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• When does a const return type interfere with template instantiation?

  - by Rimo

  From Herb Sutter's GotW #6 Return-by-value should normally be const for non-builtin return types. ... Note: Lakos (pg. 618) argues against returning const value, and notes that it is redundant for builtins anyway (for example, returning "const int"), which he notes may interfere with template instantiation. While Sutter seems to disagree on whether to return a const value or non-const value when returning an object of a non-built type by value with Lakos, he generally agrees that returning a const value of a built-in type (e.g const int) is not a good idea. While I understand why that is useless because the return value cannot be modified as it is an rvalue, I cannot find an example of how that might interfere with template instantiation. Please give me an example of how having a const qualifier for a return type might interfere with template instantiation.

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• Does a static object within a function introduce a potential race condition?

  - by Jeremy Friesner

  I'm curious about the following code: class MyClass { public: MyClass() : _myArray(new int[1024]) {} ~MyClass() {delete [] _myArray;} private: int * _myArray; }; // This function may be called by different threads in an unsynchronized manner void MyFunction() { static const MyClass _myClassObject; [...] } Is there a possible race condition in the above code? Specifically, is the compiler likely to generate code equivalent to the following, "behind the scenes"? void MyFunction() { static bool _myClassObjectInitialized = false; if (_myClassObjectInitialized == false) { _myClassObjectInitialized = true; _myClassObject.MyClass(); // call constructor to set up object } [...] } ... in which case, if two threads were to call MyFunction() nearly-simultaneously, then _myArray might get allocated twice, causing a memory leak? Or is this handled correctly somehow?

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• How to get this to compile?

  - by ShaChris23

  I have this code which compiles and works as expected: class Right {}; class Left { public: Left& operator = (Right const&) { //... Do something ... return *this; } }; int main() { Right right; Left left; // Assign individual object -- this works left = right; } But now, this one surprises me, I thought the template would work itself out since I already provided the = operator() to the Left class. int main() { ... std::list<Right> rightLst; std::list<Left> leftLst; // Assign a list of objects -- this doesn't compile leftLst = rightLst; } What can I do so that I could convert the rightLst to leftLst conversion in a single line?

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• Operator() as a subscript (C++)

  - by Ivan Gromov

  I use operator() as a subscript operator this way: double CVector::operator() (int i) const { if (i >= 0 && i < this->size) return this->data[i]; else return 0; } double& CVector::operator() (int i) { return (this->data[i]); } It works when I get values, but I get an error when I try to write assign a value using a(i) = 1; UPD: Error text: Unhandled exception at 0x651cf54a (msvcr100d.dll) in CG.exe: 0xC0000005: Access violation reading location 0xccccccc0.

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• Generic class for performing mass-parallel queries. Feedback?

  - by Aaron

  I don't understand why, but there appears to be no mechanism in the client library for performing many queries in parallel for Windows Azure Table Storage. I've created a template class that can be used to save considerable time, and you're welcome to use it however you wish. I would appreciate however, if you could pick it apart, and provide feedback on how to improve this class. public class AsyncDataQuery<T> where T: new() { public AsyncDataQuery(bool preserve_order) { m_preserve_order = preserve_order; this.Queries = new List<CloudTableQuery<T>>(1000); } public void AddQuery(IQueryable<T> query) { var data_query = (DataServiceQuery<T>)query; var uri = data_query.RequestUri; // required this.Queries.Add(new CloudTableQuery<T>(data_query)); } /// <summary> /// Blocking but still optimized. /// </summary> public List<T> Execute() { this.BeginAsync(); return this.EndAsync(); } public void BeginAsync() { if (m_preserve_order == true) { this.Items = new List<T>(Queries.Count); for (var i = 0; i < Queries.Count; i++) { this.Items.Add(new T()); } } else { this.Items = new List<T>(Queries.Count * 2); } m_wait = new ManualResetEvent(false); for (var i = 0; i < Queries.Count; i++) { var query = Queries[i]; query.BeginExecuteSegmented(callback, i); } } public List<T> EndAsync() { m_wait.WaitOne(); return this.Items; } private List<T> Items { get; set; } private List<CloudTableQuery<T>> Queries { get; set; } private bool m_preserve_order; private ManualResetEvent m_wait; private int m_completed = 0; private void callback(IAsyncResult ar) { int i = (int)ar.AsyncState; CloudTableQuery<T> query = Queries[i]; var response = query.EndExecuteSegmented(ar); if (m_preserve_order == true) { // preserve ordering only supports one result per query this.Items[i] = response.Results.First(); } else { // add any number of items this.Items.AddRange(response.Results); } if (response.HasMoreResults == true) { // more data to pull query.BeginExecuteSegmented(response.ContinuationToken, callback, i); return; } m_completed = Interlocked.Increment(ref m_completed); if (m_completed == Queries.Count) { m_wait.Set(); } } }

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• checking whether 4 points in a plane define a square ??

