Member Avatar for ahtaniv 
class A
{
        public:
                int a;
                A() : a(100) { }
               
};

class B : public A
{
        public:
                int b;
                B() : b(200){ }
};

int main()
{
        A a;
        B *ptrB =(B*)&a;

        cout<<ptrB->b<<endl; // Is there a way to get this to print 200??
}
Edited by Nick Evan because: Added [CODE] tags
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  • Latest Post Latest Post by ahtaniv

Dani AI

Generated

Short answer: no — forcing the address of an A object into a B* and then reading b is undefined behavior. As pointed out, the pointer in your original post refers to an object whose dynamic type is A, not B, so there is no b member storage to read; reading it can return garbage or crash. was right to remind about formatting, but the root problem is a misuse of downcasting.

Why this is wrong and what to do instead:

  • The C++ object model requires that a pointer-to-derived be used only when the object really is that derived type. Casting a base object’s address to a derived pointer does not create derived data.
  • To do a safe downcast at runtime, make the base polymorphic (one virtual function or a virtual destructor) and use dynamic_cast. That checks the actual type and returns nullptr if the object is not a Derived.
  • If you know an object is actually the derived type (because you created it as such), static_cast is faster but has no runtime check. Avoid C-style casts or reinterpret_cast for this — they don’t make an invalid cast safe.

Example (safe pattern using RTTI):

#include <iostream>

struct Base { virtual ~Base() = default; int a = 100; };
struct Derived : Base { int b = 200; };

int main() {
    Derived d;
    Base* pb = &d;                        // actually points to a Derived
    if (Derived* pd = dynamic_cast<Derived*>(pb))
        std::cout << pd->b << '\n';      // prints 200
}

Practical tips:

  • If you only ever need to call behavior, prefer a virtual method on Base and override it in Derived — no casts needed.
  • Watch out for object slicing (assigning a Derived to a Base by value loses Derived members).
  • If you see strange values after a cast, assume undefined behavior and rework to use real Derived objects, polymorphism, or a variant/visitor pattern.

Recommended Answers

All 3 Replies

Member Avatar for jk09

the pointer is pointing to an object of class A which has no relation with class B...
using base class object cannot print value of b.

Member Avatar for Mr. K

First use code tags. Your code is not formatted.Formatting makes your code easier to read.

class A
{
public:
     int a;
     A() : a(100) { }
};

class B : public A
{
public:
    int b;
     B() : b(200){ }
};

int main()
{
    A a; (B*)&a;

    cout << ptrB -> b << endl; // Is there a way to get this to print 200??
}
Edited by Mr. K because: n/a
Member Avatar for ahtaniv

OK. Thanks for the info. Will have it in mind for future posts.

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