Re: destruction of already destructed pointer variable when copying an object - abort error

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Sat, 11 Sep 2010 11:08:18 +0200

Message-ID:

<i6fgu2$vss$1@news.doubleSlash.org>

suresh 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.

I take it, you want to inspect the set after min_element() has run.
Otherwise, the above would not be a rationale.

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?

Don't have FO take ownership and use pointers only to manage addresses of
objects that already exist. Something like this (off the cuff and untested):

typedef std::set< int > int_set;

class FO {
  int_set * where;
public:
  FO ( int_set & ref )
    : where ( &ref )
  {}
  bool operator() ( int lhs, int rhs ) const {
    where->insert(a);
    return ( a < b );
  }
}

int main ( void ) {
  std::vector< int > v;
  ...
  int_set inspect;
  min_element( v.begin(), v.end(), FO( inspect ) );
  // now inspect contains some elements
}

Best

Kai-Uwe Bux