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• C++ Array vs vector

  - by blue_river

  when using C++ vector, time spent is 718 milliseconds, while when I use Array, time is almost 0 milliseconds. Why so much performance difference? int _tmain(int argc, _TCHAR* argv[]) { const int size = 10000; clock_t start, end; start = clock(); vector<int> v(size*size); for(int i = 0; i < size; i++) { for(int j = 0; j < size; j++) { v[i*size+j] = 1; } } end = clock(); cout<< (end - start) <<" milliseconds."<<endl; // 718 milliseconds int f = 0; start = clock(); int arr[size*size]; for(int i = 0; i < size; i++) { for(int j = 0; j < size; j++) { arr[i*size+j] = 1; } } end = clock(); cout<< ( end - start) <<" milliseconds."<<endl; // 0 milliseconds return 0; }

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• SQLite DB open time really long Problem

  - by sxingfeng

  I am using sqlite in c++ windows, And I have a db size about 60M, When I open the sqlite db, It takes about 13 second. sqlite3* mpDB; nRet = sqlite3_open16(szFile, &mpDB); And if I closed my application and reopen it again. It takse only less then 1 second. First, I thought It is because of disk cache. So I preload the 60M db file before sqlite open, and read the file using CFile, However, after preloading, the first time is still very slow. BOOL CQFilePro::PreLoad(const CString& strPath) { boost::shared_array<BYTE> temp = boost::shared_array<BYTE>(new BYTE[PRE_LOAD_BUFFER_LENGTH]); int nReadLength; try { CFile file; if (file.Open(strPath, CFile::modeRead) == FALSE) { return FALSE; } do { nReadLength = file.Read(temp.get(), PRE_LOAD_BUFFER_LENGTH); } while (nReadLength == PRE_LOAD_BUFFER_LENGTH); file.Close(); } catch(...) { } return TRUE; } My question is what is the difference between first open and second open. How can I accelerate the sqlite open-process.

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• What's the most trivial function that would benfit from being computed on a GPU?

  - by hanDerPeder

  Hi. I'm just starting out learning OpenCL. I'm trying to get a feel for what performance gains to expect when moving functions/algorithms to the GPU. The most basic kernel given in most tutorials is a kernel that takes two arrays of numbers and sums the value at the corresponding indexes and adds them to a third array, like so: __kernel void add(__global float *a, __global float *b, __global float *answer) { int gid = get_global_id(0); answer[gid] = a[gid] + b[gid]; } __kernel void sub(__global float* n, __global float* answer) { int gid = get_global_id(0); answer[gid] = n[gid] - 2; } __kernel void ranksort(__global const float *a, __global float *answer) { int gid = get_global_id(0); int gSize = get_global_size(0); int x = 0; for(int i = 0; i < gSize; i++){ if(a[gid] > a[i]) x++; } answer[x] = a[gid]; } I am assuming that you could never justify computing this on the GPU, the memory transfer would out weight the time it would take computing this on the CPU by magnitudes (I might be wrong about this, hence this question). What I am wondering is what would be the most trivial example where you would expect significant speedup when using a OpenCL kernel instead of the CPU?

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• how to write binary copy of structure array to file

  - by cerr

  I would like to write a binary image of a structure array to a binary file. I have tried this so far: #include <stdio.h> #include <string.h> #define NUM 256 const char *fname="binary.bin"; typedef struct foo_s { int intA; int intB; char string[20]; }foo_t; void main (void) { foo_t bar[NUM]; bar[0].intA = 10; bar[0].intB = 999; strcpy(bar[0].string,"Hello World!"); Save(bar); printf("%s written succesfully!\n",fname); } int Save(foo_t* pData) { FILE *pFile; int ptr = 0; int itr = 0; pFile = fopen(fname, "w"); if (pFile == NULL) { printf("couldn't open %s\n", fname); return; } for (itr = 0; itr<NUM; itr++) { for (ptr=0; ptr<sizeof(foo_t); ptr++) { fputc((unsigned char)*((&pData[itr])+ptr), pFile); } fclose(pFile); } } but the compiler is saying aggregate value used where an integer was expected fputc((unsigned char)*((&pData[itr])+ptr), pFile); and I don't quite understand why, what am I doing wrong? Thanks!

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• How do I cover unintuitive code blocks?

