c++ - Const-ness when returning reference to an internal object -

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• c++11 auto: if gets constant reference? 1 answer

i trying understand following case, behaves differently expect.

given following code example:

class consttestclass {     std::deque<unsigned long long> deque; public:     const std::deque<unsigned long long int> &getdeque() const{return deque;} };  int main() {     consttestclass consttestclass;     auto constdeque = consttestclass.getdeque();     constdeque.emplace_back(1ull); } 

i expect call constdeque.emplace_back(1ull); fail, since expecting constdeque reference const std::deque<>, compiles without error on gcc 5.4.0.

if change line above const auto constdeque = consttestclass.getdeque(); fails compile expected, since constdeque of expected type.

i trying understand why auto can remove const-ness return type , error in understanding lies.

edit: not answered duplicate. still looking way make sure caller gets const reference (or pointer) internal object, caller see state changes itnernal object without being able modify it.

auto uses same rules template argument deduction. in order clarify further, imagine have function (e.g., foo):

template<typename t> void foo(t v) {   v.emplace_back(1ull); } 

and following:

std::deque<unsigned long long> dq; const std::deque<unsigned long long> &dqr = dq; foo(dqr); 

this code pass compiler live demo. happening because according template argument deduction rules t deduced std::deque<unsigned long long> , dqr passed copy foo.

now imagine auto template argument (e.g,. t):

t constdeque = consttestclass.getdeque(); 

because auto uses same rules template argument deduction, happend in example in foo, deduce t in std::deque<unsigned long long>.

consequently, you're creating copy of const member std::deque member variable. getting behaviour you're anticipating, you'll have give little auto deduction , write:

auto &constdeque = consttestclass.getdeque(); 

or if want clarify further readers of code resulting auto deduction const qualified variable add explicitly const qualifier as:

auto const &constdeque = consttestclass.getdeque();