Re: question regarding the shared_ptr use_count
Am 03.02.14 02:12, schrieb somenath:
I am not able to understand the behavior of the following program
#include <iostream>
#include<memory>
#include<string>
using namespace std;
int main() {
auto p1 = make_shared<string> (10,'S');
cout<<"p1 use_count = "<<p1.use_count()<<endl;
shared_ptr<string> p2(new string()) ;
p1 = p2;
cout<<"p2 use_count = "<<p2.use_count()<<endl;
cout<<"second p1 use_count = "<<p1.use_count()<<endl;
return 0;
}
Output
++++++++++++++++
p1 use_count = 1
p2 use_count = 2
second p1 use_count = 2
I can understand the first two print. At beginning p1 points to one string so the reference count is 1.
When the statement p1=p2; executes p2's reference count gets incremented to 2 but at the same time I was expecting that p1's reference count to be decremented by 1 as p1 would be pointing to p2 now.
Please let me know where my understanding is going wrong?
(this posting contains some ASCII art which will look quite bad if
viewed with proportional fonts).
The situation just before the assignment "p1 = p2;" looks like this:
Each shared_pointer does not point to the object directly but to an
intermediate object which contains the reference count. Both pointers p1
and p2 are the only shared_pointers pointing to their strings, so the
reference counts have both the value 1.
,''''''''`.
| refcnt 1|
,_______ `. | | _______________
| P1 |------=: |.......... . | |
L______| - | | `. |"SSSSSSSSSS" |
| ptr |----------'|_______________|
|_________J ,'
-'
,''''''''`.
| refcnt 1|
,_______ `. | | _______________
| P2 |------=: |.......... . | |
L______| - | | `. |"" |
| ptr |----------'|_______________|
|_________J ,'
-'
In the assignment statement, p1 will decrement the ref-count of the
intermediate object, since p1 no longer points to the intermediate
object of string "SSSSSSSSSS". Instead it will point to the intermediate
object of p2:
,''''''''`.
| refcnt 0|
,_______ | | _______________
| P1 | |.......... . | |
L______| | | `. |"SSSSSSSSSS" |
`. | ptr |----------'|_______________|
`. |_________J ,'
`. -'
\ |
`.|
---- ,''''''''`.
| refcnt 2|
,_______ `. | | _______________
| P2 |------=: |.......... . | |
L______| - | | `. |"" |
| ptr |----------'|_______________|
|_________J ,'
-'
Now the assigment statement will figure out that the reference count of
the intermediate object of "SSSSSSSSSS" has become zero, so it can
delete the intermediate object and the string "SSSSSSSSSS".
Sorry, if I did not use the proper vocabulary. The intermediate object
is an implementation details, although I doubt whether there is an
implementation that differs from this scheme.
Regards,
Stuart