  - by naivedeveloper

  For some reason, I'm having a hard time trying to cover the block of code below. This code is an excerpt from the UNIX uniq command. I'm trying to write test cases to cover all blocks, but can't seem to reach this block: if (nfiles == 2) { // Generic error routine } In context: int main (int argc, char **argv) { int optc = 0; bool posixly_correct = (getenv ("POSIXLY_CORRECT") != NULL); int nfiles = 0; char const *file[2]; file[0] = file[1] = "-"; program_name = argv[0]; skip_chars = 0; skip_fields = 0; check_chars = SIZE_MAX; for (;;) { /* Parse an operand with leading "+" as a file after "--" was seen; or if pedantic and a file was seen; or if not obsolete. */ if (optc == -1 || (posixly_correct && nfiles != 0) || ((optc = getopt_long (argc, argv, "-0123456789Dcdf:is:uw:", longopts, NULL)) == -1)) { if (optind == argc) break; if (nfiles == 2) { // Handle errors } file[nfiles++] = argv[optind++]; } else switch (optc) { case 1: { unsigned long int size; if (optarg[0] == '+' && posix2_version () < 200112 && xstrtoul (optarg, NULL, 10, &size, "") == LONGINT_OK && size <= SIZE_MAX) skip_chars = size; else if (nfiles == 2) { // Handle error } else file[nfiles++] = optarg; } break; } } } Any help would be greatly appreciated. Thanks.

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• Creating a process in ASP.NET MVC controller

  - by GRKamath

  I have a requirement to run an application through my MVC controller. To get the installation path I used following link (I used answer provided by Fredrik Mörk). It worked and I could able to run the exe through a process. The problem occurred when I deployed this solution on IIS where it did not create the process as it was creating in local dev environment. Can anybody tell me how to create a windows process through a solution which is hosted on IIS ? private string GetPathForExe(string fileName) { private const string keyBase = @"SOFTWARE\Wow6432Node\MyApplication"; RegistryKey localMachine = Registry.LocalMachine; RegistryKey fileKey = localMachine.OpenSubKey(string.Format(@"{0}\{1}", keyBase, fileName)); object result = null; if (fileKey != null) { result = fileKey.GetValue("InstallPath"); } fileKey.Close(); return (string)result; } public void StartMyApplication() { Process[] pname = Process.GetProcessesByName("MyApplication"); if (pname.Length == 0) { string appDirectory = GetPathForExe("MyApplication"); Directory.SetCurrentDirectory(appDirectory); ProcessStartInfo procStartInfo = new ProcessStartInfo("MyApplication.exe"); procStartInfo.WindowStyle = ProcessWindowStyle.Hidden; Process proc = new Process(); proc.StartInfo = procStartInfo; proc.Start(); } }

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• C++: calling member functions within constructor?

  - by powerboy

  The following code raises a runtime error: #include <iostream> #include <iterator> #include <ext/slist> class IntList : public __gnu_cxx::slist<int> { public: typedef IntList::iterator iterator; IntList() { tail_ = begin(); } // seems that there is a problem here void append(const int node) { tail_ = insert_after(tail_, node); } private: iterator tail_; }; int main() { IntList list; list.append(1); list.append(2); list.append(3); for (IntList::iterator i = list.begin(); i != list.end(); ++i) { std::cout << *i << " "; } return 0; } Seems that the problem is in the constructor IntList(). Is it because it calls the member function begin()?

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• Checking if a function has C-linkage at compile-time

  - by scjohnno

  Is there any way to check if a given function is declared with C-linkage (that is, with extern "C") at compile-time? I am developing a plugin system. Each plugin can supply factory functions to the plugin-loading code. However, this has to be done via name (and subsequent use of GetProcAddress or dlsym). This requires that the functions be declared with C-linkage so as to prevent name-mangling. It would be nice to be able to throw a compiler error if the referred-to function is declared with C++-linkage (as opposed to finding out at runtime when a function with that name does not exist). Here's a simplified example of what I mean: extern "C" void my_func() { } void my_other_func() { } // Replace this struct with one that actually works template<typename T> struct is_c_linkage { static const bool value = true; }; template<typename T> void assertCLinkage(T *func) { static_assert(is_c_linkage<T>::value, "Supplied function does not have C-linkage"); } int main() { assertCLinkage(my_func); // Should compile assertCLinkage(my_other_func); // Should NOT compile } Thanks.

