C ++将已删除函数与unique_ptr，make_unique一起使用

Question

A pseudo class performing matrix additon using std::unique_ptr as the data member to represent the elements. M-rows, N columns. Class is templated for type, M, N. I am trying to perform a matrix addition (C = A+B) and keep getting this error from "error: use of deleted function". I am trying to crunch the fundamentals of C++ with a complication of smart_ptrs, any pointers regarding this error is appreciated.

All the participating functions (in the error) are shown below:

class Matrix{  
  private:  
   std::unique_ptr<T[]> array=std::make_unique<T[]>(M*N);
  public:  
   Matrix();
   Matrix( const Matrix& other );
   inline Matrix operator+( const Matrix& other ) const;
   const Matrix& operator=( const Matrix< M, N, T >& other );
}

Matrix::Matrix(const Matrix& other)
{
  *this = other;
}

inline Matrix
Matrix::operator+( const Matrix& other ) const
{
  Matrix result(*this);
  result += other;
  return result;
}

const Matrix&
Matrix::operator=( const Matrix& other )
{
  if(this != &other)
    this->array = std::move(other.array);                                                                                                                                                         
  return *this;
}

Error reporting places:

error: use of deleted function ‘matlib::Matrix<2ul, 2ul, double>::Matrix(const matlib::Matrix<2ul, 2ul, double>&)’
 Matrix< M, N, T > result(*this);

error: use of deleted function ‘std::unique_ptr<_Tp [], _Dp>& std::unique_ptr<_Tp [], _Dp>::operator=(const std::unique_ptr<_Tp [], _Dp>&) [with _Tp = double; _Dp = std::default_delete<double []>]’
   this->array = std::move(other.array);

Thanks in advance.

Answer 1

In general, most of your errors originate from that std::unique_ptr has no copy constructor. This is so that it can efficiently manage the scope of the pointer inside of it. One way around this is to create a new std::unique_ptr , and copy all individual values over.

I've written an example class for you, perhaps it can help.

#include <memory>

template <class T>
class Matrix
{  
private:  
    std::unique_ptr<T[]> data = nullptr;
    size_t height, width;
public:  
    Matrix(size_t h, size_t w)
        : height(h), width(w)
    {
        if(h*w == 0) return;
        data = std::make_unique<T[]>(h*w);
    }

    Matrix(const Matrix& other)
    {
        height = other.height;
        width = other.width;

        data = std::make_unique<T[]>(height * width);

        for(size_t i = 0; i < height; i++)
        {
            for(size_t j = 0; j < width; j++)
            {
                (*this)(j, i) = other(j,i);
            }
        }
    }

    Matrix operator=( const Matrix& other )
    {
        if(this == &other)
        {
            return *this;
        }

        height = other.height;
        width = other.width;

        data.reset(std::make_unique<T[]>(other.height * other.width));

        for(size_t i = 0; i < height; i++)
        {
            for(size_t j = 0; j < width; j++)
            {
                (*this)(j, i) = other(j,i);
            }
        }

        return *this;
    }

    T & operator()(size_t x, size_t y)
    {
        //If data is nullptr then this is undefined behaviour.
        //Consider adding a throw or assert here
        return data[y * height + x];
    }

    T operator()(size_t x, size_t y) const
    {
        //If data is nullptr then this is undefined behaviour.
        //Consider adding a throw or assert here
        return data[y * height + x];
    }

    size_t getHeight() const
    {
        return height;
    }

    size_t getWidth() const
    {
        return width;
    }
};

As a final statement, if what you want to do is to create matrices for mathematical purposes, I suggest you give them static sizes for performance reasons. Adding in mathematical operators to a class like this involves additional logic for the cases where dimensions are mismatched. Statically sized matrices will solve this by themselves due to their typing. You can still do it like this, but be wary of any edge cases.