CODEWITHSUNDEEP
c++genericspointersvector
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Reputation: 283

C++ comparing pointers to different types?

I'm having a really hard time finding information about these kind of stuff! :(

I'm confused why this isn't working: 

vector<B*> b;
vector<C*> c;
(B and C are subclasses of A) 
(both are also initialized and contain elements etc etc...) 

template <class First, class Second>
bool func(vector<First*>* vector1, vector<Second*>* vector2)
   return vector1 == vector2;

When compiling this returns:

Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast

I don't see why this wouldn't work, pointers hold addresses yeah? So why don't it just compare if the two vector pointers... point to the same address(-es)?

Upvotes: 6

Views: 6574

Answers (2)

dunc123
dunc123

Reputation: 2713

Your two vectors are different types and you cannot compare them.

If you want to check that you do not call func(b, b) then you can try:

template <typename T> bool func(vector<T> const & a, vector<T> const & b)
{
if (&a == &b) return false;
// do stuff
return true;
}

Unless you are doing something very strange then the pointers to two vectors of a different types will not be equal. If you try and call func with two vectors of a different type then you will get a compiler error.

Upvotes: 4

Useless
Useless

Reputation: 67723

Here's a simple example where what you're asking for won't work.

struct A{ int i; };
struct OhNoes { double d; };
struct B: public A {};
struct C: public OhNoes, public B {};

So here, B and C are both subclasses of A. However, an instance of C is unlikely to have the same address as its B subobject.

That is, this:

C c;
B *b = &c; // valid upcast
assert(static_cast<void*>(b) == static_cast<void *>(&c));

will fail.

Upvotes: 8

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