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• Displaying cookies as key=value for all domains?

  - by OverTheRainbow

  Hello, This question pertains to the use of the cookie-capable WebClient derived class presented in the How can I get the WebClient to use Cookies? question. I'd like to use a ListBox to... 1) display each cookie individually as "key=value" (the For Each loop displays all of them as one string), and 2) be able to display all cookies, regardless of the domain from which they came ("www.google.com", here): Imports System.IO Imports System.Net Public Class Form1 Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim webClient As New CookieAwareWebClient Const URL = "http://www.google.com" Dim response As String response = webClient.DownloadString(URL) RichTextBox1.Text = response 'How to display cookies as key/value in ListBox? 'PREF=ID=5e770c1a9f279d5f:TM=1274032511:LM=1274032511:S=1RDPaKJKpoMT9T54 For Each mycc In webClient.cc.GetCookies(New Uri(URL)) ListBox1.Items.Add(mycc.ToString) Next End Sub End Class Public Class CookieAwareWebClient Inherits WebClient Public cc As New CookieContainer() Private lastPage As String Protected Overrides Function GetWebRequest(ByVal address As System.Uri) As System.Net.WebRequest Dim R = MyBase.GetWebRequest(address) If TypeOf R Is HttpWebRequest Then With DirectCast(R, HttpWebRequest) .CookieContainer = cc If Not lastPage Is Nothing Then .Referer = lastPage End If End With End If lastPage = address.ToString() Return R End Function End Class Thank you.

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• Anything wrong with this MySQL quert? takes 10 seconds+ to load

  - by user345426

  I have a search that is taking 10 seconds+ to execute! Keep in mind it is also searching over 200,000 products in the database. I posted the explain and MySQL query here. 1 SIMPLE p ref PRIMARY,products_status,prod_prodid_status,product... products_status 1 const 9048 Using where; Using temporary; Using filesort 1 SIMPLE v ref PRIMARY,vendors_id,vendors_vendorid vendors_vendorid 4 rhinomar_rhinomartnew.p.vendors_id 1 1 SIMPLE s ref products_id products_id 4 rhinomar_rhinomartnew.p.products_id 1 1 SIMPLE pd ref PRIMARY,products,prod_desc_prodid_prodname prod_desc_prodid_prodname 4 rhinomar_rhinomartnew.p.products_id 1 1 SIMPLE p2c ref PRIMARY,ptc_catidx PRIMARY 4 rhinomar_rhinomartnew.p.products_id 1 Using where; Using index 1 SIMPLE c eq_ref PRIMARY PRIMARY 4 rhinomar_rhinomartnew.p2c.categories_id 1 Using where MySQL Query: select p.products_id, p.products_image, p.products_price, p.products_weight, p.products_unit_quantity, s.specials_new_products_price, s.status, pd.products_name, pd.products_img_alt from products p left join vendors v ON v.vendors_id = p.vendors_id left join specials s on s.products_id = p.products_id left join products_description pd on pd.products_id = p.products_id left join products_to_categories p2c on p2c.products_id = p.products_id left join categories c on c.categories_id = p2c.categories_id where ( ( pd.products_name like '%apparel%' ) or p2c.categories_id IN (773, 132, 135, 136, 119, 122, 124, 125, 126, 1749, 1753, 1747, 123, 127, 130, 131, 178, 137, 140, 164, 165, 166, 167, 168, 169, 832, 2045 ) or p.products_id = 'apparel' or p.products_model = 'apparel' or CONCAT(v.vendors_prefix, '-') = 'apparel' or CONCAT( v.vendors_prefix, '-', p.products_id ) = 'apparel' ) and p.products_status = '1' and c.categories_status = '1' group by p.products_id order by pd.products_name

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• Measuring the CPU frequency scaling effect

  - by Bryan Fok

  Recently I am trying to measure the effect of the cpu scaling. Is it accurate if I use this clock to measure it? template<std::intmax_t clock_freq> struct rdtsc_clock { typedef unsigned long long rep; typedef std::ratio<1, clock_freq> period; typedef std::chrono::duration<rep, period> duration; typedef std::chrono::time_point<rdtsc_clock> time_point; static const bool is_steady = true; static time_point now() noexcept { unsigned lo, hi; asm volatile("rdtsc" : "=a" (lo), "=d" (hi)); return time_point(duration(static_cast<rep>(hi) << 32 | lo)); } }; Update: According to the comment from my another post, I believe redtsc cannot use for measure the effect of cpu frequency scaling because the counter from the redtsc does not affected by the CPU frequency, am i right?

