use delete to destroy primitive/object types but memory is not freed

[jimjim]

Hello,

This is a simple question for you all, I guess .
int main(){
double *g= new double;
*g = 9;
delete g;
cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl;
*g = 111;
cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl;
return 0;
}

The output:
4 8 8 9
4 8 8 111

Although I delete g, why is it that I can still use it and it references to
actual memory?

The same happens when creating and deleting object types with new and
delete!

Please dont answer with what you think but with what actually happens. If
you can point me to web sources I can read on this, it would have been
great!

Thank you in advance.
Regards,
jimjim

[Christopher Benson-Manica]

jimjim <Fr*********@bl ueyonder.co.uk> spoke thus:

cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl;
cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl; Please dont answer with what you think but with what actually happens. If
you can point me to web sources I can read on this, it would have been
great!

(comp.lang.c++ is that way ----->)

Your post is off-topic for comp.lang.c. Please visit

http://www.ungerhu.com/jxh/clc.welcome.txt
http://www.eskimo.com/~scs/C-faq/top.html
http://benpfaff.org/writings/clc/off-topic.html

for posting guidelines and frequently asked questions. Thank you.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Christopher Benson-Manica]

jimjim <Fr*********@bl ueyonder.co.uk> spoke thus:

cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl;
cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl; Please dont answer with what you think but with what actually happens. If
you can point me to web sources I can read on this, it would have been
great!

(comp.lang.c++ is that way ----->)

Your post is off-topic for comp.lang.c. Please visit

http://www.ungerhu.com/jxh/clc.welcome.txt
http://www.eskimo.com/~scs/C-faq/top.html
http://benpfaff.org/writings/clc/off-topic.html

for posting guidelines and frequently asked questions. Thank you.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Eric Sosman]

jimjim wrote:

Hello,

This is a simple question for you all, I guess .
int main(){
double *g= new double;
[...]

Stop right there: The question just becaue non-simple
for us-all here in comp.lang.c. Perhaps it would be simple
for them-all over thar in comp.lang.c++. Tread carefully
as you cross the demilitarized zone.

--
Er*********@sun .com

[Eric Sosman]

jimjim wrote:

Hello,

This is a simple question for you all, I guess .
int main(){
double *g= new double;
[...]

Stop right there: The question just becaue non-simple
for us-all here in comp.lang.c. Perhaps it would be simple
for them-all over thar in comp.lang.c++. Tread carefully
as you cross the demilitarized zone.

--
Er*********@sun .com

[jimjim]

I knew it was an easy question. I should have looked at a faq first.

The answer is at http://www.eskimo.com/~scs/C-faq/q7.21.html as Christopher
pointed out.

Thanks for the reply :-)

[jimjim]

change new to malloc and delete to free and if you dont know the answer
visit http://www.eskimo.com/~scs/C-faq/q7.21.html as Christopher pointed
out.

[jimjim]

This layout http://www.lysator.liu.se/c/c-faq/c-faq-toc.html of the faq in
the previous post is easier for us the newbies in C to locate the
questions/answers ;-)

Pay attention at the top of the page. There are link to faqs that are also
extremely interesting for newbies like us to read. Most of my questions are
there!! ftp://rtfm.mit.edu/pub/usenet/news.answers/C-faq/faq as well as this
pub/usenet/comp.lang.c/ and this ftp.uu.net (directory
usenet/news.answers/C-faq/ )

[Alan Balmer]

On Wed, 07 Apr 2004 21:11:52 GMT, "jimjim"
<Fr*********@bl ueyonder.co.uk> wrote:

change new to malloc and delete to free and if you dont know the answer
visit http://www.eskimo.com/~scs/C-faq/q7.21.html as Christopher pointed
out.

I'll tell you a secret: Eric does know the answer. When you're
off-topic, you're off-topic. Don't argue about it, just take the good
advice you are given.

Even if the simple substitution you propose actually worked (it
doesn't) there are still differences between C and C++ memory
allocation, and comp.lang.c++ is where you'll get the right answers,
reviewed by the right people.

--
Al Balmer
Balmer Consulting
re************* ***********@att .net

[E. Robert Tisdale]

jimjim wrote:

This is a simple question for you all, I guess .

int main(){
double *g = new double;
double* g = (double*)malloc (sizeof(double) );
*g = 9;
delete g;
free((void*)g);
cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl;
fprinf(stdout, ...
*g = 111;
cout<< sizeof(g)<<" "<<sizeof(doubl e)<<" "<<sizeof(*g)<< " "<<*g<<" "<<endl;
fprintf(stdout, ...
return 0;
} The output:

4 8 8 9
4 8 8 111

Although I delete g,
why is it that I can still use it and it references to actual memory?

The same happens
when creating and deleting object types with new and delete!

Please don't answer with what you think but with what actually happens.
If you can point me to web sources I can read on this,
it would have been great!
delete g;

or

free((void*)g);

delete's or free's the object to which g points.
g is still a valid [pointer] object
but the object to which it points is *not*.
Any reference to *g is *undefined* --
not defined by either the C or C++ standards
after *g is delete'd or free'd.
cat main.cc #include <iostream>

int main(int argc, char* argv[]) {
double* p = new double;
*p = 9;
delete p;
double* g = new double;
std::cout << sizeof(g) << ' ' << sizeof(double) << ' '
<< sizeof(*g) << ' ' << *g << std::endl;
*p = 111;
std::cout << sizeof(g) << ' ' << sizeof(double) << ' '
<< sizeof(*g) << ' ' << *g << std::endl;
return 0;
}
g++ -Wall -ansi -pedantic -o main main.cc
./main

4 8 8 9
4 8 8 111

Notice that, after *p is delete'd (free'd),
the storage be allocated for another object (*g in this case).