Re: Wrong

From:

"Leigh Johnston" <leigh@i42.co.uk>

Newsgroups:

comp.lang.c++

Date:

Sun, 18 Apr 2010 14:19:41 +0100

Message-ID:

<-qqdnfbNoedpm1bWnZ2dnUVZ8jqdnZ2d@giganews.com>

"Branimir Maksimovic" <bmaxa@hotmail.com> wrote in message
news:20100418151700.69d7f2d8@maxa...

On Sun, 18 Apr 2010 14:11:53 +0100
"Leigh Johnston" <leigh@i42.co.uk> wrote:

"Pete Becker" <pete@versatilecoding.com> wrote in message
news:E6WdncgSL4k0nlbWnZ2dnUVZ_h2dnZ2d@giganews.com...

Leigh Johnston wrote:

"???? Tiib" <ootiib@hot.ee> wrote in message
news:bf9343ed-c952-4e26-9f51-19dc55acceaf@y17g2000yqd.googlegroups.com...

On 18 apr, 15:27, "Leigh Johnston" <le...@i42.co.uk> wrote:

Yes and even within the context you are talking about Alf
disagreed with the
fact that the code was wrong. Code that does not invoke UB is
not necessarily correct. In this particular case the merits of
the code which I
presented can be considered in isolation.

I totally agree with Pete.
Context was that OP was seeking RAII without zero-initializing
memory space, since the performance issues on specific platform.
Suggestion to use vector was yours.
Alf claimed that yes, reserve()'d raw space can be used without
causing UB and RAII works and no zero-initialization, exactly
like OP wanted.
Silly code posted here was also yours.
Alf again said that yes, it works everywhere without causing UB.
Now you totally go out of limits claiming that the code was meant
as some sort of example how to use vectors.

No, the code was an attempt to ascertain clearly what Alf was
asserting,

By your own rules, context doesn't matter, so you can't invoke
context to claim that someone has misstated what you said.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of
"The Standard C++ Library Extensions: a Tutorial and Reference"
(www.petebecker.com/tr1book)

Of course context can matter, I am claiming that a particular piece
of code is *always* wrong irrespective of context. You seem unable
to grasp this. Here it is again:

void foo()
{
    std::vector<int> v;
    v.reserve(2);
    v.push_back(41);
    *(&v[0]+1) = 42;
}

Anyone who thinks quality is important will agree with me that this
piece of code is wrong.

Well, only if reserve is not required to allocate memory!
Since vector can be used as plain C array,

void foo()
{
vector<int> v;
v.reserve(2);
int* pi = &v[0];
*pi = 1;
*(pi+1) = 2;
}
would be ok if reserve is required to allocate memory, not to be nop
eg.

Greets!

v[0] is undefined behaviour because *(v.begin() + 0) is undefined behaviour
for a vector whose size() is 0.

/Leigh