Reputation: 757
Consider this simple example:
struct Base1 {};
struct Base2 {};
struct Derived : public Base1, public Base2 {};
int main()
{
Derived foo;
Base1* foo1 = &foo;
Base2* foo2 = static_cast<Base2*>(foo1);
}
I get:
Error: static_cast from 'Base1 *' to 'Base2 *', which are not related by inheritance, is not allowed
The compiler should have enough information to figure out that Base2
can be reached from Derived
without RTTI (dynamic_cast
) and having me to do:
Derived* foo3 = static_cast<Derived*>(foo1);
Base2* foo2 = foo3;
Why isn't this allowed? (One could argue that the compiler doesn't know if foo1
is of Derived
type, but static_cast
doesn't check the type anyway even when converting from Base1
to Derived
for example)
Note: This question is similiar to mine, but it's not quite the same because here we are cross-casting base classes, not derived ones
Upvotes: 0
Views: 161
Reputation: 234875
A static_cast
will fail since, informally speaking, Base1
and Base2
are not related.
However a dynamic_cast
will work if your classes are polymorphic: which you can achieve by adding virtual destructors:
struct Base1 {virtual ~Base1() = default;};
struct Base2 {virtual ~Base2() = default;};
struct Derived : Base1, Base2 {};
int main()
{
Derived foo;
Base1* foo1 = &foo;
Base2* foo2 = dynamic_cast<Base2*>(foo1);
}
This casting from Base1
to Base2
is idiomatic when working with composition (i.e. interfaces).
Upvotes: 3