Question

#include<stdio.h>

class A { public: int a;};

class B: public A {
    int c; 
    int d;
};

int main() {

    A* pA = new B[10];
    B* pB = new B[10];

    printf("\n%d", pA->a);
    pA++;
    printf("\n%d", pA->a);  // prints junk value

    printf("\n\n%d", pB->a);
    pB++;
    printf("\n%d", pB->a);
    return 0;
}

The second printf prints a junk value.

It should figure that it is pointing to an object of type B and increment by the sizof(B).

Why does that not happen?

Answer 1

It can only know that at runtime. Imagine it slightly changed

A* a;
if(runtimevalue)
  a = new A[10];
else
  a = new B[10];

But that’s not going to happen. C++ puts emphasize in speed, but this would basically make it into a language that ensures safety of operations. There is Java, C# and others that already solve this.

Kernel and device driver developers don’t want a clever language runtime. They just want to have things run fast.

Have a look at Common undefined behavior in C++ question for all the things that will need to get “fixed” along. It won’t be C++ anymore!