Reading Memory

[Maxime Savard]

Hi there,

What I'm trying to do is quite simple and in fact I have found a way to
do it but I am not sure if it is OK. What I mean is sometime in c++ stuff
can "work" but not be "correct". Here is my code maybe you'll see something
wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

add++;

memcpy(&get, (void*)add, sizeof(char));

printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe it
is basic stuff here but I don't know why, I'm kinda newb). Then I came up
with this technique. The final question is... Is it "correct" ?

Thanks for any info!
Max.

P.s. Sorry for any typo I'm french

[Ian Collins]

Maxime Savard wrote:

Hi there,

What I'm trying to do is quite simple and in fact I have found a way to
do it but I am not sure if it is OK. What I mean is sometime in c++ stuff
can "work" but not be "correct". Here is my code maybe you'll see something
wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

Why are you casting to int? Naked casts are always a smell.

add++;

memcpy(&get, (void*)add, sizeof(char));

Again, why the casts? Also sizeof(char) is by definition 1.

printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe it
is basic stuff here but I don't know why, I'm kinda newb). Then I came up
with this technique. The final question is... Is it "correct" ?

Why didn't you just write

get = *(test+1);

--
Ian Collins.

[John Harrison]

Maxime Savard wrote:

Hi there,

What I'm trying to do is quite simple and in fact I have found a way to
do it but I am not sure if it is OK. What I mean is sometime in c++ stuff
can "work" but not be "correct".

Exactly.

Here is my code maybe you'll see something
wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

add++;

memcpy(&get, (void*)add, sizeof(char));

I think it is undefined behaviour to cast a pointer to an integer then
back to a pointer again.

>
printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe it
is basic stuff here but I don't know why, I'm kinda newb).

test + 1 would work.

&test + 1 increments the pointer by the size of the array (which is
four) because &test is a pointer to the array, but test is a pointer to
the first element of the array. It's daft but its the way C and C++ have
always worked.

Then I came up

with this technique. The final question is... Is it "correct" ?

Not really, use 'test + 1' and don't use casts.

Thanks for any info!
Max.

P.s. Sorry for any typo I'm french

john

[Alf P. Steinbach]

* John Harrison:

>
I think it is undefined behaviour to cast a pointer to an integer then
back to a pointer again.

Technically implementation defined.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Maxime Savard]

Neat! Works just fine, and good to know why my pointer increased by 4.

Thanks for the info John,
Max.

"John Harrison" <jo*************@hotmail.comwrote in message
news:dt******************@newsfe6-gui.ntli.net...

Maxime Savard wrote:

>Hi there,

What I'm trying to do is quite simple and in fact I have found a way
to do it but I am not sure if it is OK. What I mean is sometime in c++
stuff can "work" but not be "correct".

Exactly.

>Here is my code maybe you'll see something wrong :

char test[4] = {'1','2','3','4'};
char get = '0';

int add = (int)&test;

add++;

memcpy(&get, (void*)add, sizeof(char));

I think it is undefined behaviour to cast a pointer to an integer then
back to a pointer again.

>>
printf("Second char in array is : %c\n", get);

My objective here was to read the '2' in the test array without using
test[1] as the starting point to read. Then I discovered that when using
&test+1 as the starting address I was in fact increasing it by 4 (maybe
it is basic stuff here but I don't know why, I'm kinda newb).

test + 1 would work.

&test + 1 increments the pointer by the size of the array (which is four)
because &test is a pointer to the array, but test is a pointer to the
first element of the array. It's daft but its the way C and C++ have
always worked.

Then I came up

>with this technique. The final question is... Is it "correct" ?

Not really, use 'test + 1' and don't use casts.

>Thanks for any info!
Max.

P.s. Sorry for any typo I'm french

john

[Default User]

Maxime Savard wrote:

Neat! Works just fine, and good to know why my pointer increased by 4.

Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or the group FAQ list:
<http://www.parashift.com/c++-faq-lite/how-to-post.html>