casting

问题 I have spend whole day on this problem. My problem is how to make an MethodHandle.invokeExact invocation on an instance, whose class type is dynamically loaded at program runtime. To make problem more clear, i show my sample code below: Class<?> expClass = new MyClassLoader().load(....) //expClass is AddSample.class which is subclass of BaseTemplate BaseTemplate obj = expClass.getConstructor(...) .newInstance(...); MethodHandle myMH = MethodHandles.lookup().findVirtual(expClass, methodName,..

问题 In C++, the compiling the following code: std::pair <int, int> x; static_cast <std::pair <const int, int>*> (&x); gives an error: error: invalid static_cast from type ‘std::pair<int, int>*’ to type ‘std::pair<const int, int>*’ I more or less understand why it happens, as cv-qualifying a type in a template parameter list can, in principle, give an "incompatible" result. And even if in this case it doesn't, compiler has no way to know it. Anyway, is there a non-hackish way to perform this

问题 I want to convert an MKMapPoint to a NSValue. In Objective-C i can do it with the following statement: MKMapPoint point = MKMapPointForCoordinate(location.coordinate); NSValue *pointValue = [NSValue value:&point withObjCType:@encode(MKMapPoint)]; How can i do that in Swift? Thanks! 回答1: It isn't possible in Swift, but you can still create a category in ObjC and use it in your Swift project // NSValue+MKMapPoint.h @interface NSValue (MKMapPoint) + (NSValue *)valueWithMKMapPoint:(MKMapPoint

问题 Simple question but I have spent over an hour with this. My code is below. I need to make SomeClass sc dynamic. So you pass the class name as a string in a function and use that class in place of static someClass. How to go about it? SomeClass sc; if (someOtherClassObject instanceof SomeClass){ sc=(SomeClass) someOtherClassObject; What I want is public void castDynamic (String strClassName){ //cast the classname referred by strClassName to SomeClass //if it is the instance of SomeClass } EDIT

问题 I got a snippet of code from this article and I'm confused as to how it works? The snippet starts by saying: You can detect that a particular read or write operation failed by testing the result of the read. For example, to check that a valid integer is read from the user, you can do this: int x; if ( cin >> x ) { cout << "Please enter a valid number" << endl; } This works because the read operation returns a reference to the stream. I understand that the cin >> x operation returns a

问题 I got a snippet of code from this article and I'm confused as to how it works? The snippet starts by saying: You can detect that a particular read or write operation failed by testing the result of the read. For example, to check that a valid integer is read from the user, you can do this: int x; if ( cin >> x ) { cout << "Please enter a valid number" << endl; } This works because the read operation returns a reference to the stream. I understand that the cin >> x operation returns a

问题 Is there a VB.NET equivalent to this? Note in particular the bmp in the code sample. public void MyMethod(Object obj) { if (obj is Bitmap bmp) { // ... } } Or the short pattern matching syntax with is is exclusive to C#? EDIT: I already know these syntaxes: If TypeOf obj Is Bitmap Then Dim bmp As Bitmap = obj ' ... End If or Dim bmp As Bitmap = TryCast(obj, Bitmap) If bmp IsNot Nothing Then ' ... End If What I want to know is whether there is something even shorter, like that new C#7 syntax..

问题 I am inspecting code that does not require explicitly casting result of malloc call but whenever I attempt to do this, the compiler throws an error. i.e. char *somevar; somevar = malloc(sizeof(char) * n); //error somevar = (char *)malloc(sizeof(char) * n); // ok 回答1: This happens if you use C++ compiler instead of C compiler. As C++ requires explicit casting. The problem is not just with (un)casting malloc result, but any void pointer to other pointer. 回答2: Did you remember to include the

问题 I've a legacy postgres db that has date columns cast as character(50) fields (don't ask). I'd like to alter the table and columns to contain actual dates. Because this worked: select distinct to_date(date_begin, 'YYYY DD MM') from dates; I naively thought this might work: alter table dates alter column date_begin type character using to_date(date_begin, 'YYYY DD MM'); But it does not. Any clues for the clueless? 回答1: This just works as intended by the OP. What we have here is a simple thinko

问题 I need some in-memory filtering with Dynamic Linq. My objects have only a indexer: public object this[int index] { } The access to my data is like: object[0], object[1],... So my query is like this: // get FilterText from user at runtime // eg. filterText can be: [0] > 100 and [1] = "wpf" collection.AsQueryable().where(filterText); Is there any way to do this? 回答1: I have some news for you. It is actually possible... BUT! (Yes, there´s a but). I assume you are using the dynamic Linq library.