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• When are C++ macros beneficial?

  - by Motti

  The C preprocessor is justifiably feared and shunned by the C++ community. In-lined functions, consts and templates are usually a safer and superior alternative to a #define. The following macro: #define SUCCEEDED(hr) ((HRESULT)(hr) >= 0) is in no way superior to the type safe: inline bool succeeded(int hr) { return hr >= 0; } But macros do have their place, please list the uses you find for macros that you can't do without the preprocessor. Please put each use-cases in a seperate answer so it can be voted up and if you know of how to achieve one of the answers without the preprosessor point out how in that answer's comments.

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• C++ static classes & shared_ptr memory leaks

  - by HardCoder1986

  Hello! I can't understand why does the following code produce memory leaks (I am using boost::shared_ptr with static class instance). Could someone help me? #include <crtdbg.h> #include <boost/shared_ptr.hpp> using boost::shared_ptr; #define _CRTDBG_MAP_ALLOC #define NEW new(_NORMAL_BLOCK, __FILE__, __LINE__) static struct myclass { static shared_ptr<int> ptr; myclass() { ptr = shared_ptr<int>(NEW int); } } myclass_instance; shared_ptr<int> myclass::ptr; int main() { _CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF | _CRTDBG_CHECK_ALWAYS_DF | _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG)); return 0; }

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• Understanding linear linked list

  - by ArtWorkAD

  Hi, I have some problems understanding the linear linked list data structure. This is how I define a list element: class Node{ Object data; Node link; public Node(Object pData, Node pLink){ this.data = pData; this.link = pLink; } } To keep it simple we say that a list are linked nodes so we do not need to define a class list (recursion principle). My problem is that I am really confused in understanding how nodes are connected, more precisely the sequence of the nodes when we connect them. Node n1 = new Node(new Integer(2), null); Node n2 = new Node(new Integer(1), n1); What is link? Is it the previous or the next element? Any other suggestions to help me understanding this data structure?

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• Linq to SQL - How to compare against a collection in the where clause?

  - by Sgraffite

  I'd like to compare against an IEnumerable collection in my where clause. Do I need to manually loop through the collection to pull out the column I want to compare against, or is there a generic way to handle this? I want something like this: public IEnumerable<Cookie> GetCookiesForUsers(IEnumerable<User> Users) { var cookies = from c in db.Cookies join uc in db.UserCookies on c.CookieID equals uc.CookieID join u in db.Users on uc.UserID equals u.UserID where u.UserID.Equals(Users.UserID) select c; return cookies.ToList(); } I'm used to using the lambda Linq to SQL syntax, but I decided to try the SQLesque syntax since I was using joins this time. What is a good way to do this?

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• Function return type style

  - by JB

  I'm learning c++0x, at least the parts supported by the Visual C++ Express 2010 Beta. This is a question about style rather than how it works. Perhaps it's too early for style and good practice to have evolved yet for a standard that isn't even released yet... In c++0x you can define the return type of a method using - type at the end of the function instead of putting the type at the start. I believe this change in syntax is required due to lambdas and some use cases of the new decltype keyword, but you can use it anywhere as far as I know. // Old style int add1(int a, int b) { return a + b; } // New style return type auto add2(int a, int b) -> int { return a + b; } My question really then, is given that some functions will need to be defined in the new way is it considered good style to define all functions in this way for consistency? Or should I stick to only using it when necessary?

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• error reporting issues

  - by SunnyOne

  I'm still entangled in error handling problems. After having read and tried lots of different scripts, I still do not have an overall insight on the problem. I made a syntax error on purpose and get the following error message : Parse error: syntax error, unexpected '{' in /home/directory/public_html/mypage.php on line 10 To understand how it work, as a first step, I'd like to remove this message(which I'am able to do) and replace it by a custom one. How can I customize this message ? Thanks a lot.

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• T-SQL Add Column In Specific Order

  - by Aren B

  Im a bit new to T-SQL, Coming from a MySQL background Im still adapting to the different nuances in the syntax. Im looking to add a new column AFTER a specific one. I've found out that AFTER is a valid keyword but I don't think it's the right one for the job. ALTER TABLE [dbo].[InvStockStatus] ADD [Abbreviation] [nvarchar](32) DEFAULT '' NOT NULL ; This is my current query, which works well, except it adds the field at the end of the Table, Id prefer to add it after [Name]. What's the syntax im looking for to represent this?

