Member Avatar for sjcomp

Hello, I have a vector of smart pointers and I'd like to NULL smart pointers in reverse order. Smart pointers are done with boost::shared_ptr<>, this is the code:

// List of pointers
typedef boost::shared_ptr<int> TSmart;
std::vector<TSmart> Vec;
// Make sure that everything is NULL
std::transform( Vec.rbegin(), Vec.rend(), TSmart());

I get errors for this one. What am I doing wrong? I tried to use boost::bind

std::transform( Vec.rbegin(), Vec.rend(), boost::bind<TSmart>(TSmart()));

But I still get errors. I do not want to use boost::lambda there. What is the proper way to do that?
Thanks

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The compile error is because std::transform expects an output iterator as its third parameter, not a value. The overload you tried (two iterator args plus a TSmart() value) does not exist. ' approach works because it supplies a proper destination iterator and a functor, but there are simpler and clearer ways to null every boost::shared_ptr in a vector.

A concise way to set every element to a null shared_ptr (no per-element functor needed) is std::fill:

std::fill(Vec.begin(), Vec.end(), TSmart());

If the order in which the pointed-to objects are destroyed matters (you asked for reverse order), iterate with reverse iterators and call reset() on each element:

for (auto it = Vec.rbegin(); it != Vec.rend(); ++it)
    it->reset();   // or *it = TSmart();

You can also use an algorithm with a member-call binder if you prefer:

std::for_each(Vec.rbegin(), Vec.rend(), boost::bind(&TSmart::reset, _1));

Notes and cautions:

  • std::fill copies the provided value into each slot; using TSmart() gives an empty smart pointer for each element.
  • Resetting a shared_ptr only destroys the pointed object once its last owner is gone; other copies elsewhere prevent immediate destruction.
  • Cyclic references will keep objects alive even after resets; use weak_ptr to break cycles (see weak/shared_ptr docs).

Useful references: std::fill, std::transform, std::for_each, and the shared_ptr discussion on ownership and cycles (see the shared/weak pointer docs).

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Member Avatar for twomers

Well, if you use vec.size() surely the vector is empty when you're trying to null everything... right? Also, doesn't transform take four parameters?

Anyway, something like this should work:

#include <iostream>
#include <vector>
#include <algorithm>

#include <boost/shared_ptr.hpp>
#include <boost/bind.hpp>

template <class ty>
  class nullify {
  private:
  public:
    ty operator ( ) ( ty &elem ) {
      elem.reset();
      return elem;
    }
  };

int main() {
  // List of pointers
  typedef boost::shared_ptr<int> TSmart;

  std::vector<TSmart> Vec;

  // Make sure that everything is NULL
  std::transform( 
    Vec.begin(), 
    Vec.end(), 
    Vec.begin(), 
    nullify<TSmart>() );

  return 0;
}
Member Avatar for sjcomp

Thanks a lot, twomers. Your solution works!

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