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• Is it possible for an abstract class to force it's children to have a constructor in PHP?

  - by Logan Serman

  I would like to do something like this: abstract class Foo { public function __construct() { echo 'This is the parent constructor'; } abstract function __construct(); } class Bar extends Foo { // constructor is required as this class extends Foo public function __construct() { //call parent::__construct() if necessary echo 'This is the child constructor'; } } But I get a fatal error when doing this: Fatal error: Cannot redeclare Foo::__construct() in Foo.php on line 8 Is there another way to ensure child classes have a constructor?

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• Replaceable parameter syntax meaning

  - by Alexander N.

  Replaceable parameter syntax for the console object in C#. I am taking the O'Reilly C# Course 1 and it is asking for a replaceable parameter syntax and it is not very clear on what that means. Currently I used this: double trouble = 99999.0009; double bubble = 11111.0001; Console.WriteLine(trouble * bubble); Am I missing the meaning of replaceable parameter syntax? Can someone provide an example for what I am looking for? Original question for the quiz: "Create two variables, both doubles, assign them numbers greater than 10,000, and include a decimal component. Output the result of multiplying the numbers together, but use replaceable parameter syntax of the Console object, and multiply the numbers within the call to the Console.WriteLine() method."

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• Quick Outline: Navigating Your PL/SQL Packages in Oracle SQL Developer

  - by thatjeffsmith

  If you’re browsing your packages using the Connections panel, you have a nice tree navigator to click around your packages and your variable, procedure, and functions. Click, click, click all day long, click, click, click while I sing this song… But What if you drill into your PL/SQL source from the worksheet and don’t have the Tree expanded? Let’s say you’re working on your script, something like - Hmm, what goes next again? So I need to reacquaint myself with just what my beer package requires, so I’m going to drill into it by doing a DESCRIBE (via SHIFT+F4), and now I have the package open. The package is open but the tree hasn’t auto-expanded. Please don’t tell me I have to do the click-click-click thing in the tree!?! Just Open the Quick Outline Panel Do you see it? Just right click in the procedure editor – select the ‘Quick Outline’ in the context menu, and voila! The navigational power of the tree, without needing to drill down the tree itself. If I want to drill into my procedure declaration, just click on said procedure name in the Quick Outline panel. This works for both package specs and bodies. Technically you can use this for stand alone procedures and functions, but the real power is demonstrated for packages.

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• Oracle syntax - should we have to choose between the old and the new?

  - by Martin Milan

  Hi, I work on a code base in the region of about 1'000'000 lines of source, in a team of around eight developers. Our code is basically an application using an Oracle database, but the code has evolved over time (we have plenty of source code from the mid nineties in there!). A dispute has arisen amongst the team over the syntax that we are using for querying the Oracle database. At the moment, the overwhelming majority of our queries use the "old" Oracle Syntax for joins, meaning we have code that looks like this... Example of Inner Join select customers.*, orders.date, orders.value from customers, orders where customers.custid = orders.custid Example of Outer Join select customers.custid, contacts.ContactName, contacts.ContactTelNo from customers, contacts where customers.custid = contacts.custid(+) As new developers have joined the team, we have noticed that some of them seem to prefer using SQL-92 queries, like this: Example of Inner Join select customers.*, orders.date, orders.value from customers inner join orders on (customers.custid = orders.custid) Example of Outer Join select customers.custid, contacts.ContactName, contacts.ContactTelNo from customers left join contacts on (customers.custid = contacts.custid) Group A say that everyone should be using the the "old" syntax - we have lots of code in this format, and we ought to value consistency. We don't have time to go all the way through the code now rewriting database queries, and it wouldn't pay us if we had. They also point out that "this is the way we've always done it, and we're comfortable with it..." Group B however say that they agree that we don't have the time to go back and change existing queries, we really ought to be adopting the "new" syntax on code that we write from here on in. They say that developers only really look at a single query at a time, and that so long as developers know both syntax there is nothing to be gained from rigidly sticking to the old syntax, which might be deprecated at some point in the future. Without declaring with which group my loyalties lie, I am interested in hearing the opinions of impartial observers - so let the games commence! Martin. Ps. I've made this a community wiki so as not to be seen as just blatantly chasing after question points...

