Object initialization

Question

I works on embedded software. Previously we don't use too many C++ features so we use memset(this,0,sizeof(child)) to initialize(zero out) a object. However it doesn't work now since we are using virtual functions. Apparently it would destroy the vtable/virtual pointer.

So my question is: How can I initialize an object quickly and conveniently?

The class child inherits from class parent, which defines a lot virtual functions, and got many data member. If I need only to zero out all data member, any way to avoid member-by-memeber assignment in child's constructor without using memset()? or any trick to use memset without destroying vtable? (compiler-independent way)

Thank you very much.

Answer 1

You're asking to utilize the facilities of C++ but don't want the performance-hit of per-member initialization. Firstly, I'd ask myself if this is really the hit you're talking about. There are plenty of more bottlenecks you can be looking for than setting a member to 0.

But, if you want the features of C++ and still want the speed of memset() then I suggest you put the data for this class in a different class and initialize that to 0 and pass it to the class that is going to use it by reference.

Answer 2

Using placement new is definitely an option to avoid member wise zeroing out memory. Use delete[] to delete memory.

struct base{virtual ~base(){}};
struct derived : base{};

int main() 
{
   char *p = new char[sizeof(derived)];
   memset(p, 0, sizeof(derived));
   derived *pd = new (p) derived;
}