C++ for Scientists - Technische Universität Dresden

192 CHAPTER 6. INHERITANCE
dbp= static cast(bbp); // erroneous downcast performed
ddp= static cast(bdp); // correct downcast but not checked by the system
std::cout ≪ ”Erroneous downcast of bbp will not return 0, it is: ” ≪ dbp ≪ ’\n’;
std::cout ≪ ”Correct downcast of bdp but not checked at run−time, it is: ” ≪ ddp ≪ ’\n’;
Whether the referred object really allows for the downcast cannot be decided at compile time
and lies in the responsibility of the programmer.
Cross-casting
An interesting feature of dynamic cast is casting across from B to C when the referred object’s
type is a derived class of both types:
C∗ cdp= dynamic cast(bdp); // cross−cast from B to C ok: bdp points to an object of type D
std::cout ≪ ”Dynamic cross−cast of bdp should succeed and pointer should not be 0, it is: ” ≪ cdp ≪
’\n’;
Static cross-casting from B to C:
cdp= static cast(bdp); // error: cross−cast from B to C does not compile
is not possible because C is neither a base or derived class of B. It can be casted indirectly over
D:
cdp= static cast(static cast(bdp)); // error: cross−cast from B to C via D
This again is in the responsibility of the programmer whether the addressed object can be really
casted this way.
Comparing Static and Dynamic Cast
Dynamic casting is safer but slower then static casting due the run-time check of the referred
object’s type. Static casting allows for casting up and down with the programmer’s responsibility
that the referred objects are handled correctly. Dynamic casting is in some sense always
up, namely from the referred object’s type to a super-type (including itself).
Furthermore, dynamic casting can only be applied on ‘Polymorphic Types’ that are class that
define or inherit a virtual function. The following summarizes the differences between the to
forms of casting:
static cast dynamic cast
Applicability all only polymorphic classes
Cross-casting no yes
Run-time check no yes
Speed no run-time overhead overhead for checking
Table 6.1: Static vs. dynamic cast