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• Asymptotic runtime of list-to-tree function

  - by Deestan

  I have a merge function which takes time O(log n) to combine two trees into one, and a listToTree function which converts an initial list of elements to singleton trees and repeatedly calls merge on each successive pair of trees until only one tree remains. Function signatures and relevant implementations are as follows: merge :: Tree a -> Tree a -> Tree a --// O(log n) where n is size of input trees singleton :: a -> Tree a --// O(1) empty :: Tree a --// O(1) listToTree :: [a] -> Tree a --// Supposedly O(n) listToTree = listToTreeR . (map singleton) listToTreeR :: [Tree a] -> Tree a listToTreeR [] = empty listToTreeR (x:[]) = x listToTreeR xs = listToTreeR (mergePairs xs) mergePairs :: [Tree a] -> [Tree a] mergePairs [] = [] mergePairs (x:[]) = [x] mergePairs (x:y:xs) = merge x y : mergePairs xs This is a slightly simplified version of exercise 3.3 in Purely Functional Data Structures by Chris Okasaki. According to the exercise, I shall now show that listToTree takes O(n) time. Which I can't. :-( There are trivially ceil(log n) recursive calls to listToTreeR, meaning ceil(log n) calls to mergePairs. The running time of mergePairs is dependent on the length of the list, and the sizes of the trees. The length of the list is 2^h-1, and the sizes of the trees are log(n/(2^h)), where h=log n is the first recursive step, and h=1 is the last recursive step. Each call to mergePairs thus takes time (2^h-1) * log(n/(2^h)) I'm having trouble taking this analysis any further. Can anyone give me a hint in the right direction?

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• Syntax errors on Heroku, but not on local server (postgresql related?)

  - by Phil_Ken_Sebben

  I'm trying to deploy my first app on Heroku (rails 3). It works fine on my local server, but when I pushed it to Heroku and ran it, it crashes, giving a number of syntax errors. These are related to a collection of scopes I use like the one below: scope :scored, lambda { |score = nil| score.nil? ? {} : where('products.votes_count >= ?', score) } it produces errors of this form: "syntax error, unexpected '=', expecting '|' " "syntax error, unexpected '}', expecting kEND" Why is this syntax making Heroku choke and how can I correct it? Thanks! EDIT: I was using sqlite on my local machine and Heroku does not support that. Trying to make sure the db is properly configured for PG. I believe I have done that by specifying in the gemfile that sqlite only be used in development. Yet I still get these syntax errors, that interrupt even the db:migrate. EDIT: So now it seems more likely that my scope syntax doesn't work in postgreSQL. Does anyone know how to convert this properly?

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• Binary Search Tree, cannot do traversal

  - by ihm

  Please see BST codes below. It only outputs "5". what did I do wrong? #include <iostream> class bst { public: bst(const int& numb) : root(new node(numb)) {} void insert(const int& numb) { root->insert(new node(numb), root); } void inorder() { root->inorder(root); } private: class node { public: node(const int& numb) : left(NULL), right(NULL) { value = numb; } void insert(node* insertion, node* position) { if (position == NULL) position = insertion; else if (insertion->value > position->value) insert(insertion, position->right); else if (insertion->value < position->value) insert(insertion, position->left); } void inorder(node* tree) { if (tree == NULL) return; inorder(tree->left); std::cout << tree->value << std::endl; inorder(tree->right); } private: node* left; node* right; int value; }; node* root; }; int main() { bst tree(5); tree.insert(4); tree.insert(2); tree.insert(10); tree.insert(14); tree.inorder(); return 0; }

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• How exactly is an Abstract Syntax Tree created?

  - by Howcan

  I think I understand the goal of an AST, and I've build a couple of tree structures before, but never an AST. I'm mostly confused because the nodes are text and not number, so I can't think of a nice way to input a token/string as I'm parsing some code. For example, when I looked at diagrams of AST's, the variable and its value were leaf nodes to an equal sign. This makes perfect sense to me, but how would I go about implementing this? I guess I can do it case by case, so that when I stumble upon an "=" I use that as a node, and add the value parsed before the "=" as the leaf. It just seems wrong, because I'd probably have to make cases for tons and tons of things, depending on the syntax. And then I came upon another problem, how is the tree traversed? Do I go all the way down the height, and go back up a node when I hit the bottom, and do the same for it's neighbor? I've seen tons of diagrams on ASTs, but I couldn't find a fairly simple example of one in code, which would probably help.

