Why am I getting compile error "use of deleted function 'std::unique_ptr ..."

Question

Am getting a huge compile error with message

c:\mingw\include\c++\6.1.0\bits\predefined_ops.h:123:18: error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = Deduction; _Dp = std::default_delete<Deduction>]'
         { return bool(_M_comp(*__it1, *__it2)); }

when I pass a custom comparator to STL set_difference function.

My code:

struct Value{
   std::string ded_code;
   float amount;
   Value(std::string code, float amt):ded_code(code), amount(amt){}
};

struct Deduction{
  std::string p_number;
  std::vector<std::unique_ptr<Value>> values;
  Deduction(string pnum, string code, float amt):p_number(pnum){ 
    auto val = std::make_unique<Value>(code, amt);
    values.emplace_back(move(val));
  }
};

class compute{
public:
   vector<unique_ptr<Deduction>> deductions;
   void fillDeductions(){
    // fill deductions
    ...
   }

};

class CompareDiff{
public:
  bool operator()(unique_ptr<Deduction>& ded1, unique_ptr<Deductions>& ded2){
    rPtr1 = ded1.get();
    rPtr2 = ded2.get();
    return ( rPtr1->p_number < rPtr2->p_number);
 }
};

...
int main(){
  ...
  // fill two deduction vectors
  Compute compA = Compute()
  compA.fillDeductions()

  Compute compB = Compute()
  compB.fillDeductions()

  vector<unique_ptr<Deduction>> diffs

  set_difference(compA.begin(), compA.end(),
                 compB.begin(), compB.end(),
             inserter(diffs, diffs.begin()), CompareDiff());

 }

Am using gcc 6.1.0 on windows 7 machine.

What am I missing?

Regards.

PG

Answer 1

You cannot copy construct a unique_ptr since its a deleted function, you can move unique pointers to transfer ownership but since you want a functor to compare something you need to pass those unique_ptr by reference.

Answer 2

The reason why you are still getting an error:

std::set_difference does copy internally:

Copies the elements from the sorted range [first1, last1) which are not found in the sorted range [first2, last2) to the range beginning at d_first.

http://en.cppreference.com/w/cpp/algorithm/set_difference

If you want your code to compile, use std::shared_ptr instead.

Keep in mind, that the Standard Library is optimized for Objects and not for pointers. You will have to do the ownership management yourself if you use pointers.

Answer 3

The chief feature of std::unqiue_ptr is that it cannot be copied. That's by design, and the name tells you as much.

However, CompareDiff tries to take its arguments by value. That requires a copy. Instead, take a std::unique_ptr<..> const& - no copy is needed.