Memory quiz, question 2

[pheres]

Hi,

http://www.cprogramming.com/ans/ansmem.html

Question 2 is:

#


Question #2
# Assuming that myObj is less than 1000 bytes, is there anything wrong with this code?

char x[1000];
myObj *obj = reinterpret_cast(x);

new (obj) myObj;]

a) Nope, it works fine
b) Yes, there could be byte alignment issues
c) Yes, the syntax for calling new is incorrect

The correct (but incorrect) answer is b).

1. I think the reinterpret_cast need the template type argument (at least msvc complains if it's not given), true?

2. ']' is a typo in line 4, true? If so, could someone correct it?

3. What header file needs to be included to call the placement new operator?

4. Why exactly could there be byte alignement issues? If "myObj is less than 1000 bytes", doesn't that mean "it is less than 1000 bytes, including any possible padding bytes (what's what the c++ sizeof operator understands by size)"?

Thank you!

[Elysia]

The correct (but incorrect) answer is b).

I would like to say it's b).
Remember, this is essentially what all C programmers do with malloc.

1. I think the reinterpret_cast need the template type argument (at least msvc complains if it's not given), true?

Yes, C++ is a strongly typed language, so char* != MyObj*, hence a cast is necessary.

2. ']' is a typo in line 4, true? If so, could someone correct it?

Yes, it is. Just remove the "]".

3. What header file needs to be included to call the placement new operator?

Just <new>.

4. Why exactly could there be byte alignement issues? If "myObj is less than 1000 bytes", doesn't that mean "it is less than 1000 bytes, including any possible padding bytes (what's what the c++ sizeof operator understands by size)"?

There will be no alignment issues if the entire sizeof(MyObj) [this also assumes that it includes all possible padding] fits inside x, because the compiler can (and will) lay out the object as if allocated with new (not placement).
However, there may be alignment issues if x isn't aligned on a proper address. If you allocate a block of char using new, then use placement new, there would be no alignment issues.

[anon]

You could also write the same thing without a cast:

char x[1000];
MyObj* obj = new (x) MyObj;

Couldn't the potential alignment issue be that x itself doesn't begin at a word border?

[Elysia]

When you mention it, yes, I suppose that's right.
You'd need an aligned address, preferably from new and then construct something in place.
I'll update the post to avoid confusion.

[pheres]

So the only possible problem is the buffer address does not start at a multiple of the size of a machine word? What kind of behavior could be caused by that? A little example would be nice.

Yes, it is. Just remove the "]"

I can't, some site administarator has to do it. That's what I meant by "could someone correct it?"