Re: Best way to allocate memory in the constructor

From: (Carl Barron)
Sat, 19 Jan 2008 22:21:13 CST
salvatore benedetto <> wrote:


What's the best way to allocate memory in the constructor and avoid
memory leaks if the constructor fails?

Let's say that in my constructor, I have to allocate memory
with malloc (i.e. an array of char *)several times, and one
of them fails.

What's the best way to deallocate the previously allocated memory?
Is the destructor being called if I "return" in the middle of
the constructor body? If not, is it a good idea to call explicity
the constructor in order to clean the memory?

At the moment, I simply check every allocation and if the check fails
I deallocate all the memory previously allocated, but this produces
a lot of code duplication.

      if ((first = (char *)malloc(sizeof(....)) == NULL)
      if ((second = (char *)malloc.... )) == NULL) {
              free(first); return;
      if ((third = (int *)malloc... )) == NULL) {
      // and so on!

Thanks in advance for your help.

    Resource Allocation Is Initialization [RAII] con do this.
The idea is to construct the info in a ctor and destruct it in
the destructor. for the above we want to get control of the pointers
only if all three are allocated so some private nested class in your
class say foo, and foo::RAII can look like this:

class foo
    class RAII
       char *p;
       bool create(int n)
          p = (char *)std::malloc(n);
          return p != 0;
       char *p release()
         char *q = p;
         p = 0;
         return q;
         if(p) std::free(p);

    char *first,*second,*third;
    void destroy()
       if(first) std::free(first);
       if(second) std::free(second);
       if(third) std::free(third);
    void create()
        RAII f,s,t;
        if(f.create(n_1) && s.create(n_2) && t.create(n_3))
           first = f.release();
           second =;ease();
           third = t.release();
           first = second = third = 0;
// ....

note foo::create calls foo::RAII::create(int) and if all three
creates succeed it takes control of the pointers otherwise those
allocated get free'd when the RAII's go out of scope.
This is the 'bare bones solution'. Using std::vector<char> is
even easier if that is an option

clsss Foo
    std::vector<char> first,second,third;
    static void clear(std::vector<char> &x)
         std::vector<char> t();
    // if you really need char ptrs this will give
    // one pointing to the beginning of the vector's data.
    static char * char_ptr(std::vector<char> &x)
        return x.empty() ?0: &x[0];
    // ...

      [ See for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]

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