language-features

问题 I have heard/read the term but don\'t quite understand what it means. When should I use this technique and how would I use it? Can anyone provide a good code sample? 回答1: The visitor pattern is a way of doing double-dispatch in an object-oriented way. It's useful for when you want to choose which method to use for a given argument based on its type at runtime rather than compile time. Double dispatch is a special case of multiple dispatch . When you call a virtual method on an object, that's

问题 As made clear in update 3 on this answer, this notation: var hash = {}; hash[X] does not actually hash the object X ; it actually just converts X to a string (via .toString() if it\'s an object, or some other built-in conversions for various primitive types) and then looks that string up, without hashing it, in \" hash \". Object equality is also not checked - if two different objects have the same string conversion, they will just overwrite each other. Given this - are there any efficient

问题 In JavaScript, there are two values which basically say \'I don\'t exist\' - undefined and null . A property to which a programmer has not assigned anything will be undefined , but in order for a property to become null , null must be explicitly assigned to it. I once thought that there was a need for null because undefined is a primitive value and null an object. It\'s not, even if typeof null will yield \'object\' : Actually, both are primitive values - which means neither undefined nor

问题 I would like to properly understand hashes in Perl. I\'ve had to use Perl intermittently for quite some time and mostly whenever I need to do it, it\'s mostly related to text processing. And everytime, I have to deal with hashes, it gets messed up. I find the syntax very cryptic for hashes A good explanation of hashes and hash references, their differences, when they are required etc. would be much appreciated. 回答1: A simple hash is close to an array. Their initializations even look similar.

问题 Java has generics and C++ provides a very strong programming model with template s. So then, what is the difference between C++ and Java generics? 回答1: There is a big difference between them. In C++ you don't have to specify a class or an interface for the generic type. That's why you can create truly generic functions and classes, with the caveat of a looser typing. template <typename T> T sum(T a, T b) { return a + b; } The method above adds two objects of the same type, and can be used for

问题 I\'m always surprised that even after using C# for all this time now, I still manage to find things I didn\'t know about... I\'ve tried searching the internet for this, but using the \"~\" in a search isn\'t working for me so well and I didn\'t find anything on MSDN either (not to say it isn\'t there) I saw this snippet of code recently, what does the tilde(~) mean? /// <summary> /// Enumerates the ways a customer may purchase goods. /// </summary> [Flags] public enum PurchaseMethod { All =

问题 I know, I know... Eric Lippert\'s answer to this kind of question is usually something like \" because it wasn\'t worth the cost of designing, implementing, testing and documenting it \". But still, I\'d like a better explanation... I was reading this blog post about new C# 4 features, and in the section about COM Interop, the following part caught my attention : By the way, this code uses one more new feature: indexed properties (take a closer look at those square brackets after Range.) But

问题 HTML being the most widely used language (at least as a markup language) has not gotten its due credit. Considering that it has been around for so many years, things like the FORM / INPUT controls have still remained same with no new controls added. So at least from the existing features, do you know any features that are not well known but very useful. Of course, this question is along the lines of: Hidden Features of JavaScript Hidden Features of CSS Hidden Features of C# Hidden Features of

问题 Brief background: Many (most?) contemporary programming languages in widespread use have at least a handful of ADTs [abstract data types] in common, in particular, string (a sequence comprised of characters) list (an ordered collection of values), and map-based type (an unordered array that maps keys to values) In the R programming language, the first two are implemented as character and vector , respectively. When I began learning R, two things were obvious almost from the start: list is the

问题 I \'m trying to do : award_dict = { \"url\" : \"http://facebook.com\", \"imageurl\" : \"http://farm4.static.flickr.com/3431/3939267074_feb9eb19b1_o.png\", \"count\" : 1, } def award(name, count, points, desc_string, my_size, parent) : if my_size > count : a = { \"name\" : name, \"description\" : desc_string % count, \"points\" : points, \"parent_award\" : parent, } a.update(award_dict) return self.add_award(a, siteAlias, alias).award But if felt really cumbersome in the function, and I would