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• SFINAE + sizeof = detect if expression compiles

  - by FredOverflow

  I just found out how to check if operator<< is provided for a type. template<class T> T& lvalue_of_type(); template<class T> T rvalue_of_type(); template<class T> struct is_printable { template<class U> static char test(char(*)[sizeof( lvalue_of_type<std::ostream>() << rvalue_of_type<U>() )]); template<class U> static long test(...); enum { value = 1 == sizeof test<T>(0) }; typedef boost::integral_constant<bool, value> type; }; Is this trick well-known, or have I just won the metaprogramming Nobel prize? ;) EDIT: I made the code simpler to understand and easier to adapt with two global function template declarations lvalue_of_type and rvalue_of_type.

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• Setting synthesized arrays causing memory leaks using nested arrays

  - by webtoad

  Hello: Why is the following code causing a memory leak in an iPhone App? All of the initted objects below leak, including the arrays, the strings and the numbers. So, I'm thinking it has something to do with the the synthesized array property not releasing the object when I set the property again on the second and subsequent time this piece of code is called. Here is the code: "controller" (below) is my custom view controller class, which I have a reference to, and I am setting with this code snippet: sqlite3_stmt *statement; NSMutableArray *foo_IDs = [[NSMutableArray alloc] init]; NSMutableArray *foo_Names = [[NSMutableArray alloc] init]; NSMutableArray *foo_IDsBySection = [[NSMutableArray alloc] init]; NSMutableArray *foo_NamesBySection = [[NSMutableArray alloc] init]; // Get data: NSString *sql = @"select distinct p.foo_ID, p.foo_Name from foo as p "; if (sqlite3_prepare_v2(...) == SQLITE_OK) { while (sqlite3_step(statement) == SQLITE_ROW) { int p_id; NSString *foo_Name; p_id = sqlite3_column_int(statement, 0); char *str2 = (char *)sqlite3_column_text(statement, 1); foo_Name = [NSString stringWithCString:str2]; [foo_IDs addObject:[NSNumber numberWithInt:p_id]]; [foo_Names addObject:foo_Name]; } sqlite3_finalize(statement); } // Pass the array itself into another array: // (normally there is more than one array in each array) [foo_IDsBySection addObject: foo_IDs]; [foo_NamesBySection addObject: foo_Names]; [foo_IDs release]; [foo_Names release]; // Set some synthesized properties (of type NSArray, nonatomic, // retain) in controller: controller.foo_IDsBySection = foo_IDsBySection; controller.foo_NamesBySection = foo_NamesBySection; [foo_IDsBySection release]; [foo_NamesBySection release]; Thanks for any help!

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• C++: Reference and Pointer question (example regarding OpenGL)

  - by Jay

  I would like to load textures, and then have them be used by multiple objects. Would this work? class Sprite { GLuint* mTextures; // do I need this to also be a reference? Sprite( GLuint* textures ) // do I need this to also be a reference? { mTextures = textures; } void Draw( textureNumber ) { glBindTexture( GL_TEXTURE_2D, mTextures[ textureNumber ] ); // drawing code } }; // normally these variables would be inputed, but I did this for simplicity. const int NUMBER_OF_TEXTURES = 40; const int WHICH_TEXTURE = 10; void main() { std::vector<GLuint> the_textures; the_textures.resize( NUMBER_OF_TEXTURES ); glGenTextures( NUMBER_OF_TEXTURES, &the_textures[0] ); // texture loading code Sprite the_sprite( &the_textures[0] ); the_sprite.Draw( WHICH_TEXTURE ); } And is there a different way I should do this, even if it would work? Thanks.

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• pointer to a structure in a nested structure

  - by dpka6

  I have a 6 levels of nested structures. I am having problem with last three levels. The program compiles fine but when I run it crashes with Segmentation fault. There is some problem in assignment is what I feel. Kindly point out the error. typedef struct { char addr[6]; int32_t rs; uint16_t ch; uint8_t ap; } C; typedef struct { C *ap_info; } B; typedef struct { union { B wi; } u; } A; function1(char addr , int32_t rs, uint16_t ch, uint8_t ap){ A la; la.u.wi.ap_info->addr[6] = addr; la.u.wi.ap_info->rs = rs; la.u.wi.ap_info->ch = ch; la.u.wi.ap_info->ap = ap; }

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• Trying to make a plugin system in C++