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• Splitting Code into Headers/Source files

  - by cam

  I took the following code from the examples page on Asio class tcp_connection : public boost::enable_shared_from_this<tcp_connection> { public: typedef boost::shared_ptr<tcp_connection> pointer; static pointer create(boost::asio::io_service& io_service) { return pointer(new tcp_connection(io_service)); } tcp::socket& socket() { return socket_; } void start() { message_ = make_daytime_string(); boost::asio::async_write(socket_, boost::asio::buffer(message_), boost::bind(&tcp_connection::handle_write, shared_from_this(), boost::asio::placeholders::error, boost::asio::placeholders::bytes_transferred)); } private: tcp_connection(boost::asio::io_service& io_service) : socket_(io_service) { } void handle_write(const boost::system::error_code& /*error*/, size_t /*bytes_transferred*/) { } tcp::socket socket_; std::string message_; }; I'm relatively new to C++ (from a C# background), and from what I understand, most people would split this into header and source files (declaration/implementation, respectively). Is there any reason I can't just leave it in the header file if I'm going to use it across many source files? If so, are there any tools that will automatically convert it to declaration/implementation for me? Can someone show me what this would look like split into header/source file for an example (or just part of it, anyway)? I get confused around weird stuff like thistypedef boost::shared_ptr<tcp_connection> pointer; Do I include this in the header or the source? Same with tcp::socket& socket() I've read many tutorials, but this has always been something that has confused me about C++.

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• Generating an NSDictionary from an SQL statement.

  - by Ed Wist

  I am trying to generate an NSDictonary that can be used to populate a listview with data I retrieved from an SQL statement. when I go to create an array and add them it adds the arrays for ALL my keys and not just for the current key. I've tried a removeAllObjects on the array but for some reason that destroys ALL my data that I already put in the dictionary. //open the database if(sqlite3_open([dbPath UTF8String], &database) == SQLITE_OK) { const char *sql = "select alphaID, word from words order by word"; sqlite3_stmt *selectStatement; //prepare the select statement int returnValue = sqlite3_prepare_v2(database, sql, -1, &selectStatement, NULL); if(returnValue == SQLITE_OK) { NSMutableArray *NameArray = [[NSMutableArray alloc] init]; NSString *alphaTemp = [[NSString alloc] init]; //loop all the rows returned by the query. while(sqlite3_step(selectStatement) == SQLITE_ROW) { NSString *currentAlpha = [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectStatement, 1)]; NSString *definitionName = [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectStatement, 2)]; if (alphaTemp == nil){ alphaTemp = currentAlpha; } if ([alphaTemp isEqualToString:(NSString *)currentAlpha]) { [NameArray addObject:definitionName]; } else if (alphaTemp != (NSString *)currentAlpha) { [self.words setObject:NameArray forKey:currentAlpha]; [NameArray removeAllObjects]; [NameArray addObject:definitionName]; } } } The Statement above adds all the "keys" but then removes all the array elements for all keys. if I take out the removeAllKeys it adds ALL the array elements for ALL keys. I don't want this I want it to add the array elements FOR the specific key then move on to the next key. in the end I want a NSDictonary with A (array) Alpha (string) Apple (string) B (array) Beta (string) Ball (string) C (array) Code (string) ...

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• How can I tell when the Text of a System.Windows.Forms.GroupBox wraps to the next line?