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• Unsigned double in C++ ?

  - by lost3den

  Hello everyone, Why doesn't C++ support unsigned double syntax ? Thanks in advance

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• NSPredicate aggregate function [SIZE] gives 'unsupported function expression' error

  - by jinglesthula

  iOS 4: I have entities in Core Data (using SQLite, which is a requirement) of: Request Response (which has a property personId) Revision Relationships are: Request <-- Revision Request <-- Response Revision <-- Response (e.g. each request may have many responses; each request/response pair may have many revisions) I'm trying to do a predicate to get all Responses with a given personId that have zero Revisions. Using: (personId == %d) && (Request.Revision[SIZE] == 0) in my predicate string gives me a runtime exception "Unsupported function expression Request.Revision[SIZE]" The documentation seems pretty sparse on aggregate functions, only noting that they exist, but with no syntax or examples. Not sure if it's my syntax or if the SIZE function really isn't supported in iOS.

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• How can relate "block of memory" to "objects" in Java?

  - by anonymous

  How can I define an object in Java in a way that would mean 'block of memory'?

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• C++ a map to a 2 dimensional vector

  - by user1701545

  I want to create a C++ map where key is, say, int and value is a 2-D vector of double: map myMap; suppose I filled it and now I would like to update the second vector mapped by each key (for example divide each element by 2). How would I access that vector iteratively? The "itr-second[0]" syntax in the statement below is obviously wrong. What would be the right syntax for that action? for(std::map<in, vector<vector<double> > > itr = myMap.begin(); itr != myMap.end();++itr) { for(int i = 0;i < itr->second[0].size();++i) { itr->second[0][i] /= 2; } } thanks, rubi

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• [Using $this when not in object context in] php error

  - by JasonS

  I have solved this problem, I just need to know what to do. I get the above error because I just realised that the class is being run as class::function($values) instead of class-function($values). Does anyone know how to convert this function to instantiate the class then run the function with values? private function _load($values=null) { define('LOADED_CONTROLLER', $this->controller); define('LOADED_FUNCTION', $this->function); $function = $this->function; $controller = new $this->controller; ($values == null) ? $controller->$function() : call_user_func_array(array($this->controller, $function), $values); }

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• sqlite error :/* SQL error or missing database */

  - by user262325

  Hello everyone I have a project in which I stored sqlite database file "data.sqlite3" to 'Group'&files'-'resource' Below are my viewcontroller source codes //-myviewcontroller.h #import "sqlite3.h" #define kFilename @"data.sqlite3" //myviewcontroller.m -(NSString *)dataFilePath { NSArray *paths = NSSearchPathForDirectoriesInDomains( NSDocumentDirectory, NSUserDomainMask, YES); NSString *documentsDirectory = [paths objectAtIndex:0]; return [documentsDirectory stringByAppendingPathComponent:kFilename]; } -(void)f { if (sqlite3_open([[self dataFilePath] UTF8String],&database)!=SQLITE_OK) //dataFilePath returns ///Users/interdev/Library/Application Support/iPhone Simulator/User/Applications/095C6E05-4EAE-4817-883E-A72E39D439E0/Documents/data.sqlite3 { sqlite3_close(database); NSAssert(0,@"Failed to open database");//no problem } NSString *query = @"SELECT * FROM table1 ORDER BY ROW";//table1 is table name sqlite3_stmt *statement; NSInteger v=sqlite3_prepare_v2( database, [query UTF8String], -1, &statement, nil); NSString *zs= [NSString stringWithFormat:@"%d",v]; NSLog(@" The buttontitile is %@ ",zs); if ( v == SQLITE_OK) { // ... } I checked value of v in log, it always is 1 #define SQLITE_ERROR 1 /* SQL error or missing database */ I do not know why this happened.

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• does iterator come under collections in java?

  - by androidbase Praveen

  the iterator is a collection or a kind of control statement use. define plz.

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• How can I make Church numerals more human readable in lisp?