  - by Pirate for Profit

  I'm making a task-based program that needs to have plugins. Tasks need to have properties which can be easily edited, I think this can be done with Qt's Meta-Object Compiler reflection capabilities (I could be wrong, but I should be able to stick this in a QtPropertyBrowser?) So here's the base: class Task : public QObject { Q_OBJECT public: explicit Task(QObject *parent = 0) : QObject(parent){} virtual void run() = 0; signals: void taskFinished(bool success = true); } Then a plugin might have this task: class PrinterTask : public Task { Q_OBJECT public: explicit PrinterTask(QObject *parent = 0) : Task(parent) {} void run() { Printer::getInstance()->Print(this->getData()); // fictional emit taskFinished(true); } inline const QString &getData() const; inline void setData(QString data); Q_PROPERTY(QString data READ getData WRITE setData) // for reflection } In a nutshell, here's what I want to do: // load plugin // find all the Tasks interface implementations in it // have user able to choose a Task and edit its specific Q_PROPERTY's // run the TASK It's important that one .dll has multiple tasks, because I want them to be associated by their module. For instance, "FileTasks.dll" could have tasks for deleting files, making files, etc. The only problem with Qt's plugin setup is I want to store X amount of Tasks in one .dll module. As far as I can tell, you can only load one interface per plugin (I could be wrong?). If so, the only possible way to do accomplish what I want is to create a FactoryInterface with string based keys which return the objects (as in Qt's Plug-And-Paint example), which is a terrible boilerplate that I would like to avoid. Anyone know a cleaner C++ plugin architecture than Qt's to do what I want? Also, am I safely assuming Qt's reflection capabilities will do what I want (i.e. able to edit an unknown dynamically loaded tasks' properties with the QtPropertyBrowser before dispatching)?

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• How do you compare using .NET types in an NHibernate ICriteria query for an ICompositeUserType?

  - by gabe

  I have an answered StackOverflow question about how to combine to legacy CHAR database date and time fields into one .NET DateTime property in my POCO here (thanks much Berryl!). Now i am trying to get a custom ICritera query to work against that very DateTime property to no avail. here's my query: ICriteria criteria = Session.CreateCriteria<InputFileLog>() .Add(Expression.Gt(MembersOf<InputFileLog>.GetName(x => x.FileCreationDateTime), DateTime.Now.AddDays(-14))) .AddOrder(Order.Desc(Projections.Id())) .CreateCriteria(typeof(InputFile).Name) .Add(Expression.Eq(MembersOf<InputFile>.GetName(x => x.Id), inputFileName)); IList<InputFileLog> list = criteria.List<InputFileLog>(); And here's the query it's generating: SELECT this_.input_file_token as input1_9_2_, this_.file_creation_date as file2_9_2_, this_.file_creation_time as file3_9_2_, this_.approval_ind as approval4_9_2_, this_.file_id as file5_9_2_, this_.process_name as process6_9_2_, this_.process_status as process7_9_2_, this_.input_file_name as input8_9_2_, gonogo3_.input_file_token as input1_6_0_, gonogo3_.go_nogo_ind as go2_6_0_, inputfile1_.input_file_name as input1_3_1_, inputfile1_.src_code as src2_3_1_, inputfile1_.process_cat_code as process3_3_1_ FROM input_file_log this_ left outer join go_nogo gonogo3_ on this_.input_file_token=gonogo3_.input_file_token inner join input_file inputfile1_ on this_.input_file_name=inputfile1_.input_file_name WHERE this_.file_creation_date > :p0 and this_.file_creation_time > :p1 and inputfile1_.input_file_name = :p2 ORDER BY this_.input_file_token desc; :p0 = '20100401', :p1 = '15:15:27', :p2 = 'LMCONV_JR' The query is exactly what i would expect, actually, except it doesn't actually give me what i want (all the rows in the last 2 weeks) because in the DB it's doing a greater than comparison using CHARs instead of DATEs. I have no idea how to get the query to convert the CHAR values into a DATE in the query without doing a CreateSQLQuery(), which I would like to avoid. Anyone know how to do this?