  - by fre0n

  I'm creating a GroupBox at runtime and setting its Text property. Usually, the text is only on one line, but sometimes it wraps. The problem is that the controls contained in the GroupBox cover up the GroupBox's text. What I'd like to do is determine if and when the text wraps. Specifically, I'd like to determine how much extra height the wrapped text takes up as compared to a single line. That way, I can reposition the GroupBox's controls and adjust its height. Initially, I thought I'd do this by calling the GroupBox's CreateGraphics() method, and using the Graphics to measure the string. Something like this: private void SetGroupBoxText(GroupBox grp, string text) { const int somePadding = 10; Graphics g = grp.CreateGraphics(); SizeF textSize = g.MeasureString(text, grp.Font); if (textSize.Width > (grp.Width - somePadding)) { // Adjust height, etc. } } The problem is that the size generated by g.MeasureString(text, grp.Font) doesn't seem to be accurate. I determined that it wasn't accurate by putting enough of a single character to cause a wrap, then measuring the resulting string. For example, it took 86 pipes (|) to until a wrap happened. When I measured that string, its width was ~253. And it took 16 capital W's to force a wrap - its string had a width of ~164. These were the two extremes that I tested. My GroupBox's width was 189. (a's took 29 and had a width of ~180, O's took 22 and had a width of ~189) Does anyone have any ideas? (hacks, WinAPI, etc. are welcome solutions)

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• How is the C++ synthesized move constructor affected by volatile and virtual members?

  - by user1827766

  Look at the following code: struct node { node(); //node(const node&); //#1 //node(node&&); //#2 virtual //#3 ~node (); node* volatile //#4 next; }; main() { node m(node()); //#5 node n=node(); //#6 } When compiled with gcc-4.6.1 it produces the following error: g++ -g --std=c++0x -c -o node.o node.cc node.cc: In constructor node::node(node&&): node.cc:3:8: error: expression node::next has side-effects node.cc: In function int main(): node.cc:18:14: note: synthesized method node::node(node&&) first required here As I understand the compiler fails to create default move or copy constructor on line #6, if I uncomment either line #1 or #2 it compiles fine, that is clear. The code compiles fine without c++0x option, so the error is related to default move constructor. However, what in the node class prevents default move constructor to be created? If I comment any of the lines #3 or #4 (i.e. make the destructor non-virtual or make data member non-volatile) it compiles again, so is it the combination of these two makes it not to compile? Another puzzle, line #5 does not cause an compilation error, what is different from line #6? Is it all specific for gcc? or gcc-4.6.1?

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• Declaring an enum within a class

  - by bporter

  In the following code snippet, the Color enum is declared within the Car class in order to limit the scope of the enum and to try not to "pollute" the global namespace. class Car { public: enum Color { RED, BLUE, WHITE }; void SetColor( Car::Color color ) { _color = color; } Car::Color GetColor() const { return _color; } private: Car::Color _color; }; (1) Is this a good way to limit the scope of the Color enum? Or, should I declare it outside of the Car class, but possibly within its own namespace or struct? I just came across this article today, which advocates the latter and discusses some nice points about enums: http://gamesfromwithin.com/stupid-c-tricks-2-better-enums. (2) In this example, when working within the class, is it best to code the enum as Car::Color, or would just Color suffice? (I assume the former is better, just in case there is another Color enum declared in the global namespace. That way, at least, we are explicit about the enum to we are referring.) Thanks in advance for any input on this.

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• How to update a string property of an sqlite database item

  - by Thomas Joos

  hi all, I'm trying to write an application that checks if there is an active internet connection. If so it reads an xml and checks every 'lastupdated' item ( php generated string ). It compares it to the database items and if there is a new value, this particular item needs to be updated. My code seems to work ( compiles, no error messages, no failures, .. ) but I notice that the particular property does not change, it becomese (null). When I output the binded string value it returns the correct string value I want to update into the db.. Any idea what I'm doing wrong? const char *sql = "update myTable Set last_updated=? Where node_id =?"; sqlite3_stmt *statement; // Preparing a statement compiles the SQL query into a byte-code program in the SQLite library. // The third parameter is either the length of the SQL string or -1 to read up to the first null terminator. if (sqlite3_prepare_v2(database, sql, -1, &statement, NULL) == SQLITE_OK){ NSLog(@"last updated item: %@", [d lastupdated]); sqlite3_bind_text(statement, 1, [d lastupdated],-1,SQLITE_TRANSIENT); sqlite3_bind_int (statement, 2, [d node_id]); }else { NSLog(@"SQLite statement error!"); } if(SQLITE_DONE != sqlite3_step(statement)){ NSAssert1(0, @"Error while updating. '%s'", sqlite3_errmsg(database)); }else { NSLog(@"SQLite Update done!"); }

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• Reflection and Generics get value of property

