then what functionally for constant was intended.

paulsm4 · 03-04-2010, 05:11 PM

Hi -

Here's the problem:

Quote:

Offending code:
944: cString NewName = recording->FileName();
945: char *ext = strrchr(NewName, '.');

Quote:

Prototype for C strchr:
char *strchr(const char *s, int c);

Here's a reasonable solution:

Quote:

char *ext = strrchr((const char *)NewName, '.');

There are a number of stylist and/or technical issues one could debate (for example, using an "old fashioned" C-style cast instead of a C+ "const_cast<>") but, frankly, I think the above snippet should fix the problem, and be perfectly OK for you.

'Hope that helps .. PSM

primerib · 03-04-2010, 05:23 PM

It seems to compile now, thank you. I'm not sure what the author is using a constant at all. I've tried emailing him but there was no response. Unfortunately I'm guessing the intent is something a more knowledgeable person then myself would have to look into.

Thanks again for the help!

primerib · 03-04-2010, 05:26 PM

Quote:

Originally Posted by paulsm4

Hi -

Here's the problem:

Here's a reasonable solution:

There are a number of stylist and/or technical issues one could debate (for example, using an "old fashioned" C-style cast instead of a C+ "const_cast<>") but, frankly, I think the above snippet should fix the problem, and be perfectly OK for you.

'Hope that helps .. PSM

I tried your solution of char *ext = strrchr((const char *)NewName, '.'); but it actually gave me the same error as in the original post.

Sergei Steshenko · 03-04-2010, 05:50 PM

Quote:

Originally Posted by primerib

It seems to compile now, thank you. I'm not sure what the author is using a constant at all. I've tried emailing him but there was no response. Unfortunately I'm guessing the intent is something a more knowledgeable person then myself would have to look into.

Thanks again for the help!

Which of the two solutions worked - suggested by paulsm4 or myself ?

The approach is the same, the point is to understand what the compiler actually meant. I.e. to learn how to identify the exact problematic spot.

paulsm4 · 03-04-2010, 05:57 PM

One more thing: Sergei Steshenko and I are both curious which of these work for you:

Code:

  const char *ext = strrchr((const char *)NewName, '.');  // A

Code:

  char *ext = strrchr((const char *)NewName, '.');  // B

Code:

  const char *ext = strrchr(NewName, '.');  // and/or C

Thanx in advance .. PSM

PS:
Again, the prototype I have for "strrchr()" looks like this:

Quote:

man strrchr =>
#include <string.h>
...
char *strrchr(const char *s, int c);

It sounds like yours is different

Sergei Steshenko · 03-04-2010, 06:00 PM

Quote:

Originally Posted by paulsm4

One more thing: I'm curious which of these work for you:
...

- we've just asked the same question.

primerib · 03-04-2010, 06:09 PM

doesn't work:
const char *ext = strrchr((const char *)NewName, '.'); // A
char *ext = strrchr((const char *)NewName, '.'); // B
const char *ext = strrchr(NewName, '.'); // and/or C

works
char *ext = (char *)strrchr(NewName, '.');

So what does that tell us?

smeezekitty · 03-04-2010, 06:14 PM

Quote:

const char *ext = strrchr(NewName, '.');

Should have infact worked, HMMM...

Sergei Steshenko · 03-04-2010, 06:15 PM

Quote:

Originally Posted by primerib

doesn't work:
const char *ext = strrchr((const char *)NewName, '.'); // A
char *ext = strrchr((const char *)NewName, '.'); // B
const char *ext = strrchr(NewName, '.'); // and/or C

works
char *ext = (char *)strrchr(NewName, '.');

So what does that tell us?

It tells that the function returns 'const char *' (if I'm not mistaken).

But my point was that it looks like when the error message is tied to ');', it is about function return type, not argument type.