Re: Question about destructors

From:
88888 Dihedral <dihedral88888@googlemail.com>
Newsgroups:
comp.lang.c++
Date:
Sat, 11 Feb 2012 08:17:46 -0800 (PST)
Message-ID:
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On Feb 7, 6:13 pm, Gus Gassmann wrote:

Some of the code is not written by me, so I am afraid Fred`s solution
does not work. Let me then summarize, just to be absolutely clear:

4. Object **obj = new Object[n];
     for (int i=0; i<n; i++) obj[i] = new Object();


        for (int i=0; i<n; i++) delete obj[i];
        delete[] obj;

5. Object *obj = new Object();
     obj->intArray = new int[10];


        delete[] obj->intArray;
        delete obj;

6. Object **obj = new Object[n];
     for (int i=0; i<n; i++) {
            obj[i] = new Object();
            obj[i]->intArray = new int[10];
     }


        for (int i=0; i<n; i++) {
              delete[] obj[i]->intArray;
              delete obj[i];
        }
        delete[] obj;

Will that do it?

 
Not really. This is pretty fragile C++ code and doomed to break
eventually. For example, it's not exception-safe. Try to exploit the
strengths of C++. One of the biggest corner stones of modern C++
design is letting objects "own"/take care of other resources. C++
allows you to delegate the responsibility of cleaning up things so you
don't have to do it yourself.
 
  Fragile:
    int* p = new int[10];
    ...
    delete[] p;
 
  Good:
    vector<int> v (10);
    ...
 
To put it differently: If you want memory to be released if a pointer
goes out of scope, the raw pointer type would be a misuse because it
doesn't have the semantics you actually want.
 
Instead of new[] and delete[], prefer std::vector. Instead of "naked"
new, prefer the use of std::unique_ptr (or std::shared_ptr) so that
every heap-allocated object is "owned" by at least one other object.
This makes "delete" in user code rather obsolete. And even "new"
should be avoided in preference of std::make_shared and
std::make_unique (make_unique is not yet standard but you can write it
yourself
 
  template<class T, class...Args>
  std::unique_ptr<T> make_unique(Args&&...args)
  {
    return {new T(std::forward<Args>(args)...)};
  }
 
). In case you are worried about unnecessary copying with respect to
vectors and functions returning function-local vectors by value,
you'll be pleased to know that C++2011 will make sure this is about as
efficient as returning a raw pointer.
 
Cheers!
SG


In C++ objects in classes with several vectors or other containers
are allowed. Even copying an object is not so trivial.

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