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reinterpret_cast

Posted on 2003-12-10
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Last Modified: 2012-08-13
I was wondering why this casting does'nt work ...
Is there an other way to cast this... in C++
I know that it's working with
char ** p1 = (char **) elem1;    but this is C casting...

Help me


int compare(const void* elem1, const void* elem2)
{
      

      
      const char **p1 = reinterpret_cast< const char**>(elem1);

...

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Question by:Prog_
5 Comments
 
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Expert Comment

by:SteH
ID: 9911589
static_cast should work in almost any case. Not completly sure whether the const specifier doesn't allow for reinterpret_cast. If so you can remove the const'ness with const_cast.
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rstaveley earned 125 total points
ID: 9911842
I think you need this:
--------8<--------
int compare(const void* elem1, const void* elem2)
{
      const char * const *p1 = reinterpret_cast<const char* const *>(elem1);
--------8<--------

const void* elem1 means that elem1 is a pointer to something const (as opposed to a const pointer). If you reinterpret that as an array of char pointers to something const, you are casting off the const-ness of elem1. You have to say that the contents of the array are const.

If you want to cast off const-ness, you need to use a const cast.
--------8<--------
int compare(const void* elem1, const void* elem2)
{
      const char** p1 = reinterpret_cast<const char**>(const_cast<void*>(elem1));
--------8<--------
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Expert Comment

by:travd
ID: 9914592
By trying the cast from <const void *> to <const char **> you are trying to cast away the constness since it is the value pointed to be the first dereference of elem1 that is const.  Instead you should be casting to <char * const *>.  Also you should probably use static_cast, since the semantics of this cast are more well defined and guaranteed to be portable (although in this case reinterpret_cast is probably equivalent).

const char * const *p1 = static_cast< char * const * >(elem1);

Try that.
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Expert Comment

by:MAYURNATH
ID: 9970747

reinterpret_cast - internally interpret the pointers ( source pointer and target pointer ) and while compile time it embeds extra code if neccessary.

Here your treatment on pointers is converting from void* to char **. Which are not compatable

You are trying to copy only the 4 bytes of pointer data ( ex. 0x023FD2F) to the target pointer.

 const char **p1 = reinterpret_cast< const char**>(elem1);  - this casting should not be used here

reinterpret_cast is used when some situations like virtual pointer hirerchy casting
For only pointer copy it is not neccessary an extra interpretation is neccessary by the compiler then this casting is supportable

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Expert Comment

by:rstaveley
ID: 9971142
void* is a strange beast. It is a pointer to something unspecified. There is nothing wrong with saying that the unspecified thing is a char**, in fact we'd be lost if we weren't able to typecast void* in thread code.

You can do the following with a static_cast per the standard. I was surprised that the standard does not guarantee reinterpret_cast to work for void* too, but it does indeed say that it is implementation specific. Neither static nor reinterpret casts cast off constness though.

---------8<--------
#include <cstdio>
#include <memory>

int main()
{
      // malloc() - more comfortable in C code, but
      // what the hell. It is a simple example of
      // something that returns a void*, when you
      // really mean it to return a pointer to something
      // different
      void *ptr = malloc(sizeof(char*) * 2);

      // However, we were allocating an array of 2 char* s
      char **arr = static_cast<char**>(ptr);

      // Just to show it works
      arr[0] = "one";
      arr[1] = "two";
      for (int i = 0;i < 2;i++)
            printf("String %d is \"%s\"\n",i+1,arr[i]);

      free(ptr);
}
---------8<--------

NB: it looks odd written like this, because you are more used to seeing a C cast in C-ish C++.

      char** arr = (char**)malloc(sizeof(char*) * 2);
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