C++ memory leak with class members

问题:

Debug_VLD in VS2010 reveals some memory leaks that come from class member creation / initialization / deletion.

my_member is a data member with type double*. In constructor, I have

my_member = NULL ;

Then in some method, i need to allocate memory for my_member. I cannot do so in constructor, since i dont know size of array yet, and/or size may different for different calls of the method. What i do in this method is checking if member is NULL. if so, i allocate space for it, if not, i can operate on array (changing value for its element with accesor []). It looks like

void MyClass::my_method()
{
    if( my_member == NULL )
        my_member = new double[n_dim] ;

    for(int k = 0 ; k < n_dim ; k++ )
         my_member[k] = k ;
}

and memory leak occurs at line my_member = new double[n_dim] ;.

In destructor, i have

delete[] my_member ;

what is wrong? how to do allocation properly ?

thanks!

回答1:

Using std::vector<double> is the preferred way, but if you want to have raw double* and write copy constructor, move constructor and operator= "by hand", you should do something like this:

#include <assert.h>  // for assert

#include <algorithm> // for std::swap

class MyClass
{
  //
  // Raw array
  //
  double * m_ptr; // raw pointer
  size_t m_dim;   // number of items in the array

public:

  // Default constructor - creates empty vector
  MyClass()
    : m_ptr(nullptr)
    , m_dim(0)
  {
  }


  // Copy constructor
  MyClass(const MyClass& src)
    : m_ptr(nullptr)
    , m_dim(0)
  {
    // Special case of empty source
    if (src.m_dim == 0)
    {
      assert(src.m_ptr == nullptr);
      return;
    }

    // Allocate and deep-copy from source
    m_ptr = new double[src.m_dim];
    m_dim = src.m_dim;
    for (size_t i = 0; i < m_dim; i++)
      m_ptr[i] = src.m_ptr[i];
  }

  // Move constructor: steal the "guts" from src
  MyClass(MyClass&& src)
  {
    m_ptr = src.m_ptr;
    src.m_ptr = nullptr;

    m_dim = src.m_dim;
    src.m_dim = 0;
  }

  // Destructor
  ~MyClass()
  {
    delete [] m_ptr;
  }

  // Unified operator=
  MyClass& operator=(MyClass src)
  {
    std::swap(m_ptr, src.m_ptr);
    std::swap(m_dim, src.m_dim);
    return *this;
  }
};

回答2:

If there's any other place in the code where you set my_member to NULL without calling delete[], then yes. If you don't obey the rule of three (properly implemented copy constructor and assignment operator), you'll run into all sorts of trouble.

To prevent this, use a std::vector<double> instead, where you can do:

void MyClass::my_method()
{
    my_member.resize(n_dim); // yay, learned something new here 
    for(int k = 0 ; k < n_dim ; k++ )
         my_member[k] = k ;
}

That way, you're not managing memory, so there's no need for a destructor (unless it's virtual, in which case it can be empty).