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• How to determine which inheriting class is using an abstract class' methods.

  - by Kin

  In my console application have an abstract Factory class "Listener" which contains code for listening and accepting connections, and spawning client classes. This class is inherited by two more classes (WorldListener, and MasterListener) that contain more protocol specific overrides and functions. I also have a helper class (ConsoleWrapper) which encapsulates and extends System.Console, containing methods for writing to console info on what is happening to instances of the WorldListener and MasterListener. I need a way to determine in the abstract ListenerClass which Inheriting class is calling its methods. Any help with this problem would be greatly appreciated! I am stumped :X Simplified example of what I am trying to do. abstract class Listener { public void DoSomething() { if(inheriting class == WorldListener) ConsoleWrapper.WorldWrite("Did something!"); if(inheriting class == MasterListener) ConsoleWrapper.MasterWrite("Did something!"); } } public static ConsoleWrapper { public void WorldWrite(string input) { System.Console.WriteLine("[World] {0}", input); } } public class WorldListener : Listener { public void DoSomethingSpecific() { ConsoleWrapper.WorldWrite("I did something specific!"); } } public void Main() { new WorldListener(); new MasterListener(); } Expected output [World] Did something! [World] I did something specific! [Master] Did something! [World] I did something specific!

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• How to determine which inheriting class is using an abstract class's methods.

  - by Kin

  In my console application have an abstract Factory class "Listener" which contains code for listening and accepting connections, and spawning client classes. This class is inherited by two more classes (WorldListener, and MasterListener) that contain more protocol specific overrides and functions. I also have a helper class (ConsoleWrapper) which encapsulates and extends System.Console, containing methods for writing to console info on what is happening to instances of the WorldListener and MasterListener. I need a way to determine in the abstract ListenerClass which Inheriting class is calling its methods. Any help with this problem would be greatly appreciated! I am stumped :X Simplified example of what I am trying to do. abstract class Listener { public void DoSomething() { if(inheriting class == WorldListener) ConsoleWrapper.WorldWrite("Did something!"); if(inheriting class == MasterListener) ConsoleWrapper.MasterWrite("Did something!"); } } public static ConsoleWrapper { public void WorldWrite(string input) { System.Console.WriteLine("[World] {0}", input); } } public class WorldListener : Listener { public void DoSomethingSpecific() { ConsoleWrapper.WorldWrite("I did something specific!"); } } public void Main() { new WorldListener(); new MasterListener(); } Expected output [World] Did something! [World] I did something specific! [Master] Did something! [World] I did something specific!

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• Is there a case for parameterising using Abstract classes rather than Interfaces?

  - by Chris

  I'm currently developing a component based API that is heavily stateful. The top level components implement around a dozen interfaces each. The stock top-level components therefore sit ontop of a stack of Abstract implementations which in turn contain multiple mixin implementations and implement multiple mixin interfaces. So far, so good (I hope). The problem is that the base functionality is extremely complex to implement (1,000s of lines in 5 layers of base classes) and therefore I do not wish for component writers to implement the interfaces themselves but rather to extend my base classes (where all the boiler plate code is already written). If the API therefore accepts interfaces rather than references to the Abstract implementation that I wish for component writers to extends, then I have a risk that the implementer will not perform the validation that is both required and assumed by other areas of code. Therefore, my question is, is it sometimes valid to paramerise API methods using an abstract implementation reference rather than a reference to the interface(s) that it implements? Do you have an example of a well-designed API that uses this technique or am I trying to talk myself into bad-practice?

