DAtum: Explicit construction of class object (not pointer to class object)

Explicit construction of class object (not pointer to class object)

Consider the following piece of code:



#include <stdio.h> 

#include <stdlib.h> 
using namespace std;

class SSS{
    public:
       SSS(){
        printf("Calling constructor\n");
       }
};

typedef struct test{
SSS s1;
}test;

int main(){
    test *t1;
    t1=(test*)malloc( sizeof(test)*1);
    return 0;
}

Does the constructor of the class get called?
Nope.

Then how do you do it. The problem is, how do you explicitly call the constructor of a class object that is defined but not constructed. Usually, class objects are constructed as soon as they are declared. For e.g.

SSS s1;

The above will immediately call the constructor of SSS. However, this is a case where objects are declared, but not constructed. And there is no easy way to construct it now.

First of all, this is bad design. If there is ever a necessity to explicitly declare a class object but construct it only later, then a pointer to a class should be used.
However, this is comes from another problem that I encountered.
Legacy code.
Ah yes. the bane of any code writers existence. While code can almost be thought of a s poetry, this, ladies and gentlemen, is nothing short of blasphemy.

In any case, the solution is not very difficult - placement new operator.
Take a look at the modified code:



#include <stdio.h> 

#include <stdlib.h> 
using namespace std;

class SSS{
    public:
       SSS(){
        printf("Calling constructor\n");
       }
       void* operator new(size_t size, SSS* s2){ //<-- need to overload

        printf("Calling operator new\n");
        return (malloc(sizeof(SSS)*1)); 
       }
       private:
        int n;
};

typedef struct test{
SSS s1;
}test;

int main(){
    test *t1;
    t1=(test*)malloc( sizeof(test)*1);
    new(&t1->s1)SSS;  //<---- how cool is that

    return 0;
}

The key is overloading the new operator and calling the placement new operator in a weird way:

new(&t1->s1)SSS

Again... bad design.

This entry was posted on 10/22/2005 at 10/22/2005 05:48:00 PM.

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