Re: Using malloc in C++

From:

Nick Keighley <nick_keighley_nospam@hotmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 6 Jul 2010 02:29:59 -0700 (PDT)

Message-ID:

<33182ea1-4e81-4709-abe8-039aafb8ef8a@s9g2000yqd.googlegroups.com>

On 29 June, 19:55, Bharath <bharath...@gmail.com> wrote:

On Jun 28, 5:31 pm, Stuart Golodetz <b...@blah.com> wrote:

On 28/06/2010 12:20, Bharath wrote:

Hi all,

I am trying to use malloc to allocate memory to an object. Something
like:

TestClass *obj = (TestClass *) malloc(sizeof(TestClass));

Its obvious that the constructor doesn't get called in the above case=

nor the destructor when I free it.

Is there a way that I can invoke the constructor after the malloc
statement?

I believe that's what 'new' does isn't it? It allocates memory and
then invokes the constructor.

Thanks,
-Bharath

#include <iostream>
#include <memory>

struct X
{
        X() { std::cout << "X()\n"; }
        ~X() { std::cout << "~X()\n"; }

};

void test_malloc()
{
        X *p = static_cast<X*>(malloc(sizeof(X)));
        new (p) X;
        p->~X();
        free(p);

}

void test_operator_new()
{
        X *p = static_cast<X*>(::operator new(sizeof(X)));
        new (p) X;
        p->~X();
        ::operator delete(p);

}

void test_new()
{
        X *p = new X;
        delete p;

}

int main()
{
        test_malloc();
        test_operator_new();
        test_new();
        return 0;

}

Cheers,
Stu

Wow Stu, that clears up the confusion. I had an idea that I had to use
the new operator to call the constructor again, but wasn't sure how.

it isn't calling it again. Its invoking it once. You don't want it to
call it again.