Invalid addition of constness? Error: Cannot use char** to initialize const char**

Question

Solaris Studio is generating the most puzzling of error messages.

158 char* inbufptr = buffer;
159 char* outbufptr = outbuf;
160 
161 const char** inbufptrpos = &inbufptr;

And the error message is:

line 161: Error: Cannot use char** to initialize const char**.

Why should it be a problem to add constness? I am stuck, please help me out...

---

 memory: [m y _ c h a r _ a r r a y | inbufptr | inbufptr_pos]
          ^                           ^
          | (1)                       | (2)
          inbufptr                    inbufptrpos

The pointer char* inbufptr points to the beginning of the array, and does not promise to keep anything constant.

Now if I now have a pointer char const **inbufptr_pos this type promises not the change the contents of the array, but I can still change where the pointer points to. If I do that I have not changed the array and I don't see a problem with that.

Answer 1

This is an age-old problem, whereby intuitively you think you can "add constness", but in fact adding constness indirectly violates const-correctness.

The standard itself even has an example of it to help set people back on the correct path:

#include <cassert>  

int main() {  
  char* p = 0;  

  //char const** a = &p; // not allowed, but let's pretend it is  
  char const** a = (char const**)&p; // instead force the cast to compile  

  char const* orig = "original";  
  *a = orig; // type of *a is char const*, which is the type of orig, this is allowed  

  assert(p == orig); // oops! char* points to a char const*  
}

• http://kera.name/articles/2009/12/a-question-on-indirect-constness/

Answer 2

Assume this was legal.

char* inbufptr = buffer;

const char** inpufptrpos = &inbufptr;

Now you can change inbufptr, but inpufptrpos is const, thus it should not be changed. You see that this doesn't make much sense. It's like const is not respected!

With the help of this answer, I hope this helped! :)