  - by osabri

  how to check whether 4 points in the plane define a square? what's the function which given a point and a value of the area of a square as input parameters returns four squares(define a corresponding type) with sides parallel to the x axis and y axis this how i start: #include <stdio.h> #include<math.h> struct point{ float x; float y; } typedef struct point POINT; struct square{ struct point p1; struct point p2; struct point p3; struct point p4; } typedef struct square SQUARE; int main() { int point; printf("point coordinate"); printf("\n\n"); printf("enter data\n");

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• How did this get 8?

  - by David

  Here's the code: class qual { public static int fibonacci(int n) { if (n == 0 || n == 1) { return 1; } else { return fibonacci(n-1) + fibonacci(n-2); } } public static void main(String[] arg) { System.out.println(fibonacci(5)); } } The output was 8. The output should be 8 but when I look at this I think it should be 7 ((5-1) +(5-2)). Why was the output 8? I think the reasoning behind getting 8 will make recursion maybe stop being confusing for me.

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• Runtime error in sml function

  - by Rpant

  I am writing this function in sml fun removeNodeswithNoIncoming((x:int,y)::xs,element:int) = if x=element then removeNodeswithNoIncoming (xs , element) else (x,y) :: removeNodeswithNoIncoming (xs , element) | removeNodeswithNoIncoming(nil,element) = nil; the function takes list of tuples [(0,0),(0,1)(1,2)] and another element 0 if first element of the tuples is same as second parameter , it removes it from the list The o/p for the above list should be [(1,2)] Unfortunately , the code is not working for the above input. Though there are other inputs for which it works - removeNodeswithNoIncomingEdge([(0,1),(0,2),(1,2)],0); stdIn:30.1-30.30 Error: unbound variable or constructor: removeNodeswithNoIncomi ngEdge - removeNodeswithNoIncomingEdge([(0,1),(0,2),(1,2),(1,4)],0); stdIn:1.2-1.31 Error: unbound variable or constructor: removeNodeswithNoIncoming Edge

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• Multidimensional Array Initialization: Any benefit from Threading?

  - by Alan

  say I have the following code: char[5][5] array; for(int i =0; i < 5; ++i) { for(int j = 0; j < 5; ++i) { array[i][j] = 'X'; } } Would there be a benefit for initializing each row in this array in a separate thread? Imagine instead of a 5 by 5 array, we have a 10 by 10? n x n? Also, this is done once, during application startup.

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• How does this iterative Tower of Hanoi work? C

  - by Nitesh Panchal

  Hello, while surfing google, i found this interesting solution to Tower Of Hanoi which doesn't even use stack. Can anybody explain me in brief, what is it actually doing? And this solution really acceptable? #include <stdio.h> #include <stdlib.h> int main() { int n, x; printf( "How many disks? " ); scanf( "%d", &n ); printf("\n"); for (x=1; x < (1 << n); x++) printf( "move from tower %i to tower %i.\n", (x&x-1)%3, ((x|x-1)+1)%3 ); return 0; }

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• Interface -> Entity Mapping

  - by KiD0M4N

  Hi guys, Suppose I have a few definitions like so: public interface ICategory { int ID { get; set; } string Name { get; set; } ICategory Parent { get; set; } } public class Category : ICategory { public virtual int ID { get; set; } public virtual string Name { get; set; } public virtual ICategory Parent { get; set; } } How do I map such a scenario in NHibernate? I am trying to separate the implementation of the DAL. I am learning NHibernate. Regards, Karan

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• How to simplify my code... 2D NSArray in Objective C...?