  - by GigaPr

  Hi i am trying to implement a generic method which allows to output csv file public static void WriteToCsv<T>(List<T> list) where T : new() { const string attachment = "attachment; filename=PersonList.csv"; HttpContext.Current.Response.Clear(); HttpContext.Current.Response.ClearHeaders(); HttpContext.Current.Response.ClearContent(); HttpContext.Current.Response.AddHeader("content-disposition", attachment); HttpContext.Current.Response.ContentType = "text/csv"; HttpContext.Current.Response.AddHeader("Pragma", "public"); bool isFirstRow = true; foreach (T item in list) { //Get public properties PropertyInfo[] propertyInfo = item.GetType().GetProperties(); while (isFirstRow) { WriteColumnName(propertyInfo); isFirstRow = false; } Type type = typeof (T); StringBuilder stringBuilder = new StringBuilder(); foreach (PropertyInfo info in propertyInfo) { //string value ???? I am trying to get the value of the property info for the item object } HttpContext.Current.Response.Write(stringBuilder.ToString()); HttpContext.Current.Response.Write(Environment.NewLine); } HttpContext.Current.Response.End(); } but I am not able to get the value of the object's property Any suggestion? Thanks

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• c++ programming- Tryin to get a number within an array that is twice the average

  - by max

  i have been assigned to set up an array with points. I am told to get the maximum value, average, nd within this same array, if any point in the array is twice the average, i should cout an "outlier." So far i have gotten the average and maximum numbers in the array. but i am unable to set the programme to cout the outlier. Instead it gives me a multiple of the average. pls help! here is the programme; int main() { const int max = 10; int ary[max]={4, 32, 9, 7, 14, 12, 13, 17, 19, 18}; int i,maxv; double out,sum=0; double av; maxv= ary[0]; for(i=0; i } cout<<"maximum value: "< for(i=0; i sum = sum + ary[i]; av = sum / max; } cout<<"average: "< out = av * 2; if(ary[i]out) { cout<<"outlier: "< return 0; }

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• Problems with variadic function (C)

  - by morpheous

  I have the following function from some legacy code that I am maintaining. long getMaxStart(long start, long count, const myStruct *s1, ...) { long i1, maxstart; myStruct *s2; va_list marker; maxstart = start; /*BUGFIX: 003 */ /*(va_start(marker, count);*/ va_start(marker, s1); for (i1 = 1; i1 <= count; i1++) { s2 = va_arg(marker, myStruct *); /* <- s2 is assigned null here */ maxstart = MAX(maxstart, s2->firstvalid); /* <- SEGV here */ } va_end(marker); return (maxstart); } When the function is called with only one myStruct argument, it causes a SEGV. The code compiled and run without crashing on an XP, when I compiled it using VS2005. I have now moved the code to Ubuntu Karmic and I am having problems with the stricter compiler on Linux. Is anyone able to spot what is causing the parameter not to be read correctly in the var_arg() statement? I am compiling using gcc version 4.4.1

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• Filling a region draws it off canvas

  - by Xanyx

  Hi Using the following code in Delphi 2007: procedure TfrmTest.PaintBox1Paint(Sender: TObject); const Rect_Size = 10; begin PaintBox1.Canvas.Brush.Color := clYellow; PaintBox1.Canvas.FillRect(Rect(0, 0, PaintBox1.width, PaintBox1.height)); PaintBox1.Canvas.Brush.Color := clRed; DrawARect(PaintBox1.Canvas, 0, 0, Rect_Size, Rect_Size); end; procedure TfrmTest.DrawARect(ACanvas: TCanvas; iLeft, iTop, iWidth, iHeight: Integer); var rgnMain: HRGN; begin rgnMain := CreateRectRgn(iLeft, iTop, iLeft + iWidth, iTop + iHeight); try SelectClipRgn(ACanvas.handle, rgnMain); ACanvas.FillRect(ACanvas.ClipRect); SelectClipRgn(ACanvas.handle, 0); finally DeleteObject(rgnMain); end; end; I get this: (Yellow area shows boundaries of PaintBox1). HMMM, NOT ALLOWED TO POST IMAGE Please go to: http://www.freeimagehosting.net/uploads/62cf687d29.jpg (Linked image shows a form with a yellow box [PaintBox1] in the center. However my red rectange [rgnMain] has been drawn at pos 0,0 on the form) My expectation was that the red rectangle would be at the top left of the PaintBox1 canvas, not the form's canvas. Why is it not? Can regions only be used with controls that have a Windows handle? Thanks

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• SFINAE and detecting if a C++ function object returns void.