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• Passing an array of structs in C

  - by lelouch

  I'm having trouble passing an array of structs to a function in C. I've created the struct like this in main: int main() { struct Items { char code[10]; char description[30]; int stock; }; struct Items MyItems[10]; } I then access it like: MyItems[0].stock = 10; etc. I want to pass it to a function like so: ReadFile(MyItems); The function should read the array, and be able to edit it. Then I should be able to access the same array from other functions. I've tried heaps of declarations but none of them work. e.g. void ReadFile(struct Items[10]) I've had a look around for other questions, but the thing is they're all done different, with typedefs and asterisks. My teacher hasn't taught us pointers yet, so I'd like to do it with what I know. Any ideas? :S EDIT: Salvatore's answer is working after I fixed my prototype to: void ReadFile(struct Items[9]);

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• Aligning decimal points in HTML

  - by ijw

  I have a table containing decimal numbers in one column. I'm looking to align them in a manner similar to a word processor's "decimal tab" feature, so that all the points sit on a vertical line. I have two possible solutions at the moment but I'm hoping for something better... Solution 1: Split the numbers within the HTML, e.g. <td><div>1234</div><div class='dp'>.5</div></td> with .dp { width: 3em; } (Yes, this solution doesn't quite work as-is. The concept is, however, valid.) Solution 2: I found mention of <COL ALIGN="CHAR" CHAR="."> This is part of HTML4 according to the reference page, but it doesn't work in FF3.5, Safari 4 or IE7, which are the browsers I have to hand. It also has the problem that you can't pull out the numeric formatting to CSS (although, since it's affecting a whole column, I suppose that's not too surprising). Thus, anyone have a better idea?

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• [C++] My First Go With Function Templates

  - by bobber205

  Thought it was pretty straight forward. But I get a "iterator not dereferencable" errro when running the below code. What's wrong? template<typename T> struct SumsTo : public std::binary_function<T, T, bool> { int myInt; SumsTo(int a) { myInt = a; } bool operator()(const T& l, const T& r) { cout << l << " + " << r; if ((l + r) == myInt) { cout << " does add to " << myInt; } else { cout << " DOES NOT add to " << myInt; } return true; } }; void main() { list<int> l1; l1.push_back(1); l1.push_back(2); l1.push_back(3); l1.push_back(4); list<int> l2; l2.push_back(9); l2.push_back(8); l2.push_back(7); l2.push_back(6); transform(l1.begin(), l1.end(), l2.begin(), l2.end(), SumsTo<int>(10) ); }

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• memcmp,strcmp,strncmp in C

  - by el10780

  I wrote this small piece of code in C to test memcmp() strncmp() strcmp() functions in C. Here is the code that I wrote: #include <stdio.h> #include <stdlib.h> #include <string.h> int main(int argc, char** argv) { char *word1="apple",*word2="atoms"; if (strncmp(word1,word2,5)==0) printf("strncmp result.\n"); if (memcmp(word1,word2,5)==0) printf("memcmp result.\n"); if (strcmp(word1,word2)==0) printf("strcmp result.\n"); } Can somebody explain me the differences because I am confused with these three functions?My main problem is that I have a file in which I tokenize its line of it,the problem is that when I tokenize the word "atoms" in the file I have to stop the process of tokenizing.I first tried strcmp() but unfortunately when it reached to the point where the word "atoms" were placed in the file it didn't stop and it continued,but when I used either the memcmp() or the strncmp() it stopped and I was happy.But then I thought,what if there will be a case in which there is one string in which the first 5 letters are a,t,o,m,s and these are being followed by other letters.Unfortunately,my thoughts were right as I tested it using the above code by initializing word1 to "atomsaaaaa" and word2 to atoms and memcmp() and strncmp() in the if statements returned 0.On the other hand strcmp() it didn't.It seems that I must use strcmp(). I have done google searches but I got more confused as I have seen sites and other forums to define these three differently.If it is possible for someone to give me correct explanations/definitions so I can use them correctly in my source code,I would be really grateful.

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• Suggestions for a django db structure

  - by rh0dium

  Hi Say I have the unknown number of questions. For example: Is the sky blue [y/n] What date were your born on [date] What is pi [3.14] What is a large integ [100] Now each of these questions poses a different but very type specific answer (boolean, date, float, int). Natively django can happily deal with these in a model. class SkyModel(models.Model): question = models.CharField("Is the sky blue") answer = models.BooleanField(default=False) class BirthModel(models.Model): question = models.CharField("What date were your born on") answer = models.DateTimeField(default=today) class PiModel(models.Model) question = models.CharField("What is pi") answer = models.FloatField() But this has the obvious problem in that each question has a specific model - so if we need to add a question later I have to change the database. Yuck. So now I want to get fancy - How do a set up a model where by the answer type conversion happens automagically? ANSWER_TYPES = ( ('boolean', 'boolean'), ('date', 'date'), ('float', 'float'), ('int', 'int'), ('char', 'char'), ) class Questions(models.model): question = models.CharField(() answer = models.CharField() answer_type = models.CharField(choices = ANSWER_TYPES) default = models.CharField() So in theory this would do the following: When I build up my views I look at the type of answer and ensure that I only put in that value. But when I want to pull that answer back out it will return the data in the format specified by the answer_type. Example 3.14 comes back out as a float not as a str. How can I perform this sort of automagic transformation? Or can someone suggest a better way to do this? Thanks much!!

