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

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.

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

你不能复制构造unique_ptr因为它是一个已删除的函数 ，你可以移动唯一的指针来转移所有权，但是你需要一个函子来比较你需要通过引用传递那些unique_ptr的东西。

Answer 4

Use a unique_ptr<x> to express that a function assumes ownership of an x .

Use a shared_ptr<x> to express that a function is part owner of an x .

If you indeed want to pass a unique_ptr and transfer ownership, you should move the smart pointer into the function argument.

More notes on passing smart pointers , and Herb Sutter has some good thoughts in this CppCon talk .