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• How to cleanly add after-the-fact commits from the same feature into git tree

  - by Dennis

  I am one of two developers on a system. I make most of the commits at this time period. My current git workflow is as such: there is master branch only (no develop/release) I make a new branch when I want to do a feature, do lots of commits, and then when I'm done, I merge that branch back into master, and usually push it to remote. ...except, I am usually not done. I often come back to alter one thing or another and every time I think it is done, but it can be 3-4 commits before I am really done and move onto something else. Problem The problem I have now is that .. my feature branch tree is merged and pushed into master and remote master, and then I realize that I am not really done with that feature, as in I have finishing touches I want to add, where finishing touches may be cosmetic only, or may be significant, but they still belong to that one feature I just worked on. What I do now Currently, when I have extra after-the-fact commits like this, I solve this problem by rolling back my merge, and re-merging my feature branch into master with my new commits, and I do that so that git tree looks clean. One clean feature branch branched out of master and merged back into it. I then push --force my changes to origin, since my origin doesn't see much traffic at the moment, so I can almost count that things will be safe, or I can even talk to other dev if I have to coordinate. But I know it is not a good way to do this in general, as it rewrites what others may have already pulled, causing potential issues. And it did happen even with my dev, where git had to do an extra weird merge when our trees diverged. Other ways to solve this which I deem to be not so great Next best way is to just make those extra commits to the master branch directly, be it fast-forward merge, or not. It doesn't make the tree look as pretty as in my current way I'm solving this, but then it's not rewriting history. Yet another way is to wait. Maybe wait 24 hours and not push things to origin. That way I can rewrite things as I see fit. The con of this approach is time wasted waiting, when people may be waiting for a fix now. Yet another way is to make a "new" feature branch every time I realize I need to fix something extra. I may end up with things like feature-branch feature-branch-html-fix, feature-branch-checkbox-fix, and so on, kind of polluting the git tree somewhat. Is there a way to manage what I am trying to do without the drawbacks I described? I'm going for clean-looking history here, but maybe I need to drop this goal, if technically it is not a possibility.

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• How can I implement a splay tree that performs the zig operation last, not first?

  - by Jakob

  For my Algorithms & Data Structures class, I've been tasked with implementing a splay tree in Haskell. My algorithm for the splay operation is as follows: If the node to be splayed is the root, the unaltered tree is returned. If the node to be splayed is one level from the root, a zig operation is performed and the resulting tree is returned. If the node to be splayed is two or more levels from the root, a zig-zig or zig-zag operation is performed on the result of splaying the subtree starting at that node, and the resulting tree is returned. This is valid according to my teacher. However, the Wikipedia description of a splay tree says the zig step "will be done only as the last step in a splay operation" whereas in my algorithm it is the first step in a splay operation. I want to implement a splay tree that performs the zig operation last instead of first, but I'm not sure how it would best be done. It seems to me that such an algorithm would become more complex, seeing as how one needs to find the node to be splayed before it can be determined whether a zig operation should be performed or not. How can I implement this in Haskell (or some other functional language)?

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• How do I easily highlight the syntax of PHP code in Word?

  - by christian

  How do I easily write and syntax hightlight PHP (or any other language really) code in a Word 2009 document?

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• Syntactic sugar in PHP with static functions

  - by Anna

  The dilemma I'm facing is: should I use static classes for the components of an application just to get nicer looking API? Example - the "normal" way: // example component class Cache{ abstract function get($k); abstract function set($k, $v); } class APCCache extends Cache{ ... } class application{ function __construct() $this->cache = new APCCache(); } function whatever(){ $this->cache->add('blabla'); print $this->cache->get('blablabla'); } } Notice how ugly is this->cache->.... But it gets waay uglier when you try to make the application extensible trough plugins, because then you have to pass the application instance to its plugins, and you get $this->application->cache->... With static functions: interface CacheAdapter{ abstract function get($k); abstract function set($k, $v); } class Cache{ public static $ad; public function setAdapter(CacheAdapter $a){ static::$ad = $ad; } public static function get($k){ return static::$ad->get($k); } ... } class APCCache implements CacheAdapter{ ... } class application{ function __construct(){ cache::setAdapter(new APCCache); } function whatever() cache::add('blabla', 5); print cache::get('blabla'); } } Here it looks nicer because you just call cache::get() everywhere. The disadvantage is that I loose the possibility to extend this class easily. But I've added a setAdapter method to make the class extensible to some point. I'm relying on the fact that I won't need to rewrite to replace the cache wrapper, ever, and that I won't need to run multiple application instances simultaneously (it's basically a site - and nobody works with two sites at the same time) So, am doing it wrong?

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• Which of these design patterns is superior?

