How can I delete a resource owned by shared pointer from the vector C++

Actual Aim: Delete/Destroy the network from the vector and reintroduce it if needed.

So I wrote a small program to try the same.

I am facing problems in deleteing a shared pointer and setting it to nullptr. Directly setting to nullptr will lead to dangling pointer. How can I free that position without causing a memory leak?

#include <iostream>
#include <vector>
#include <memory>
using namespace std;
class ptr{
  public:
  int x;
  ptr(int a):x(a){  }
  ~ptr()
  {
       cout << "destructor\n";
  }
};
int main()
{
    std::vector<shared_ptr<ptr>>v;
    v.push_back(shared_ptr<ptr>(new ptr(100)));
    v.push_back(shared_ptr<ptr>(new ptr(200)));
    v.push_back(shared_ptr<ptr>(new ptr(300)));
    v.push_back(shared_ptr<ptr>(new ptr(400)));
    v.push_back(shared_ptr<ptr>(new ptr(500)));
    auto it = v.begin();
    int i = 0;
    while(i < 5)
    {
        if((*it)->x == 300)
        {
            cout << "300 found \n";
            delete (*it);  // -> Problem is here.
        }
        it++;
        i++;
    }
    return 0;
}

Solution

Its a little odd that you are using smart pointers and at the same time want to manually manage the memory.

However, there are several ways to let a smart pointer release the managed object:

the most natural: destroy the smart pointer. You could erase it from the vector
reset(), effectively the same as...
shared_ptr<int>().swap(v[i])
... and more ...

None of that results in a leak or a dangling pointer. You can misuse shared_ptr to create a dangling pointer or to get a memory leak, but you would have to really push hard to do so.

Actual Aim: Delete/Destroy the network from the vector and reintroduce it if needed.

This doesn't sound like smart pointers but rather like std::vector<std::optional<ptr>>. Alternatively consider to use a std::unordered_map<int,ptr> which can model a sparse std::vector where you can erase elements while the others keep their key (=index).