Is circumventing a class' constructor legal or does it result in undefined behaviour?

Question

Consider following sample code:

class C
{
public:
    int* x;
};

void f()
{
    C* c = static_cast(malloc(sizeof(C)));
    c->x = nullptr; // &         


        

Answer 1

This particular code is fine, because C is a POD. As long as C is a POD, it can be initialized that way as well.

Your code is equivalent to this:

struct C
{
   int *x;
};

C* c = (C*)malloc(sizeof(C)); 
c->x = NULL;

Does it not look like familiar? It is all good. There is no problem with this code.