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c++design-patternsraii

Pattern name for create in constructor, delete in destructor (C++)


Traditionally, in C++, you would create any dependencies in the constructor and delete them in the destructor.

class A
{
 public:
    A() { m_b = new B(); }
    ~A() { delete m_b; }
 private:
    B* m_b;
};

This technique/pattern of resource acquisition, does it have a common name?
I'm quite sure I've read it somewhere but can't find it now.

Edit:
As many has pointed out, this class is incomplete and should really implement a copy constructor and assignment operator.
Originally, I intentionally left it out since it wasn't relevant to the actual question: the name of the pattern. However, for completeness and to encourage good practices, the accepted answer is what it is.


Solution

  • The answer to your question is RAII (Resource Acquisition Is Initialization).

    But your example is dangerous:

    Solution 1 use a smart pointer:

    class A
    {
      public:
         A(): m_b(new B) {}
      private:
         boost::shared_ptr<B> m_b;
    };
    

    Solution 2: Remember the rule of 4:
    If your class contains an "Owned RAW pointer" then you need to override all the compiler generated methods.

    class A
    {
      public:
         A():              m_b(new B)           {}
         A(A const& copy): m_b(new B(copy.m_b)) {}
         A& operator=(A const& copy)
         {
             A  tmp(copy);
             swap(tmp);
             return *this;
         }
        ~A()
         {
             delete m_b;
         }
         void swap(A& dst) throw ()
         {
             using std::swap;
             swap(m_b, dst.m_b);
         }
      private:
         B* m_b;
    };
    

    I use the term "Owned RAW Pointer" above as it is the simplest example. But RAII is applicable to all resources and when your object contains a resource that you need to manage ('Owned RAW Poiner', DB Handle etc).