I'm sure a lot of you have seen the following code which demonstrates why=
=20
we can't implicitly convert from:-

  char**

to:

  const char**


Here's the code (written originally by James Kanze):

const char     c =3D 'a';
char*          p;
char**         pp =3D &p;
const char**   ppc =3D pp;     //  Supposing that this were not illegal.

*ppc =3D &c;                   //  Oops: where does p point?
*p  =3D 'b';                   //  And what is wrong here?


I've read the above code many times but never understood it. Even when I=20
thought about it thoroughly and slowly, I couldn't comprehend what was=20
going on.

Anyway, I came across the code again today, and finally I resolved to=20
grasp an understanding of it once and for all. So I drew a diagram, which=
,=20
after a few minutes of analysing, made me understand. With the hope of=20
helping others, here's the diagram:

http://img315.imageshack.us/img315/2876/undconst6kk.jpg

I've posted this to more than one newsgroup, so if you would like to=20
reply, I'd appreciate if you would keep the discussion in comp.lang.c++.

-Tom=E1s

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