pointers - c++ push_back copying object instead of reference -


i'm having issues understanding problem i've mentioned in comments below:

class node {     public:         node(int value): value(value), neighbors() {}         int value;         std::vector<std::shared_ptr<node>> neighbors; };      std::vector<node> nodes;     nodes.reserve(50);     for(int j = 0 ; j < 50; ++j) {         nodes.push_back(node(j));     }     for(int j = 1 ; j < 25; ++j) {         nodes.at(0).neighbors.push_back(std::make_shared<node>(nodes.at(j)));     }      //the following line copying nodes.at(0) instead of      //copying reference nodes.at(0). why?     nodes.at(15).neighbors.push_back(std::make_shared<node>(nodes.at(0)));      for(int j = 25 ; j < 50; ++j) {         nodes.at(0).neighbors.push_back(std::make_shared<node>(nodes.at(j)));     }      std::cout << nodes.at(15).neighbors.at(0).neighbors.size();     //prints out 24      std::cout << nodes.at(0).neighbors.size();     //prints out 49

why following line copying nodes.at(0) (which returns reference first element of nodes vector) instead of storing reference it?

    nodes.at(15).neighbors.push_back(std::make_shared<node>(nodes.at(0)));

nothing in code stores references. code copies node because allocate new node in std::make_shared , invoke copy constructor.

  std::vector<node> nodes;

it's local function. impossible keep either pointers (shared or not) or references elements of vector. perhaps want use vector of shared pointers instead:

 std::vector<std::shared_ptr<node>> nodes;

bear in mind shared_ptr doesn't work in presence of cyclic referenced. if data structure general graph, perhaps shared_ptr not appropriate storing neighbours. may want consider weak_ptr instead (you have keep container of shared pointers nodes separately).


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