  - by durron597

  I find I tend to design class structures where several subclasses have nearly identical functionality, but one piece of it is different. So I write nearly all the code in the abstract class, and then create several subclasses to do the one different thing. Does this pattern have a name? Is this the best way for this sort of scenario? Option 1: public interface TaxCalc { String calcTaxes(); } public abstract class AbstractTaxCalc implements TaxCalc { // most constructors and fields are here public double calcTaxes(UserFinancials data) { // code double diffNumber = getNumber(data); // more code } abstract protected double getNumber(UserFinancials data); protected double initialTaxes(double grossIncome) { // code return initialNumber; } } public class SimpleTaxCalc extends AbstractCalc { protected double getNumber(UserFinancials data) { double temp = intialCalc(data.getGrossIncome()); // do other stuff return temp; } } public class FancyTaxCalc extends AbstractTaxCalc { protected double getNumber(UserFinancials data) { int temp = initialCalc(data.getGrossIncome()); // Do fancier math return temp; } } Option 2: This version is more like the Strategy pattern, and should be able to do essentially the same sorts of tasks. public class TaxCalcImpl implements TaxCalc { private final TaxMath worker; public DummyImpl(TaxMath worker) { this.worker = worker; } public double calcTaxes(UserFinancials data) { // code double analyzedDouble = initialNumber; int diffNumber = worker.getNumber(data, initialNumber); // more code } protected int initialTaxes(double grossIncome) { // code return initialNumber; } } public interface TaxMath { double getNumber(UserFinancials data, double initial); } Then I could do: TaxCalc dum = new TaxCalcImpl(new TaxMath() { @Override public double getNumber(UserFinancials data, double initial) { double temp = data.getGrossIncome(); // do math return temp; }); And I could make specific implementations of TaxMath for things I use a lot, or I could make a stateless singleton for certain kinds of workers I use a lot. So the question I'm asking is: Which of these patterns is superior, when, and why? Or, alternately, is there an even better third option?

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• Sentence Tree v/s Words List

  - by Rohit Jose

  I was recently tasked with building a Name Entity Recognizer as part of a project. The objective was to parse a given sentence and come up with all the possible combinations of the entities. One approach that was suggested was to keep a lookup table for all the know connector words like articles and conjunctions, remove them from the words list after splitting the sentence on the basis of the spaces. This would leave out the Name Entities in the sentence. A lookup is then done for these identified entities on another lookup table that associates them to the entity type, for example if the sentence was: Remember the Titans was a movie directed by Boaz Yakin, the possible outputs would be: {Remember the Titans,Movie} was {a movie,Movie} directed by {Boaz Yakin,director} {Remember the Titans,Movie} was a movie directed by Boaz Yakin {Remember the Titans,Movie} was {a movie,Movie} directed by Boaz Yakin {Remember the Titans,Movie} was a movie directed by {Boaz Yakin,director} Remember the Titans was {a movie,Movie} directed by Boaz Yakin Remember the Titans was {a movie,Movie} directed by {Boaz Yakin,director} Remember the Titans was a movie directed by {Boaz Yakin,director} Remember the {the titans,Movie,Sports Team} was {a movie,Movie} directed by {Boaz Yakin,director} Remember the {the titans,Movie,Sports Team} was a movie directed by Boaz Yakin Remember the {the titans,Movie,Sports Team} was {a movie,Movie} directed by Boaz Yakin Remember the {the titans,Movie,Sports Team} was a movie directed by {Boaz Yakin,director} The entity lookup table here would contain the following data: Remember the Titans=Movie a movie=Movie Boaz Yakin=director the Titans=Movie the Titans=Sports Team Another alternative logic that was put forward was to build a crude sentence tree that would contain the connector words in the lookup table as parent nodes and do a lookup in the entity table for the leaf node that might contain the entities. The tree that was built for the sentence above would be: The question I am faced with is the benefits of the two approaches, should I be going for the tree approach to represent the sentence parsing, since it provides a more semantic structure? Is there a better approach I should be going for solving it?

