CODEWITHSUNDEEP
c++pointerscasting
St.Antario
St.Antario

Reputation: 27385

Why can I cast pointers of unrelated classes without error?

I've been experimenting with pointers and written the following code:

#include <iostream>

using std::cout;

struct A
{
    int a;
    char b;
    A(){ a = 4; }
};

struct B
{
    int c;
    B(){ c = 5; }
};

A *a = new A;
B *b = new B;

int main()
{ 
    a = (A*)b; 
    cout << a -> a; //5
}

Why B* can be converted to A*? Can any pointer be converted to any other pointer?

Upvotes: 1

Views: 136

Answers (1)

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

"Can any pointer be converted to any other pointer?"

If you use a c-style cast like this, yes.

a = (A*)b;

The structure pointed to by b will be just (re-)interpreted, just as it would be an A. The correct c++ equivalent is 

a = reinterpret_cast<A*>(b);

What you'll experience from such casts by means of consistency is unlikely to fit what's expected.
In other words: You'll experience all kinds of undefined behavior, accessing any members of a after doing such cast.

You should use a static_cast<> to let the compiler check, if those types are related, and can be casted somehow reasonably

a = static_cast<A*>(b);

Check these online samples to see the differences of static_cast<> and reinterpret_cast<>.

Upvotes: 1

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