maria-developers team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, Aleksey!

On Oct 20, Aleksey Midenkov wrote:
> Recent refactorings of replacing C strings with LEX_CSTRING which is
> no doubt a good thing raise some questions:
> 
> 1. Is it still guaranteed that Field::field_name.str is NULL-terminated?

I think it's still the case, yes. What code relies on it?

> 2. It is still passed as a pointer to functions. Why is that?
> 
> The main feature of C++ references is that it cannot be NULL, so we
> get segfault on top of the stack (closer to a cause), not the bottom
> of it. I see that pointers are now widely used and mainly assumed to
> be always non-NULL (i.e. dereferenced without assertion). But placing
> such implicit contract on data is not evident and bug-prone. IMHO it's
> much better to use references whenever it is possible (and when there
> is no need in cosy NULL semantic). What do you think?

This is something that gets raised over and over.
Some prefer C-style pointers over references, so that when you look at
the function call:

   func(a,&b);

you can immediately see that `a` is passed by value and cannot be
modified by `func`, while `b` can.

Others prefer C++ references.

I personally reside on the middle ground, where one uses pointers to
pass "out" parameters, and const references for "in" parameters. 

> But for such lightweight structs like LEX_CSTRING it is even better to
> pass by value, so we could have the conventience of type cast.

What do you mean by that?

> 3. LEX_CSTRING and LEX_STRING are now non-convertible. Why not to make:
> 
> template <typename char_t>
> struct st_mysql_lex_string
> {
>   char_t *str;
>   size_t length;
> };
> 
> typedef st_mysql_lex_string<char *> LEX_STRING;
> typedef st_mysql_lex_string<const char *> LEX_CSTRING;
> typedef st_mysql_lex_string<const unsigned char *> LEX_CUSTRING;
> 
> ?

What would that change?

> 4. There are some duplicate types: MYSQL_LEX_STRING,
> MYSQL_CONST_LEX_STRING. Why?

These are names used in the plugin API. They start from MYSQL_* to avoid
possible name clashes with third-party code.

Regards,
Sergei
Chief Architect MariaDB
and security@xxxxxxxxxxx