C++: Comparing pointers of base and derived classes
I'd like some information about best practices when comparing pointers in cases such as this one:
class 开发者_StackOverflowBase {
};
class Derived
: public Base {
};
Derived* d = new Derived;
Base* b = dynamic_cast<Base*>(d);
// When comparing the two pointers should I cast them
// to the same type or does it not even matter?
bool theSame = b == d;
// Or, bool theSame = dynamic_cast<Derived*>(b) == d?
If you want to compare arbitrary class hierarchies, the safe bet is to make them polymorphic and use dynamic_cast
class Base {
virtual ~Base() { }
};
class Derived
: public Base {
};
Derived* d = new Derived;
Base* b = dynamic_cast<Base*>(d);
// When comparing the two pointers should I cast them
// to the same type or does it not even matter?
bool theSame = dynamic_cast<void*>(b) == dynamic_cast<void*>(d);
Consider that sometimes you cannot use static_cast or implicit conversion from a derived to a base class:
struct A { };
struct B : A { };
struct C : A { };
struct D : B, C { };
A * a = ...;
D * d = ...;
/* static casting A to D would fail, because there are multiple A's for one D */
/* dynamic_cast<void*>(a) magically converts your a to the D pointer, no matter
* what of the two A it points to.
*/
If A
is inherited virtually, you can't static_cast to a D
either.
You do not require any cast in the above case, a simple Base* b = d;
will work. Then you can compare the pointers like you are comparing now.
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