Consider the following piece of code:

#include <stdio.h>

#include <stdlib.h> using namespace std;

class SSS{ public: SSS(){ printf(“Calling constructorn”); } };

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 constructorn”); } void* operator new(size_t size, SSS* s2){ //<— need to overload

printf(“Calling operator newn”); 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.

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