  - by Jason Baker

  I can define church numerals fairly easy using scheme: > (define f (lambda (x) x)) > (f f) ;0 #<procedure:f> > (f (f f)) ;1 #<procedure:f> However, this doesn't make it very easy to recognize that (f f) is 0 and (f (f f)) is 1. Is there a way that I can make these numerals more readable? What would be ideal is this: > (f f) 0 > (f (f f)) 1 The example is in scheme, but I'll take an answer in any lisp.

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• Template class + virtual function = must implement?

  - by sold

  This code: template <typename T> struct A { T t; void DoSomething() { t.SomeFunction(); } }; struct B { }; A<B> a; is easily compiled without any complaints, as long as I never call a.DoSomething(). However, if I define DoSomething as a virtual function, I will get a compile error saying that B doesn't declare SomeFunction. I can somewhat see why it happens (DoSomething should now have an entry in the vtable), but I can't help feeling that it's not really obligated. Plus it sucks. Is there any way to overcome this? EDIT 2: Okay. I hope this time it makes sence: Let's say I am doing intrusive ref count, so all entities must inherit from base class Object. How can I suuport primitive types too? I can define: template <typename T> class Primitive : public Object { T value; public: Primitive(const T &value=T()); operator T() const; Primitive<T> &operator =(const T &value); Primitive<T> &operator +=(const T &value); Primitive<T> &operator %=(const T &value); // And so on... }; so I can use Primitive<int>, Primitive<char>... But how about Primitive<float>? It seems like a problem, because floats don't have a %= operator. But actually, it isn't, since I'll never call operator %= on Primitive<float>. That's one of the deliberate features of templates. If, for some reason, I would define operator %= as virtual. Or, if i'll pre-export Primitive<float> from a dll to avoid link errors, the compiler will complain even if I never call operator %= on a Primitive<float>. If it would just have fill in a dummy value for operator %= in Primitive<float>'s vtable (that raises an exception?), everything would have been fine.

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• Difference between Class Abstraction and Object Interfaces in PHP?

  - by Mark Tomlin

  What is the difference between a Class Abstraction and an Object Interfaces in PHP? I ask because, I don't really see the point to both of them, they both do the same thing! So, what are the advantages of disadvantages using both against one or the other? Class Abstraction: abstract class aClass { // Force extending class to define these methods abstract public function setVariable($name, $var); abstract public function getHtml($template); } Object Interface: interface iClass { // Force impementing class to define these methods public function setVariable($name, $var); public function getHtml($template); }

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• In PLT scheme, can I export functions after another function has been called?

  - by Jason Baker

  I'm trying to create a binding to libpython using scheme's FFI. To do this, I have to get the location of python, create the ffi-lib, and then create functions from it. So for instance I could do this: (module pyscheme scheme (require foreign) (unsafe!) (define (link-python [lib "/usr/lib/libpython2.6.so"]) (ffi-lib lib)) This is all well and good, but I can't think of a way to export functions. For instance, I could do something like this: (define Py_Initialize (get-ffi-obj "Py_Initialize" libpython (_fun -> _void))) ...but then I'd have to store a reference to libpython (created by link-python) globally somehow. Is there any way to export these functions once link-python is called? In other words, I'd like someone using the module to be able to do this: (require pyscheme) (link-python) (Py_Initialize) ...or this: (require pyscheme) (link-python "/weird/location/for/libpython.so") (Py_Initialize) ...but have this give an error: (require pyscheme) (Py_Initialize) How can I do this?

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• Preprocessor directive #ifndef for C/C++ code

  - by Leif Andersen

  In eclipse, whenever I create a new C++ class, or C header file, I get the following type of structure. Say I create header file example.h, I get this: /*Comments*/ #ifndef EXAMPLE_H_ #define EXAMPLE_H_ /* Place to put all of my definitions etc. */ #endif I think ifndef is saying that if EXAMPLE_H_ isn't defined, define it, which may be useful depending on what tool you are using to compile and link your project. However, I have two questions: Is this fairly common? I don't see it too often. And is it a good idea to use that rubric, or should you just jump right into defining your code. What is EXAMPLE_H_ exactly? Why not example.h, or just example? Is there anything special about that, or could is just be an artifact of how eclipse prefers to autobuild projects? Thank you for your help.

