Re: destruction of already destructed pointer variable when copying an object - abort error
In article
<9d2fec72-3729-42d6-8088-f9a78c131515@b4g2000pra.googlegroups.com>,
suresh <suresh.amritapuri@gmail.com> wrote:
Hi,
Kindly consider the code segment below: I have a function object
containing a pointer variable which is passed to min_element
algorithm. My problem is the pointer variable is deleted twice and I
do not know how to fix this issue.
class FO{
public:
set<int>::size_type size(){return s->size()}
bool operator()(int a, int b);
~FO();
private:
set<int> * s;
};
FO::FO(){
s = new set<int>;
}
FO::~FO(){
delete s;
}
bool FO::operator()(int a,int b){
s->insert(a);
return (a<b);
}
int main(){
vector<int> v;
//vector populated
FO fo;
min_element(v.begin(),v.end(),fo);
}
The variable 's' is defined as a pointer bcz min_element algorithm
takes a copy of its function object argument. Now inside the
min_element algorithm, the copy of 'fo' is deleted which results in
freeing of the memory associated with 's'. But in the main, the
original object fo is destructed and then also the same memory is
freed and this gives a abort error.
How to solve this kind of a problem?
Maybe this is a bad example, but the fundamental error here is that you
are trying to use min_element to fill a set. Better would be something
like:
void fn(vector<int>& v) {
set<int> s(v.begin(), v.end());
vector<int>::iterator it = min_element(v.begin(), v.end());
// now you can use both 's' and 'it'
}
If you are doing something inside the functor and you need the functor
back when it is done visiting all the elements in the container, then
use std::for_each (which returns a copy of the functor as its result.)
Don't pass FO to it though, it doesn't have a correct copy c_tor or op=.
An example:
struct count_vowels : unary_function<char, void> {
int result;
count_vowels(): result(0) { }
void operator()(char c) {
if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
++result;
}
}
};
int main(){
vector<char> v;
//vector populated
int count = for_each(v.begin(), v.end(), count_vowels()).result;
cout << count;
}