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• c program pointer

  - by sandy101

  Hello , I am trying some programs in c face a problem with this program #include<stdio.h> int main() { int a=9,*x; float b=3.6,*y; char c='a',*z; printf("the value is %d\n",a); printf("the value is %f\n",b); printf("the value is %c\n",c); x=&a; y=&b; z=&c; printf("%u\n",a); printf("%u\n",b); printf("%u\n",c); x++; y++; z++; printf("%u\n",a); printf("%u\n",b); printf("%u\n",c); return 0; } can any one tell me what is the problem with this and i also want to know that when in the above case if the pointer value is incremented then will it over write the previous value address as suppose that the value we got in the above program (without the increment in the pointer value )is 65524 65520 65519 and after the increment the value of the pointer is 65526(as 2 increment for the int ) 65524(as 4 increment for the float ) 65520(as 1 increment for the char variable ) then if in that case will the new pointer address overwrite the content of the previous address and what value be contained at the new address ......plz help

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• Howcome some C++ functions with unspecified linkage build with C linkage?

  - by christoffer

  This is something that makes me fairly perplexed. I have a C++ file that implements a set of functions, and a header file that defines prototypes for them. When building with Visual Studio or MingW-gcc, I get linking errors on two of the functions, and adding an 'extern "C"' qualifier resolved the error. How is this possible? Header file, "some_header.h": // Definition of struct DEMO_GLOBAL_DATA omitted DWORD WINAPI ThreadFunction(LPVOID lpData); void WriteLogString(void *pUserData, const char *pString, unsigned long nStringLen); void CheckValid(DEMO_GLOBAL_DATA *pData); int HandleStart(DEMO_GLOBAL_DATA * pDAta, TCHAR * pLogFileName); void HandleEnd(DEMO_GLOBAL_DATA *pData); C++ file, "some_implementation.cpp" #include "some_header.h" DWORD WINAPI ThreadFunction(LPVOID lpData) { /* omitted */ } void WriteLogString(void *pUserData, const char *pString, unsigned long nStringLen) { /* omitted */ } void CheckValid(DEMO_GLOBAL_DATA *pData) { /* omitted */ } int HandleStart(DEMO_GLOBAL_DATA * pDAta, TCHAR * pLogFileName) { /* omitted */ } void HandleEnd(DEMO_GLOBAL_DATA *pData) { /* omitted */ } The implementations compile without warnings, but when linking with the UI code that calls these, I get a normal error LNK2001: unresolved external symbol "int __cdecl HandleStart(struct _DEMO_GLOBAL_DATA *, wchar_t *) error LNK2001: unresolved external symbol "void __cdecl CheckValid(struct _DEMO_MAIN_GLOBAL_DATA * What really confuses me, now, is that only these two functions (HandleStart and CheckValid) seems to be built with C linkage. Explicitly adding "extern 'C'" declarations for only these two resolved the linking error, and the application builds and runs. Adding "extern 'C'" on some other function, such as HandleEnd, introduces a new linking error, so that one is obviously compiled correctly. The implementation file is never modified in any of this, only the prototypes.

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• Coordinating typedefs and structs in std::multiset (C++)

  - by Sarah

  I'm not a professional programmer, so please don't hesitate to state the obvious. My goal is to use a std::multiset container (typedef EventMultiSet) called currentEvents to organize a list of structs, of type Event, and to have members of class Host occasionally add new Event structs to currentEvents. The structs are supposed to be sorted by one of their members, time. I am not sure how much of what I am trying to do is legal; the g++ compiler reports (in "Host.h") "error: 'EventMultiSet' has not been declared." Here's what I'm doing: // Event.h struct Event { public: bool operator < ( const Event & rhs ) const { return ( time < rhs.time ); } double time; int eventID; int hostID; }; // Host.h ... void calcLifeHist( double, EventMultiSet * ); // produces compiler error ... void addEvent( double, int, int, EventMultiSet * ); // produces compiler error // Host.cpp #include "Event.h" ... // main.cpp #include "Event.h" ... typedef std::multiset< Event, std::less< Event > > EventMultiSet; EventMultiSet currentEvents; EventMultiSet * cePtr = &currentEvents; ... Major questions Where should I include the EventMultiSet typedef? Are my EventMultiSet pointers obviously problematic? Is the compare function within my Event struct (in theory) okay? Thank you very much in advance.