  - by Tattat

  self.myArray = [NSArray arrayWithObjects: [NSArray arrayWithObjects: [self d], [self generateMySecretObject],nil], [NSArray arrayWithObjects: [self generateMySecretObject], [self generateMySecretObject],nil],nil]; for (int k=0; k<[self.myArray count]; k++) { for(int s = 0; s<[[self.myArray objectAtIndex:k] count]; s++){ [[[self.myArray objectAtIndex:k] objectAtIndex:s] setAttribute:[self generateSecertAttribute]]; } } As you can see this is a simple 2*2 array, but it takes me lots of code to assign the NSArray in very first place, because I found that the NSArray can't assign the size at very beginning. Also, I want to set attribute one by one. I can't think of if my array change to 10*10. How long it could be. So, I hope you guys can give me some suggestions on shorten the code, and more readable. thz

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• Switch to BigInteger if necessary

  - by fahdshariff

  I am reading a text file which contains numbers in the range [1, 10^100]. I am then performing a sequence of arithmetic operations on each number. I would like to use a BigInteger only if the number is out of the int/long range. One approach would be to count how many digits there are in the string and switch to BigInteger if there are too many. Otherwise I'd just use primitive arithmetic as it is faster. Is there a better way? Is there any reason why Java could not do this automatically i.e. switch to BigInteger if an int was too small? This way we would not have to worry about overflows.

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• stxxl Assertion `it != root_node_.end()' failed

  - by Fabrizio Silvestri

  I am receiving this assertion failed error when trying to insert an element in a stxxl map. The entire assertion error is the following: resCache: /usr/include/stxxl/bits/containers/btree/btree.h:470: std::pair , bool stxxl::btree::btree::insert(const value_type&) [with KeyType = e_my_key, DataType = unsigned int, CompareType = comp_type, unsigned int RawNodeSize = 16384u, unsigned int RawLeafSize = 131072u, PDAllocStrategy = stxxl::SR, stxxl::btree::btree::value_type = std::pair]: Assertion `it != root_node_.end()' failed. Aborted Any idea? Edit: Here's the code fragment void request_handler::handle_request(my_key& query, reply& rep) { c_++; strip(query.content); std::cout << "Received query " << query.content << " by thread " << boost::this_thread::get_id() << ". It is number " << c_ << "\n"; strcpy(element.first.content, query.content); element.second = c_; testcache_.insert(element); STXXL_MSG("Records in map: " << testcache_.size()); } Edit2 here's more details (I omit constants, e.g. MAX_QUERY_LEN) struct comp_type : std::binary_function<my_key, my_key, bool> { bool operator () (const my_key & a, const my_key & b) const { return strncmp(a.content, b.content, MAX_QUERY_LEN) < 0; } static my_key max_value() { return max_key; } static my_key min_value() { return min_key; } }; typedef stxxl::map<my_key, my_data, comp_type> cacheType; cacheType testcache_; request_handler::request_handler() :testcache_(NODE_CACHE_SIZE, LEAF_CACHE_SIZE) { c_ = 0; memset(max_key.content, (std::numeric_limits<unsigned char>::max)(), MAX_QUERY_LEN); memset(min_key.content, (std::numeric_limits<unsigned char>::min)(), MAX_QUERY_LEN); testcache_.enable_prefetching(); STXXL_MSG("Records in map: " << testcache_.size()); }

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• Python win32api not moving mouse cursor in VirtualBox

  - by wes

  I'm trying to use this Python code: import math for i in xrange(500): x = 500 + math.sin(math.pi * i / 100) * 500 y = 500 + math.cos(i) * 100 x, y = int(x), int(y) win32api.SetCursorPos((x, y)) time.sleep(.01) taken from here to move the mouse cursor in an XP VirtualBox. The mouse icon will flicker to the appropriate graphic (when it hits the edge of a window it turns into the <- resize image, for instance), but it doesn't actually move the visible cursor. I can move the mouse around while the code is running. Same result using the ctypes example in the above link. It works fine in the Win7 host. I have Guest Additions installed, if that matters.

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• How to process <input type="file" name="test1[f]" /> in PHP?

  - by user198729

  var_dump($_FILES) gives the following: array 'test1' => array 'name' => array 'f' => string 'ntuser.dat.LOG' (length=14) 'type' => array 'f' => string 'application/octet-stream' (length=24) 'tmp_name' => array 'f' => string 'D:\wamp\tmp\php223.tmp' (length=22) 'error' => array 'f' => int 0 'size' => array 'f' => int 0 Why is http designed this way?How can I get the file by $_FILES['test1']['f']?

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• Dictionary.ContainsKey return False, but a want True

  - by SkyN

  namespace Dic { public class Key { string name; public Key(string n) { name = n; } } class Program { static string Test() { Key a = new Key("A"); Key b = new Key("A"); System.Collections.Generic.Dictionary<Key, int> d = new System.Collections.Generic.Dictionary<Key, int>(); d.Add(a, 1); return d.ContainsKey(b).ToString(); } static void Main(string[] args) { System.Console.WriteLine(Test()); } } } I want TRUE!!!

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• How do I clear the contents of a file using c?