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• Is the "==" operator required to be defined to use std::find

  - by user144182

  Let's say I have: class myClass std::list<myClass> myList where myClass does not define the == operator and only consists of public fields. In both VS2010 and VS2005 the following does not compile: myClass myClassVal = myList.front(); std::find( myList.begin(), myList.end(), myClassVal ) complaining about lack of == operator. I naively assumed it would do a value comparison of the myClass object's public members, but I am almost positive this is not correct. I assume if I define a == operator or perhaps use a functor instead, it will solve the problem. Alternatively, if my list was holding pointers instead of values, the comparison would work. Is this right or should I be doing something else?

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• NoMethodError using Memcached Sinatra-style

  - by sevennineteen

  I've just installed Memcached on my Mac and updated my Sinatra app configuration as described in Heroku's documentation, but I'm getting a NoMethodError when trying to use the hash-based syntax they specify: >> CACHE['color'] = 'blue' >> CACHE['color'] Using explicit get and set methods as below seems to work fine. >> CACHE.set('color', 'blue') >> CACHE.get('color') If necessary I can use the latter syntax, but the former seems more elegant. I haven't tested this on Heroku's environment since I'd like whatever implementation I use to work on my local environment as well. Thanks!

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• crc24 from c to python

  - by biiiiiaw

  can someone please translate this code to python? i have tried and tried again, but have not managed it: #define CRC24_INIT 0xB704CEL #define CRC24_POLY 0x1864CFBL typedef long crc24; crc24 crc_octets(unsigned char *octets, size_t len) { crc24 crc = CRC24_INIT; int i; while (len--) { crc ^= (*octets++) << 16; for (i = 0; i < 8; i++) { crc <<= 1; if (crc & 0x1000000) crc ^= CRC24_POLY; } } return crc & 0xFFFFFFL; } i have the rotate left function (ROL24(value,bits_to_rotate_by)), which i know works since i got it from a source code of a reputable programmer, but i dont get the * and ++ on octet. i only sort of understand how ++ works in c++, and i dont know what * is at all

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• Partially parse C++ for a domain-specific language

  - by PierreBdR

  I would like to create a domain specific language as an augmented-C++ language. I will need mostly two types of contructs: Top-level constructs for specialized types or declarations In-code constructs, i.e. to add primitives to make functions calls or idiom easier The language will be used for scientific computing purposes, and will ultimately be translated into plain C++. C++ has been chosen as it seems to offer a good compromise between: ease of use, efficiency and availability of a wide range of libraries. A previous attempt using flex and bison failed due to the complexity of the C++ syntax. The existing parser can still fail on some constructs. So we want to start over, but on better bases. Do you know about similar projects? And if you attempted to do so, what tools would you use? What would be the main pitfalls? Would you have recommendations in term of syntax?

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• typedef fixed length array

  - by Rajorshi

  Hi, I have to define a 24-bit data type.I am using char[3] to represent the type. Can I typedef char[3] to type24? I tried it in a code sample. I put typedef char[3] type42; in my header file. The compiler did not complain about it. But when I defined a function void foo(type24 val) {} in my C file, it did complain. I would like to be able to define functions like type24_to_int32(type24 val) instead of type24_to_int32(char value[3]).

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• Joining two select queries and ordering results

  - by user1

  Basically I'm just unsure as to why this query is failing to execute: (SELECT replies.reply_post, replies.reply_content, replies.reply_date AS d, members.username FROM (replies) AS a INNER JOIN members ON replies.reply_by = members.id) UNION (SELECT posts.post_id, posts.post_title, posts.post_date AS d, members.username FROM (posts) as b WHERE posts.post_set = 0 INNER JOIN members ON posts.post_by = members.id) ORDER BY d DESC LIMIT 5 I'm getting this error: #1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'a INNER JOIN members ON replies.re' at line 2 All I'm trying to do is select the 5 most recent rows (dates) from these two tables. I've tried Join, union etc and I've seen numerous queries where people have put another query after the FROM statement and that just makes no logical sense to me as to how that works? Am I safe to say that you can join the same table from two different but joined queries? Or am I taking completely the wrong approach, because frankly I can't seem see how this query is failing despite reading the error message. (The two queries on there own work fine)

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