std::ref(val) is used to create object of type std::reference_wrapper that holds reference of val. Used in cases when usual reference passing using & does not compile or & is dropped due to type deduction. std::cref is similar but created reference wrapper holds a const reference to val.

// create a container to store reference of objects.
auto val = 99;
auto _ref = std::ref(val);
_ref++;
auto _cref = std::cref(val);
//_cref++; does not compile
std::vector<std::reference_wrapper<int>>vec; // vector<int&>vec does not compile
vec.push_back(_ref); // vec.push_back(&i) does not compile
cout << val << endl; // prints 100
cout << vec[0] << endl; // prints 100
cout << _cref; // prints 100