  - by Eddy

  I'm writing some code so that at each iteration of a for loop it runs a functions which writes data into a file, like this: int main() { int i; /* Write data to file 100 times */ for(i = 0; i < 100; i++) writedata(); return 0; } void writedata() { /* Create file for displaying output */ FILE *data; data = fopen("output.dat", "a"); /* do other stuff */ ... } How do I get it so that when I run the program it will delete the file contents at the beginning of the program, but after that it will append data to the file? I know that using the "w" identifier in fopen() will open a new file that's empty, but I want to be able to 'append' data to the file each time it goes through the 'writedata()' function, hence the use of the "a" identifier.

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• Visual Studio 2008 compiles anything in C++ file?

  - by Brad Pepers

  I noticed today that a source code file in a project was compiling even though it had junk at the top of it. It got me wondering what all would pass without error through the compiler. Here is an example of code that will not generate any error messages: what kind of weird behaviour is this??? #include "stdafx.h" // what is up? int foo(int bar) { bla bla bla????? return bar; } and more junk??? What in the world is the compiler doing to allow this code to compile without giving any error messages? I'm using Visual Studio 2008 and this is unmanaged C++ code. The foo function isn't actually generated in the object file so it can't be used but why no errors???

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• How to change a primary key in SQL to auto_increment?

  - by Jian Lin

  I have a table in MySQL that has a primary key: mysql> desc gifts; +---------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------------+-------------+------+-----+---------+-------+ | giftID | int(11) | NO | PRI | NULL | | | name | varchar(80) | YES | | NULL | | | filename | varchar(80) | YES | | NULL | | | effectiveTime | datetime | YES | | NULL | | +---------------+-------------+------+-----+---------+-------+ but I wanted to make it auto_increment. The following statement failed. How can it be modified so that it can work? thanks mysql> alter table gifts modify giftID int primary key auto_increment; ERROR 1068 (42000): Multiple primary key defined

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• How do Arrays work in C#?

  - by Slateboard

  I know that they are used to store data, but I have a difficult time understanding exactly how to use them in a program. In addition, I found this site with a tetris clone tutorial (the actual tutorial is missing). It uses arrays, but I can't really make sense of how it works. Here's an example of the code - public int[, ,] TShape = new int[4, 4, 2] { {{1, 0}, {0, 1}, {1, 1}, {2, 1}}, {{1, 0}, {0, 1}, {1, 1}, {1, 2}}, {{0, 0}, {1, 0}, {2, 0}, {1, 1}}, {{0, 0}, {0, 1}, {1, 1}, {0, 2}}}; Could it be that I'm looking too hard into this, or perhaps there's something I'm not grasping about it?

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• Need some help with a join on ContentProviders

  - by Pentium10

  The documentation says Columns from the associated aggregated contact are also available through an implicit join. What's that implicit join? `Join with Contacts` String LOOKUP_KEY read-only See ContactsContract.Contacts String DISPLAY_NAME read-only See ContactsContract.Contacts long PHOTO_ID read-only See ContactsContract.Contacts. int IN_VISIBLE_GROUP read-only See ContactsContract.Contacts. int HAS_PHONE_NUMBER read-only See ContactsContract.Contacts. I am querying ContactsContract.Data, and I need to access as where clauses on the query IN_VISIBLE_GROUP and HAS_PHONE_NUMBER, that are defined in ContactsContract.Contacts. How can I make this possible?

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• How do I get the NextVal from an oracle Sequence thru NHibernate

  - by trainer

  I am working on c# .net 4.0 and using NHibernate to talk with an Oracle DB. You would think something as simple as this is already addressed somewhere but sadly its not. I need the NextVal from an Oracle sequence. I do not need to insert it a database as part of an Id or Primary key. I just need to use the next val on the c# side. Can somebody help me out with xml mapping and C# file(or a link) to achieve this. Thanks. Something like int NextValueOfSequence = GetNextValueofSequence(); public int GetNextValueOfSequence() { // Access NHibernate to return the next value of the sequence. }

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• Java Prepared Statement Error

