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  1. #include <cstddef>
  2. #include <iostream>
  3. #include <vector>
  4.  
  5. class Field;
  6.  
  7. class Vector3d
  8. {
  9.     friend class Field;
  10.  
  11.     public:
  12.         Vector3d() : components_(new double[3]) {}
  13.         ~Vector3d() { delete[] components_; }
  14.  
  15.         Vector3d(const Vector3d &v) : components_(new double[3])
  16.         {
  17.             components_[0] = v.components_[0]; 
  18.             components_[1] = v.components_[1]; 
  19.             components_[2] = v.components_[2];
  20.         }
  21.  
  22.         double & operator()(const std::size_t index) { return components_[index]; }
  23.  
  24.     private:
  25.         Vector3d(double *ptr) : components_(ptr) {};
  26.         double *components_;
  27. };
  28.  
  29. class Field
  30. {
  31.     public:    
  32.         Field(std::size_t size);
  33.         ~Field();
  34.  
  35.         Vector3d & operator()(const std::size_t index) { return *(vectors_[index]); }
  36.  
  37.     private:
  38.         std::vector<double> components_;
  39.         std::vector<Vector3d *> vectors_;
  40. };
  41.  
  42. Field::Field(std::size_t size)
  43. : components_(std::vector<double>(3 * size))
  44. // the line below is still evil in a subtle way, see below
  45. , vectors_(std::vector<Vector3d *>(size, new Vector3d(0)))
  46. {
  47.     double *ptr = &components_[0];
  48.     for (auto it = vectors_.begin(); it != vectors_.end(); ++it)
  49.     {
  50.         (*it)->components_ = ptr;
  51.         ptr += 3;
  52.     }
  53. } 
  54.  
  55. Field::~Field()
  56. {
  57.     for (auto it = vectors_.begin(); it != vectors_.end(); ++it)
  58.         (*it)->components_ = 0;
  59. }
  60.  
  61. int main(int argc, char *argv[])
  62. {
  63.     Field field(2);
  64.     std::cout << field(0)(0) << " " << field(1)(0) << std::endl;
  65.  
  66.     field(0)(0) = 5;
  67.  
  68.     std::cout << field(0)(0) << " " << field(1)(0) << std::endl;
  69.  
  70.     // both were changed because you initialized every single vectors_ entry with a pointer
  71.     // to the same Vector3d object.
  72.  
  73.     return 0;
  74. }
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https://ideone.com/BRLsXL
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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