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• vector rotations for branches of a 3d tree

  - by freefallr

  I'm attempting to create a 3d tree procedurally. I'm hoping that someone can check my vector rotation maths, as I'm a bit confused. I'm using an l-system (a recursive algorithm for generating branches). The trunk of the tree is the root node. It's orientation is aligned to the y axis. In the next iteration of the tree (e.g. the first branches), I might create a branch that is oriented say by +10 degrees in the X axis and a similar amount in the Z axis, relative to the trunk. I know that I should keep a rotation matrix at each branch, so that it can be applied to child branches, along with any modifications to the child branch. My questions then: for the trunk, the rotation matrix - is that just the identity matrix * initial orientation vector ? for the first branch (and subsequent branches) - I'll "inherit" the rotation matrix of the parent branch, and apply x and z rotations to that also. e.g. using glm::normalize; using glm::rotateX; using glm::vec4; using glm::mat4; using glm::rotate; vec4 vYAxis = vec4(0.0f, 1.0f, 0.0f, 0.0f); vec4 vInitial = normalize( rotateX( vYAxis, 10.0f ) ); mat4 mRotation = mat4(1.0); // trunk rotation matrix = identity * initial orientation vector mRotation *= vInitial; // first branch = parent rotation matrix * this branches rotations mRotation *= rotate( 10.0f, 1.0f, 0.0f, 0.0f ); // x rotation mRotation *= rotate( 10.0f, 0.0f, 0.0f, 1.0f ); // z rotation Are my maths and approach correct, or am I completely wrong? Finally, I'm using the glm library with OpenGL / C++ for this. Is the order of x rotation and z rotation important?

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• Cloning from a given point in the snapshot tree

  - by Fat Bloke

  Although we have just released VirtualBox 4.3, this quick blog entry is about a longer standing ability of VirtualBox when it comes to Snapshots and Cloning, and was prompted by a question posed internally, here in Oracle: "Is there a way I can create a new VM from a point in my snapshot tree?". Here's the scenario: Let's say you have your favourite work VM which is Oracle Linux based and as you installed different packages, such as database, middleware, and the apps, you took snapshots at each point like this: But you then need to create a new VM for some other testing or to share with a colleague who will be using the same Linux and Database layers but may want to reconfigure the Middleware tier, and may want to install his own Apps. All you have to do is right click on the snapshot that you're happy with and clone: Give the VM that you are about to create a name, and if you plan to use it on the same host machine as the original VM, it's a good idea to "Reinitialize the MAC address" so there's no clash on the same network: Now choose the Clone type. If you plan to use this new VM on the same host as the original, you can use Linked Cloning else choose Full.  At this point you now have a choice about what to do about your snapshot tree. In our example, we're happy with the Linux and Database layers, but we may want to allow our colleague to change the upper tiers, with the option of reverting back to our known-good state, so we'll retain the snapshot data in the new VM from this point on: The cloning process then chugs along and may take a while if you chose a Full Clone: Finally, the newly cloned VM is ready with the subset of the Snapshot tree that we wanted to retain: Pretty powerful, and very useful.  Cheers, -FB 

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• How can I make an "abstract" enum in a .NET class library?

  - by Lazlo

  I'm making a server library in which the packet association is done by enum. public enum ServerOperationCode : byte { LoginResponse = 0x00, SelectionResponse = 0x01, BlahBlahResponse = 0x02 } public enum ClientOperationCode : byte { LoginRequest = 0x00, SelectionRequest = 0x01, BlahBlahRequest = 0x02 } That works fine when you're working in your own project - you can compare which enum member is returned (i.e. if (packet.OperationCode == ClientOperationCode.LoginRequest)). However, since this is a class library, the user will have to define its own enum. Therefore, I have two enums to add as "abstract" - ServerOperationCode and ClientOperationCode. I know it's not possible to implement abstract enums in C#. How would I go doing this?

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• How do I ensure my abstract class's function can only operate on extenders of the same type as the c

  - by incrediman

  For example, let's say this is my abstract class: abstract class A{ int x; int y; void foo(A fooMe); } ...and B and C are two classes which extend A. What I want is for B to only be able to call foo() on other Bs, and for C to only be able to call foo() on other Cs. But I want this to be out of the hands of the programmer who's extending my A class - that is, I want a way to ensure this functionality within As code alone. What can I do? (If possible) I'd like to avoid any hack or generics solution that's too messy - I still want foo to be able to be called like this, for example: B b=new B(); B bb=new B(); bb.foo(b);

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• Using Lazy<T> and abstract wrapper class to lazy-load complex system parameters

