[info] Calling an object's constructor

[Zaei]

I found out a neat trick earlier. Consider the following code:

#include <iostream.h>
#include <malloc.h>

class c
{
public:
	c();
	~c();
};

c::c() {	cout << "c Constructed\n"; }

c::~c() {	cout << "c Destructed\n"; }

int main()
{
	c* p_c = NULL;
	p_c = (c*)malloc(sizeof(c));
	return 1;
}

In this code, the constructor of class c will never be called. How do we solve this problem. There is another new operator, called placement new. Consider:

#include <iostream.h>
#include <new>
#include <malloc.h>

class c
{
public:
	c();
	~c();
};

c::c() {	cout << "c Constructed\n"; }

c::~c() { 	cout << "c Destructed\n"; }

int main()
{
	c* mem = NULL;
	c* p_c = NULL;
	mem = (c*)malloc(sizeof(c));
	p_c = new(mem) c; //note how the placement new operator takes in a pointer to a memory address
	return (unsigned long(mem)) == (unsigned long)p_c;
}

In the above, new takes in an extra parameter to a memory pointer, and constructs an object in that memory.

I found this interesting, and I was looking for a solution to this problem a few weeks ago, and though someone might need it, or be interested.

Z.