c++ - Why returning a std::vector is still making a copy? -

i have class going create pretty big array of data don't want copied around. considerations, immutable (read only) outside class.

here's concept code shows how want implement it:

class c { public:     c();     std::vector<int> const& get_vector() { return m_vector; } private:     std::vector<int> m_vector; };  c::c() {     m_vector.push_back(1);     m_vector.push_back(2); }  void displayvector(std::vector<int> const& new_v) {     (int = 0; i<new_v.size(); i++)         std::cout << new_v[i] << std::endl; }  int _tmain(int argc, _tchar* argv[]) {     c myobj;     std::vector<int> v = myobj.get_vector();     v.push_back(3);      displayvector(v); } 

when step through code, see given memory address m_vector , first element of array (using begin iterator).

when return get_vector, vector "v" , first element have different memory address. v.push_back(3) doesn't have problem modifying v (since looks it's copy)

when call displayvector, works expected , new_v addresses same main v.

what doing wrong?

you explicitly created copy with

std::vector<int> v = myobj.get_vector(); 

which can modify since vis plain vector<int>. here v got copy-constructed.

to instead bind reference (which has const because of get_vector's return type) use

const std::vector<int>& v = myobj.get_vector(); 

or just

const auto& v = myobj.get_vector();