  - by DigiMortal

  .NET Framework 4.0 introduced new class called Lazy<T> and I wrote blog post about it: .Net Framework 4.0: Using System.Lazy<T>. One thing is annoying for me – we have to keep lazy loaded value and its value loader as separate things. In this posting I will introduce you my Lazy<T> wrapper for complex to get system parameters that uses template method to keep lazy value loader in parameter class. Problem with original implementation Here’s the sample code that shows you how Lazy<T> is usually used. This is just sample code, don’t focus on the fact that this is dummy console application. class Program {     static void Main(string[] args)     {         var temperature = new Lazy<int>(LoadMinimalTemperature);           Console.WriteLine("Minimal room temperature: " + temperature.Value);         Console.ReadLine();     }       protected static int LoadMinimalTemperature()     {         var returnValue = 0;           // Do complex stuff here           return true;     } } The problem is that our class with many lazy loaded properties will grow messy if it has all value loading code inside it. This code may be complex for more than one parameter and in this case it is better to use separate class for this parameter. Defining base class for parameters As a first step I will define base class for all lazy-loaded parameters. This class is wrapper around Lazy<T> and it also offers one template method that parameter classes have to override to provide loaded data. public abstract class LazyParameter<T> {     private Lazy<T> _lazyParam;       public LazyParameter()     {         _lazyParam = new Lazy<T>(Load);     }       protected abstract T Load();       public T Value     {         get { return _lazyParam.Value; }     } } It is also possible to extend Lazy<T> but I don’t prefer to do it as Lazy<T> has six constructors we have to take care of. Also I don’t like to expose Lazy<T> public interface to users of my parameter classes. Creating parameter class Now it’s time to create our first parameter class. Notice how few stuff we have in this class besides overridden Load() method. public class MinimalRoomTemperature : LazyParameter<int> {     protected override int Load()     {         var returnValue = 0;           // Do complex stuff here           return returnValue;     } } Using parameter class is simple. Here’s my test code. class Program {     static void Main(string[] args)     {         var parameter = new MinimalRoomTemperature();         Console.WriteLine("Minimal room temperature: " + parameter.Value);         Console.ReadLine();     } } Conclusion Lazy<T> is useful class that you usually don’t want to use outside from API-s. I like this class but I don’t like when people are using this class directly in application code. In this posting I showed you how to use Lazy<T> with wrapper class to get complex parameter loading code out from classes that use this parameter. We ended up with generic base class for parameters that you can also use as base for other similar classes (you have to find better name to base class in this case).

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• Pure virtual or abstract, what's in a name?

  - by Steven Jeuris

  While discussing a question about virtual functions on Stack Overflow, I wondered whether there was any official naming for pure (abstract) and non-pure virtual functions. I always relied on wikipedia for my information, which states that pure and non-pure virtual functions are the general term. Unfortunately, the article doesn't back it up with a origin or references. To quote Jon Skeet's answer to my reply that pure and non-pure are the general term used: @Steven: Hmm... possibly, but I've only ever seen it in the context of C++ before. I suspect anyone talking about them is likely to have a C++ background :) Did the terms originate from C++, or were they first defined or implemented in a earlier language, and are they the 'official' scientific terms?

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• Absent Code attribute in method that is not native or abstract

  - by kerry

  I got the following, quite puzzling error today when running a unit test: java.lang.ClassFormatError: Absent Code attribute in method that is not native or abstract in class file javax/servlet/http/Cookie A google search found this post, which explains that it is caused by having an interface in the classpath, and not an actual implementation. In this case it’s the java-ee interface. To fix this I added the jetty servlet api implementation to my pom: jetty javax.servlet test Piece of cake. I have run in to this before, so I figured I would capture the fix here in case I run in to it again.

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• Abstract Data Type and Data Structure

  - by mark075

  It's quite difficult for me to understand these terms. I searched on google and read a little on Wikipedia but I'm still not sure. I've determined so far that: Abstract Data Type is a definition of new type, describes its properties and operations. Data Structure is an implementation of ADT. Many ADT can be implemented as the same Data Structure. If I think right, array as ADT means a collection of elements and as Data Structure, how it's stored in a memory. Stack is ADT with push, pop operations, but can we say about stack data structure if I mean I used stack implemented as an array in my algorithm? And why heap isn't ADT? It can be implemented as tree or an array.

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