post  A request I guess

mcleano (535)   Link to this post
Hey, I was wondering if someone could write an article on const correctness and all the uses of const, with what they do. I think this would be a good addition to the article collection and I know i would find it extremely useful.

Any takers?
jsmith (3099)   Link to this post
Here's a start. It documents what the keyword "const" means in various places within a declaration.
It does not go into the reasons why const-correctness is important. I wrote this several years ago.


The following declarations are identical:
const char* p;
char const* p;

Both declare a pointer to a constant character. The second is slightly
better in the sense that the declaration can be read from right-to-left:
"p is a pointer to a const char". Read as such, it is easy to see that
the line *p = 'c'; will not compile.

The following declaration:
char* const p;

declares p to be a constant pointer to a character. That is:
p = "foo"; // Does not compile
*p = 'f'; // Compiles!

And thus:
const char* const p;
char const* const p;

both declare p to be a constant pointer to a constant character, and
so none of the following lines of code compile:
p = "foo";
*p = 'f';


Now throw another pointer into the mix:
const char** p;
char const** p;

These are equivalent and declare p to be a pointer to a pointer to a
constant character. That is:
p = ptr-to-ptr-to-char; // Compiles
*p = ptr-to-char; // Compiles
**p = 'f'; // Does not compile

Or how about creative placement of const:
char* const* p;

This declares p to be a pointer to a constant pointer to a character.
That is:
p = ptr-to-constptr-to-char; // Compiles
*p = ptr-to-char; // Does not compile
*p = constptr-to-char; // Does not compile
**p = 'f'; // Compiles

And the ever-popular:
char** const p;

Which declares p to be a constant pointer to a pointer to a character.
Or:
p = ptr-to-ptr-to-char; // Does not compile
p = constptr-to-ptr-to-char; // Does not compile
*p = ptr-to-char; // Compiles
**p = 'f'; // Compiles

And now we get just plain const happy:
const char* const* p;

p is a pointer to a constant pointer to a constant character. The only
thing you can do with this one (besides remove the code and rewrite) is:
p = ptr-to-constptr-to-constchar;

const char** const p;
p is a constant pointer to a pointer to a constant character. The only
thing you can do with this is:
*p = ptr-to-constchar;

And this beast:
const char* const* const p;

Well, it won't pass code review since nobody will understand it, but at
any rate... We've achieved maximum constant-ness with this line. You
can't do anything at all with p, what it points to, what that points to,
or what "what that" points to. You can print it. That's about it.


Ho-ho, and the fun is just beginning. Now throw in REFERENCES!

const char& p;
char const& p;

These both declare p to be a reference to a constant character. That is,
p cannot change.

char& const p;
const char& const p;
char const& const p;
char*& const p;
const char*& const p;
const char* const& const p;

These all generate compiler errors, because there is no such thing as
a constant reference I guess.

const char*& p;
char const*& p;

p is a reference to a pointer to a constant character. One can change p,
but not *p.

char* const& p;

p is a reference to a constant pointer to a character.

const char* const& p;

p is a reference to a constant pointer to a constant character.

const char&* p;
char const&* p;
char& const* p;
char&* const p;
const char& const* p;
const char&* const p;
const char& const* const p;

Fortunately pointers to references are not allowed. The above declarations
are illegal.

guestgulkan (1040)   Link to this post
1
2
3
 
const char* p;//allowed
char const* p; // I always thought this second version is the officially correct method - that is 
//the const keyword is put after the thing that is being made constant
Last edited on
guestgulkan (1040)   Link to this post
I think mcleano should do some research and write the article.
Last edited on
mcleano (535)   Link to this post
I think mcleano should do some research and write the article.

You know what, I might just do that.
kevinchkin (363)   Link to this post
A very good article on constant correctness

http://www.possibility.com/Cpp/const.html
mcleano (535)   Link to this post
Thanks, but as guestgulkan suggested, I think I might give this a try.

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