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  • Can I use an abstract class instead of a private __construct() when creating a singleton in PHP?

    - by Pheter
    When creating a Singleton in PHP, I ensure that it cannot be instantiated by doing the following: class Singleton { private function __construct() {} private function __clone() {} public static function getInstance() {} } However, I realised that defining a class as 'abstract' means that it cannot be instantiated. So is there anything wrong with doing the following instead: abstract class Singleton { public static function getInstance() {} } The second scenario allows me to write fewer lines of code which would be nice. (Not that it actually makes much of a difference.)

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  • Behavior of virtual function in C++

    - by Summer_More_More_Tea
    Hi everyone: I have a question, here are two class below: class Base{ public: virtual void toString(); // generic implementation } class Derive : public Base{ public: ( virtual ) void toString(); // specific implementation } The question is: If I wanna subclass of class Derive perform polymophism using a pointer of type Base, is keyword virtual in the bracket necessary? If the answer is no, what's the difference between member function toString of class Derive with and without virtual?

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  • Objective C: Compare timeInMillis with current time

    - by Srivathsan Canchi
    Hello, In my iPhone application, I need to calculate the time difference between the time a message was created on the server, and the time my phone received it. The server (Java) puts in a number returned by System.currentTimeMillis() as metadata along with the message. How do I compare this number with the current time on the device? Could not find a suitable NSDate method to do this comparison. Thanks in advance!

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  • Will First() perform the OrderBy()?

    - by Martin
    Is there any difference in (asymptotic) performance between Orders.OrderBy(order => order.Date).First() and Orders.Where(order => order.Date == Orders.Max(x => x.Date)); i.e. will First() perform the OrderBy()? I'm guessing no. MSDN says enumerating the collection via foreach och GetEnumerator does but the phrasing does not exclude other extensions.

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  • Using Variables for Class Names in Python?

    - by Sam McAfee
    I want to know how to use variables for objects and function names in Python. In PHP, you can do this: $className = "MyClass"; $newObject = new $className(); How do you do this sort of thing in Python? Or, am I totally not appreciating some fundamental difference with Python, and if so, what is it?

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  • How to implement pure virtual function in C++

    - by skydoor
    Hi, I think the implementation of virtual function is talked about a lot. My question is what about pure virtual function? However it is implemented? In virtual table, how to tell it is a pure or non-pure? What the difference between pure virtual function and virtual function with implementation?

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  • Why can one null return be tested but another throws an exception?

    - by mickey
    I want to test if an xml attribute is present. Given this: XmlAttributeCollection PG_attrColl = SomeNodeorAnother.Attributes; This first test works: if (null != PG_attrColl["SomeAttribute"]) "GetNamedItem" is supposed to return null, but the following test throws an exception complaining about the null it returns. if (null != PG_attrColl.GetNamedItem("SomeAttribute").Value;) Why the difference? Just curious.

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  • Emacs bulk indent for Python

    - by Vernon
    Working with Python in Emacs if I want to add a try/catch to a block of code, I often find that I am having to indent the whole block, line by line. In Emacs, how do you indent the whole block at once. I am not an experienced Emacs user, but just find it is the best tool for working through ssh. I am using Emacs on the command line(Ubuntu), not as a gui, if that makes any difference.

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  • Qt releasing memory occopied by unneeded images

    - by Gerhman
    I have a QML Flickable with 3 QML image elements that cycle through 8 image files. My problem is for each new image it loads into memory it does not release the the unused image. This is causing memory usage to become double what is necessary at times. I would like to know if there is some function I can use to force it to unload all the unused images? If it makes a difference, this is mainly for Symbian.

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  • List comprehension, map, and numpy.vectorize performance

    - by mcstrother
    I have a function foo(i) that takes an integer and takes a significant amount of time to execute. Will there be a significant performance difference between any of the following ways of initializing a: a = [foo(i) for i in xrange(100)] a = map(foo, range(100)) vfoo = numpy.vectorize(foo) a = vfoo(range(100)) (I don't care whether the output is a list or a numpy array.) Is there a better way?

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  • speed of map() vs. list comprehension vs. numpy vectorized function in python

    - by mcstrother
    I have a function foo(i) that takes an integer and takes a significant amount of time to execute. Will there be a significant performance difference between any of the following ways of initializing 'a': a = [foo(i) for i in xrange(100)] , a = map(foo, range(100)) , and vfoo = numpy.vectorize(foo) vfoo(range(100)) ? (I don't care whether the output is a list or a numpy array). Is there some other better way of doing this? Thanks.

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  • __proto__ of a function

    - by alter
    if I have a class called Person. var Person = function(fname, lname){ this.fname = fname; this.lname = lname; } Person.prototype.mname = "Test"; var p = new Person('Alice','Bob'); Now, p.proto refers to prototype of Person but, when I try to do Person.proto , it points to function(), and Person.constructor points to Function(). can some1 explain what is the difference between function() and Function() and why prototype of a Function() class is a function()

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  • Update one list from another

    - by lacqui
    I have two multi-select lists in HTML - one of categories, and another of items. What I want is that, when an item selected in the "category" list, everything in the "items" list from that category is also selected. For example, if "Categories" contains "Animal", "Mineral", and "Vegetable", and "Items" contains "Dog", "Cat", "Monkey", "Diamond", "Granite", "Carrot", "Tomato": If "Animal" is selected in one list, "Dog", "Cat", and "Monkey" should be selected; any other selections should remain as they are. I'm using a Symfony PHP backend on the server, if that makes any difference

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