c++ - Assigning value to returned shared_ptr doesn't behave as expected -

i have private 3 dimensional vector of shared_ptr<room> objects follows:

private:     vector<vector<vector<shared_ptr<room>>>> world; 

in same class, provide access room objects:

public:     shared_ptr<room> room_at(const int & x, const int & y, const int & z) const     {         return world.at(x).at(y).at(z);     } 

also in same class, initialize world structure:

for (int x = 0; x < c::world_x_dimension; ++x) {     vector<vector<shared_ptr<room>>> row;     (int y = 0; y < c::world_y_dimension; ++y)     {         vector<shared_ptr<room>> vertical_stack; // "stack"         (int z = 0; z < c::world_z_dimension; ++z)         {             vertical_stack.push_back(shared_ptr<room>(nullptr));         }         row.push_back(vertical_stack);     }     world.push_back(row); } 

later, want save room object world:

void add_room_to_world(const int & x, const int & y, const int & z) {     shared_ptr<room> room = make_shared<room>(); // create empty room      /* (populate room's member fields) */      // add room world     room_at(x, y, z) = room; // doesn't work expected } 

the shared_ptr within world starts out nullptr expected, doesn't change on last line above.

based on i've found on so, i've tried operator= (above), .reset(room), , make_shared<room>(room) (using actual room object rather shared_ptr<room>) in cases, shared_ptr within world stays set nullptr.

what correct way assign objects world?

room_at returns value. when returned function copied, operations on returned value don't affect original value. if want change original value have return reference this:

shared_ptr<room>& room_at(const int & x, const int & y, const int & z) const {    return world.at(x).at(y).at(z); } 

if don't want users of class able this, declare method private , keep original is.