  - by Suresh S

  Hi Guys the following code throws me an error i have an insert statement created once and in the while loop i am dynamically setting parameter , and at the end i says ps2.addBatch() again while ( (eachLine = in.readLine()) != null)) { for (int k=stat; k <=45;k++) { ps2.setString (k,main[(k-2)]); } stat=45; for (int l=1;l<= 2; l++) { ps2.setString((stat+l),pdp[(l-1)]);// Exception } ps2.addBatch(); } This is the error java.lang.ArrayIndexOutOfBoundsException: 45 at oracle.jdbc.dbaccess.DBDataSetImpl._getDBItem(DBDataSetImpl.java:378) at oracle.jdbc.dbaccess.DBDataSetImpl._createOrGetDBItem(DBDataSetImpl.java:781) at oracle.jdbc.dbaccess.DBDataSetImpl.setBytesBindItem(DBDataSetImpl.java:2450) at oracle.jdbc.driver.OraclePreparedStatement.setItem(OraclePreparedStatement.java:1155) at oracle.jdbc.driver.OraclePreparedStatement.setString(OraclePreparedStatement.java:1572) at Processor.main(Processor.java:233)

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• Java: Switch statements with methods involving arrays

  - by Shane

  Hi guys I'm currently creating a program that allows the user to create an array, search an array and delete an element from an array. Looking at the LibraryMenu Method, the first case where you create an array in the switch statement works fine, however the other ones create a "cannot find symbol error" when I try to compile. My question is I want the search and delete functions to refer to the first switch case - the create Library array. Any help is appreciated, even if its likely from a simple mistake. import java.util.*; public class EnterLibrary { public static void LibraryMenu() { java.util.Scanner scannerObject =new java.util.Scanner(System.in); LibraryMenu Menu = new LibraryMenu(); Menu.displayMenu(); switch (scannerObject.nextInt() ) { case '1': { System.out.println ("1 - Add Videos"); Library[] newLibrary; newLibrary = createLibrary(); } break; case '2': System.out.println ("2 - Search Videos"); searchLibrary(newLibrary); break; case '3': { System.out.println ("3 - Change Videos"); //Change video method TBA } break; case '4': System.out.println ("4 - Delete Videos"); deleteVideo(newLibrary); break; default: System.out.println ("Unrecognized option - please select options 1-3 "); break; } } public static Library[] createLibrary() { Library[] videos = new Library[4]; java.util.Scanner scannerObject =new java.util.Scanner(System.in); for (int i = 0; i < videos.length; i++) { //User enters values into set methods in Library class System.out.print("Enter video number: " + (i+1) + "\n"); String number = scannerObject.nextLine(); System.out.print("Enter video title: " + (i+1) + "\n"); String title = scannerObject.nextLine(); System.out.print("Enter video publisher: " + (i+1) + "\n"); String publisher = scannerObject.nextLine(); System.out.print("Enter video duration: " + (i+1) + "\n"); String duration = scannerObject.nextLine(); System.out.print("Enter video date: " + (i+1) + "\n"); String date= scannerObject.nextLine(); System.out.print("VIDEO " + (i+1) + " ENTRY ADDED " + "\n \n"); //Initialize arrays videos[i] = new Library (); videos[i].setVideo( number, title, publisher, duration, date ); } return videos; } public static void printVidLibrary( Library[] videos) { //Get methods to print results System.out.print("\n======VIDEO CATALOGUE====== \n"); for (int i = 0; i < videos.length; i++) { System.out.print("Video number " + (i+1) + ": \n" + videos[i].getNumber() + "\n "); System.out.print("Video title " + (i+1) + ": \n" + videos[i].getTitle() + "\n "); System.out.print("Video publisher " + (i+1) + ": \n" + videos[i].getPublisher() + "\n "); System.out.print("Video duration " + (i+1) + ": \n" + videos[i].getDuration() + "\n "); System.out.print("Video date " + (i+1) + ": \n" + videos[i].getDate() + "\n "); } } public static Library searchLibrary( Library[] videos) { //User enters values to setSearch Library titleResult = new Library(); java.util.Scanner scannerObject =new java.util.Scanner(System.in); for (int n = 0; n < videos.length; n++) { System.out.println("Search for video number:\n"); String newSearch = scannerObject.nextLine(); titleResult.getSearch( videos, newSearch); if (!titleResult.equals(-1)) { System.out.print("Match found!\n" + newSearch + "\n"); } else if (titleResult.equals(-1)) { System.out.print("Sorry, no matches found!\n"); } } return titleResult; } public static void deleteVideo( Library[] videos) { Library titleResult = new Library(); java.util.Scanner scannerObject =new java.util.Scanner(System.in); for (int n = 0; n < videos.length; n++) { System.out.println("Search for video number:\n"); String deleteSearch = scannerObject.nextLine(); titleResult.deleteVideo(videos, deleteSearch); System.out.print("Video deleted\n"); } } public static void main(String[] args) { Library[] newLibrary; new LibraryMenu(); } }

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