static-variables

问题 I just hit very strange (to me) behaviour of java. I have following classes: public abstract class Unit { public static final Unit KM = KMUnit.INSTANCE; public static final Unit METERS = MeterUnit.INSTANCE; protected Unit() { } public abstract double getValueInUnit(double value, Unit unit); protected abstract double getValueInMeters(double value); } And: public class KMUnit extends Unit { public static final Unit INSTANCE = new KMUnit(); private KMUnit() { } //here are abstract methods

问题 I have some static variables in a class which extending the Application class .Now, somewhere in app ,if crash takes place then these are becoming null why it is so. can i say clearly like, I am declaring those two variables in Application sub class and initializing in other java class when crash occurs these becoming null.my app requirement is after login only those two should be initialized. For any answer/advice Thanks in advance 回答1: Static variables are associated with a class and they

问题 Lets say I have function like this: void processElement() { doSomething(someArray[lastProcessedElement + 1]); } The thing is, every time this function called, I need to the store the last element that I called doSomething on. So in here I have two choices: I can create a private class variable named lastProcessedElement and increment it's value every time that function is called. This approach seems most common. So the code can be something like this: class Foo { int lastProcessedElement = 0;

问题 Lets say I have function like this: void processElement() { doSomething(someArray[lastProcessedElement + 1]); } The thing is, every time this function called, I need to the store the last element that I called doSomething on. So in here I have two choices: I can create a private class variable named lastProcessedElement and increment it's value every time that function is called. This approach seems most common. So the code can be something like this: class Foo { int lastProcessedElement = 0;

问题 I understand that a constexpr variable can be used at compiletime. For a template, or static asser for instance. But if I want to do that without constexpr I can with static const . What is since C++11/14 introduced constexpr the difference between constexpr int a = 3; //AND static const int a = 3; Thank you! Another way to see this question is which should I use? 回答1: The main difference that I know is, the value of constexpr must be known in compile-time while a const static can be assigned

问题 Let's say I have this program: class Foo { public: unsigned int bar () { static unsigned int counter = 0; return counter++; } }; int main () { Foo a; Foo b; } (Of course this example makes no sense since I'd obviously declare "counter" as a private attribute, but it's just to illustrate the problem). I'd like to know how C++ behaves in this kind of situation: will the variable "counter" in the bar() method be the same for every instance? 回答1: Yes, counter will be shared across all instances

问题 I know .OBJ is the result of compiling a unit of compilation and .LIB is a static library that can be created from several .OBJ, but this difference seems to be only in the number of units of compilation. Is there any other difference? Is it the same or different file format? I have come to this question when wondering if the same static variable defined in two (or more) .LIBs is merged or not during linking into the final executable. For .OBJs the variables are merged. But is it the same in

问题 I don't quite understand static variables when defined in the implementation of an interface. In methods I do understand how they differ from local variables, but not when defined directly in an implementation. Look at these examples. What difference do these two make practically? #include "MyClass.h" @implementation MyClass int myInt; ... @end And: #include "MyClass.h" @implementation MyClass static int myInt; ... @end myInt is in both cases visible to all the methods, and if I interpreted a

问题 I am currently porting some code from Visual Studio to Mingw GCC. It seems there is a problem when I attempt to use a static variable . This code works fine in Visual studio so I am not sure what the problem might be here. I have pasted an outline of what the code looks like. If this is insufficient I could put in more. Header file: .h namespace LG_Wrapper { template <LG_Thread Thread> class EffectApplication : public ktApplication { public: static EffectApplication<Thread>& GetInstance();

问题 I am building a POP3 mailbox in PHP. I have the following files : server_access.php (fetch mails from the POP3 server) data_access.php (which fetches/writes mails to local DB) mime_parser.php (parses MIME content) core.php (uses above files and stores parsed mail as an assoc array called $inbox) Now, I have the pages mailbox.php to show the inbox and showmail.php to display each mail. The user's credentials are stored in a .ini file and used as necessary. The thing is, I do a require_once(