Re: Best way to allocate memory in the constructor

From:
Ole Hinz <ohinz@cs.tu-berlin.de>
Newsgroups:
comp.lang.c++.moderated
Date:
Sun, 20 Jan 2008 12:10:15 CST
Message-ID:
<5vgk60F1mivgdU1@mid.dfncis.de>
Hi,

Salvatore Benedetto wrote:

Hi,

first of all, thanks to everybody for taking some time and aswering
my question.

Now back to my problem.

I have used char* and int* and my previous email, but that was just an
example. In my constructor, I actually allocate array of structures, or
single structure. I do that with malloc, instead of new(), because there
is not constructor I need to call, and malloc should be faster if I
don't mistake.

Since in my code, speed and low-memory used are a _must_, I wanted to
avoid to use STL components (std::vector,std::auto_ptr, etc..) as they
introduce some overhead, even if little.


You can use a single malloc() with aggregated size. I assume, the
structs are named s1, s2 and s3 and you want space for 100x s1, 200x s2
and 150x s3.

The code looks like this:

class Foo
{
public:
    Foo() : p1(0), p2(0), p3(0)
    {
       char* p = static_cast<char*>(
                   malloc(100*sizeof(s1)+200*sizeof(s2)+150*sizeof(s3))
                 );
       if (p)
       {
          p1 = reinterpret_cast<s1*>(p);
          p2 = reinterpret_cast<s1*>(p+100*sizeof(s1));
          p3 = reinterpret_cast<s1*>(p+100*sizeof(s1)+200*sizeof(s2));
       }
       else
       {
          // ...
       }
    }

    ~Foo()
    {
       free(p1);
    }
private:
    s1* p1;
    s2* p2;
    s3* p3;
};

Best regards,
Ole Hinz

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