  - by Tom Swirly

  I've read the various authorities on this, include Dewhurst and yet haven't managed to get anywhere with this seemingly simple question. What I want to do is to call a C++ function object, (basically, anything you can call, a pure function or a class with ()), and return its value, if that is not void, or "true" otherwise. #include <stdio.h> struct Foo { void operator()() {} }; struct Bar { bool operator()() { return false; } }; Foo foo; Bar bar; bool baz() { return false; } void bang() {} const char* print(bool b) { printf(b ? "true, " : "false, "); } template <typename Functor> bool magicCallFunction(Functor f) { return true; // lots of template magic occurs here... } int main(int argc, char** argv) { print(magicCallFunction(foo)); print(magicCallFunction(bar)); print(magicCallFunction(baz)); print(magicCallFunction(bang)); printf("\n"); }

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• How to negate a predicate function using operator ! in C++?

  - by Chan

  Hi, I want to erase all the elements that do not satisfy a criterion. For example: delete all the characters in a string that are not digit. My solution using boost::is_digit worked well. struct my_is_digit { bool operator()( char c ) const { return c >= '0' && c <= '9'; } }; int main() { string s( "1a2b3c4d" ); s.erase( remove_if( s.begin(), s.end(), !boost::is_digit() ), s.end() ); s.erase( remove_if( s.begin(), s.end(), !my_is_digit() ), s.end() ); cout << s << endl; return 0; } Then I tried my own version, the compiler complained :( error C2675: unary '!' : 'my_is_digit' does not define this operator or a conversion to a type acceptable to the predefined operator I could use not1() adapter, however I still think the operator ! is more meaningful in my current context. How could I implement such a ! like boost::is_digit() ? Any idea? Thanks, Chan Nguyen

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• C++ class derivation and superconstructor confusion

  - by LukeN

  Hey, in a tutorial C++ code, I found this particular piece of confusion: PlasmaTutorial1::PlasmaTutorial1(QObject *parent, const QVariantList &args) : Plasma::Applet(parent, args), // <- Okay, Plasma = namespace, Applet = class m_svg(this), // <- A member function of class "Applet"? m_icon("document") // <- ditto? { m_svg.setImagePath("widgets/background"); // this will get us the standard applet background, for free! setBackgroundHints(DefaultBackground); resize(200, 200); } I'm not new to object oriented programming, so class derivation and super-classes are nothing complicated, but this syntax here got me confused. The header file defines the class like this: class PlasmaTutorial1 : public Plasma::Applet { Similar to above, namespace Plasma and class Applet. But what's the public doing there? I fear that I already know the concept but don't grasp the C++ syntax/way of doing it. In this question I picked up that these are called "superconstructors", at least that's what stuck in my memory, but I don't get this to the full extend. If we glance back at the first snippet, we see Constructor::Class(...) : NS::SuperClass(...), all fine 'till here. But what are m_svg(this), m_icon("document") doing there? Is this some kind of method to make these particular functions known to the derivated class? Is this part of C++ basics or more immediate? While I'm not completly lost in C++, I feel much more at home in C :) Most of the OOP I have done so far was done in D, Ruby or Python. For example in D I would just define class MyClass : MySuperClass, override what I needed to and call the super class' constructor if I'd need to.

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• Why is T() = T() allowed?

  - by Rimo

  I believe the expression T() creates an rvalue (by the Standard). However, the following code compiles (at least on gcc4.0): class T {}; int main() { T() = T(); } I know technically this is possible because member functions can be invoked on temporaries and the above is just invoking the operator= on the rvalue temporary created from the first T(). But conceptually this is like assigning a new value to an rvalue. Is there a good reason why this is allowed? Edit: The reason I find this odd is it's strictly forbidden on built-in types yet allowed on user-defined types. For example, int(2) = int(3) won't compile because that is an "invalid lvalue in assignment". So I guess the real question is, was this somewhat inconsistent behavior built into the language for a reason? Or is it there for some historical reason? (E.g it would be conceptually more sound to allow only const member functions to be invoked on rvalue expressions, but that cannot be done because that might break some existing code.)

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