char table / pointer memory allocation

[mehafi@gmail.com]

Hi,

Why this program work?

#include<iostre am.h>
class test
{
public:
char *ptr;
void setPtr(char* p){
ptr = p;
}
void print(){
cout << ptr << endl;
}
};

main()
{
test t;
t.setPtr("abc") ;
t.print();
system("pause") ;
}

Is argument of function setPtr - "abc" - a temporary char table, made
on stack, which is deleted when setPtr function ends? If it is, the
pointer - ptr - point then to the memory addres which was deleted, so
print method should print random characters or may crash the program.
When I made char table using new:

main()
{
char* tab = new char[4];
strcpy(tab, "abc");
test t;
t.setPtr(tab);
delete[] tab;
t.print();
system("pause") ;
}

print() method prints random characters.
But why the first program doesn't crash / prints random chars ?

thanks in advance
best regards
mehafi

[sk_usenet]

Re: char table / pointer memory allocation


<mehafi@gmail.c omwrote in message Hi,

>
Why this program work?
>
#include<iostre am.h>

Non standard header. Get any recent book on C++ that would explain why this
is not portable.

class test
{
public:
char *ptr;
void setPtr(char* p){
ptr = p;
}
void print(){
cout << ptr << endl;
}
};
>
main()

You need an explicit "int main()".

{
test t;
t.setPtr("abc") ;
t.print();
system("pause") ;
}
>
Is argument of function setPtr - "abc" - a temporary char table, made
on stack, which is deleted when setPtr function ends? If it is, the

"abc" is not on stack, it's a string literal and has static storage
duration.
The storage for these objects shall last for the duration of the program

pointer - ptr - point then to the memory addres which was deleted, so
print method should print random characters or may crash the program.
When I made char table using new:

What's a char table?

main()
{
char* tab = new char[4];
strcpy(tab, "abc");
test t;
t.setPtr(tab);
delete[] tab;
t.print();
system("pause") ;
}
>
print() method prints random characters.

Because of undefined behavior.

But why the first program doesn't crash / prints random chars ?

Read above.
--

http://techytalk.googlepages.com

[mehafi@gmail.com]

Re: char table / pointer memory allocation

thx.

[Paavo Helde]

Re: char table / pointer memory allocation

mehafi@gmail.co m wrote in news:b1ee3325-348a-41ec-88bd-d3aea0a2c784
@f36g2000hsa.go oglegroups.com:

Hi,
>
Why this program work?
>
#include<iostre am.h>
class test
{
public:
char *ptr;
void setPtr(char* p){
ptr = p;
}
void print(){
cout << ptr << endl;
}
};
>
main()
{
test t;
t.setPtr("abc") ;
t.print();
system("pause") ;
}
>
Is argument of function setPtr - "abc" - a temporary char table, made
on stack, which is deleted when setPtr function ends? If it is, the

"abc" is a string literal which has static duration (compiled into the
executable code and not going anywhere from there). Thus it will be alive
during the whole program.

Besides, I recommend to get familiar with std::string and forget the
string lifetime issues forever.

Regards
Paavo

[James Kanze]

Re: char table / pointer memory allocation

On May 7, 9:47 pm, Paavo Helde <nob...@ebi.eew rote:

meh...@gmail.co m wrote in news:b1ee3325-348a-41ec-88bd-d3aea0a2c784
@f36g2000hsa.go oglegroups.com:

Besides, I recommend to get familiar with std::string and
forget the string lifetime issues forever.

I can't let that pass. The only times I use C style strings is
when lifetime is an issue; a static char[] with a constant
initializer is static initialized, and effectively has an
infinite lifetime. A static std::string can easily be accessed
before it is constructed, or after it is destructed.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Paavo Helde]

Re: char table / pointer memory allocation

James Kanze <james.kanze@gm ail.comwrote in news:65660d3f-3e1f-4924-93e7-
8e2fe084edc0@j2 2g2000hsf.googl egroups.com:

[...]

>
Nobody suggested using char[] for mutable objects.
>

And nobody has objected using string literals in the code.

Paavo