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The difference between "new" and "malloc" is that, if you use new, you are not only allocate the memory for the object, but also call the constructor for the object.

For example

Class Simple
{
public:
Simple()
{//constructor code.
}
}

If you "new" the class, the constructor code will be called, if you use malloc, only memory for the class will be allocated, no constructor code will be called.

If you directly cast the memory location, it will give you un-expected results.. during the construction of object, thing like virtual table pointers are set.
following should work..
if A is class
CreateInstance()
{
obj = (A*)malloc(size);
obj->A::A();
return obj;
}

One key thing is: you need to know the size of memory in order to use malloc. Also, it may fail.
You also need to call free() to free the memory instead of using delete.

i guess there is one more issue with using malloc...it doesnot take care of alignment issues, I mean that if you do malloc(40) for 10 integers, it doesnot guarantee 4-byte alignment..PLZ COMMENT if I am wrong

true, malloc will allocate the space you have requested. the question mentions c++. please note that malloc returns a void pointer. in c++ you cannot type cast a void pointer unlike c. hence we use new operator in c++ because new returns the pointer corresponding to the class being instantiated.