casting

问题 Consider the following code (also available at C++ Shell). Basically, we have an Employee base class with FullTime and PartTime subclasses. The organization class HAS a list of Employees (who could be Fulltime or PartTime). The problem arises when I want to define a getEmployee(int) method in the Organization class. What type of Employee should it return? If it returns a Employee*: first of all, is it safe to return a pointer? I assume that its address is permanent, because it will send the

问题 I'm trying to implement a linked list like this: typedef struct SLnode { void* item; void* next; } SLnode; typedef struct DLnode { void* item; void* next; struct DLnode* prev; } DLnode; typedef struct LinkedList { void* head; /*SLnode if doubly_linked is false, otherwise DLnode*/ void* tail; /* here too */ bool doubly_linked; } LinkedList; And I want to access it like this: void* llnode_at(const LinkedList* ll, size_t index) { size_t i; SLnode* current; current = ll->head; for(i = 0; i <

问题 I'm trying to implement a linked list like this: typedef struct SLnode { void* item; void* next; } SLnode; typedef struct DLnode { void* item; void* next; struct DLnode* prev; } DLnode; typedef struct LinkedList { void* head; /*SLnode if doubly_linked is false, otherwise DLnode*/ void* tail; /* here too */ bool doubly_linked; } LinkedList; And I want to access it like this: void* llnode_at(const LinkedList* ll, size_t index) { size_t i; SLnode* current; current = ll->head; for(i = 0; i <

问题 I'm trying to cast a generic type to its super class. class Foo : NSObject { } class Test<T> { let value: T init(_ value: T) { self.value = value } } let c = Test(Foo()) let v = c as Test<AnyObject> But on the line let v = c as Test<AnyObject> I get 'Foo' is not identical to 'AnyObject' While I can do that with built-in Arrays let array1 = [Foo(), Foo()] let array2 = array1 as [AnyObject] 回答1: Arrays are special-cased in Swift to be able to have this behaviour. You won't be able to replicate

问题 In the following sample code two classes EventA and EventB both implement the interface Historical . Java can automatically cast an EventA or EventB to Historical when one of these objects is passed as a parameter, as in the examineEvent method below. However, Java is no longer able to cast when a generic is introduced ie. from List<EventA> to List<Historical> -- Unless the target function (in this case findClosestValidEventIndex ) is declared using List<? extends Historical> . Can someone

问题 What is the difference between DYNAMIC_DOWNCAST from MFC library and standard C++ dynamic_cast operator? Can I use safety dynamic_cast instead of DYNAMIC_DOWNCAST for MFC objects? When my classes contain DECLARE_DYNAMIC and IMPLEMENT_DYNAMIC macros, can I use dynamic_cast operator or I must use DYNAMIC_DOWNCAST macro for this type of objects? 回答1: What is the difference between DYNAMIC_DOWNCAST from MFC library and standard C++ dynamic_cast operator? DYNAMIC_DOWNCAST and dynamic_cast achieve

问题 As I understand it, what makes dynamic cast different from a static cast is its use of RTTI, and the fact that it fails if the dynamic type of a variable- when casting from base to derived- does not fit. But why does the class have to be polymorphic for that to be done if we have the RTTI anyway? EDIT: Since there was some confusion about the use of the word "polymorphic", here's the entry in cplusplus.com that prompted me to ask this: dynamic_cast can be used only with pointers and

问题 I am getting this compile error: error: invalid conversion from 'const MyClass*' to 'MyClass*' Here is the code: std::tr1::shared_ptr<MyClass> myClassA; const MyClass* myClassB; myClassA = std::tr1::shared_ptr<MyClass>(myClassB); // error here I think I understand the error, just don't know how to fix. I need myClassB to be a const so how to convert/copy classB to a shared_ptr? 回答1: You'll need a shared pointer to a const object: std::tr1::shared_ptr<const MyClass> myClassA; ^^^^^ 回答2: You

问题 Why the following works float f = 10l // OK 32 bits to 64 ? float is only 32 bits and long 64 bits so there is NO room for float in the long and this one doesn't long l = 10f //compile error Could someone explain to me how kind of casting works? 回答1: The conversion from long to float is specifically allowed for assignment by the JLS, Section 5.1.2: 19 specific conversions on primitive types are called the widening primitive conversions: byte to short, int, long, float, or double short to int,

问题 Just wondering if it's safe to cast like this: char **cpp; // ... allocation and what not void *vp = (void *)cpp; // ... cpp = (char **)vp; should a void ** be used or is void * fine? This works on a couple of my boxes without issue but was wondering if it could cause problems on certain systems. 回答1: The cast is always safe, dereferencing it is safe as long as the pointer is valid. The only case where you'd use a void ** is when you plan to dereference it to get a void * . However, unless