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• Prog error: for replacing spaces with "%20"

  - by assasinC

  below is the prog i am compiling for replacing spaces with "%20" but when I run it output window shows blank and a message "arrays5.exe has occurred a prob" #include <iostream> #include<cstring> using namespace std; void method(char str[], int len) //replaces spaces with "%20" { int spaces, newlen,i; for (i=0;i<len;i++) if(str[i]==' ') spaces++; newlen=len+spaces*2; str[newlen]=0; for (i=len-1;i>=0;i--) { if(str[i]==' ') { str[newlen-1]='0'; str[newlen-2]='2'; str[newlen-3]='%'; newlen=newlen-3; } else { str[newlen-1]=str[i]; newlen=newlen-1; } } } int main() { char str[20]="sa h "; method(str,5); cout <<str<<endl; return 0; } Please help me finding the error.Thanks

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• Saving ntext data from SQL Server to file directory using asp

  - by April

  A variety of files (pdf, images, etc.) are stored in a ntext field on a MS SQL Server. I am not sure what type is in this field, other than it shows question marks and undefined characters, I am assuming they are binary type. The script is supposed to iterate through the rows and extract and save these files to a temp directory. "filename" and "contenttype" are given, and "data" is whatever is in the ntext field. I have tried several solutions: 1) data.SaveToFile "/temp/"&filename, 2 Error: Object required: '????????????????????' ??? 2) File.WriteAllBytes "/temp/"&filename, data Error: Object required: 'File' I have no idea how to import this, or the Server for MapPath. (Cue: what a noob!) 3) Const adTypeBinary = 1 Const adSaveCreateOverWrite = 2 Dim BinaryStream Set BinaryStream = CreateObject("ADODB.Stream") BinaryStream.Type = adTypeBinary BinaryStream.Open BinaryStream.Write data BinaryStream.SaveToFile "C:\temp\" & filename, adSaveCreateOverWrite Error: Arguments are of the wrong type, are out of acceptable range, or are in conflict with one another. 4) Response.ContentType = contenttype Response.AddHeader "content-disposition","attachment;" & filename Response.BinaryWrite data response.end This works, but the file should be saving to the server instead of popping up save-as dialog. I am not sure if there is a way to save the response to file. Thanks for shedding light on any of these problems!

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• Better code for accessing fields in a matlab structure array?

  - by John

  I have a matlab structure array Modles1 of size (1x180) that has fields a, b, c, ..., z. I want to understand how many distinct values there are in each of the fields. i.e. max(grp2idx([foo(:).a])) The above works if the field a is a double. {foo(:).a} needs to be used in the case where the field a is a string/char. Here's my current code for doing this. I hate having to use the eval, and what is essentially a switch statement. Is there a better way? names = fieldnames(Models1); for ix = 1 : numel(names) className = eval(['class(Models1(1).',names{ix},')']); if strcmp('double', className) || strcmp('logical',className) eval([' values = [Models1(:).',names{ix},'];']); elseif strcmp('char', className) eval([' values = {Models1(:).',names{ix},'};']); else disp(['Unrecognized class: ', className]); end % this line requires the statistics toolbox. [g, gn, gl] = grp2idx(values); fprintf('%30s : %4d\n',names{ix},max(g)); end

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• genStrAsCharArray optimisation benefits

  - by Rich

  Hi I am looking into the options available to me for optimising the performance of JBoss 5.1.0. One of the options I am looking at is setting genStrAsCharArray to true in <JBOSS_HOME>/server/<PROFILE>/deployers/jbossweb.deployer/web.xml. This affects the generation of .java code from .JSPs. The comment describes this flag as: Should text strings be generated as char arrays, to improve performance in some cases? I have a few questions about this. Is this the generation of Strings in the dynamic parts of the JSP page (ie each time the page is called) or is it the generation of Strings in the static parts (ie when the .java is built from the JSP)? "in some cases" - which cases are these? What are the situations where the performance is worse? Does this speed up the generation of the .java, the compilation of the .class or the execution of the .class? At a more technical level (and the answer to this will probably depend on the answer to part 1), why can the use of char arrays improve performance? Thanks in advance Rich

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• Memory leaks after using typeinfo::name()

  - by icabod

  I have a program in which, partly for informational logging, I output the names of some classes as they are used (specifically I add an entry to a log saying along the lines of Messages::CSomeClass transmitted to 127.0.0.1). I do this with code similar to the following: std::string getMessageName(void) const { return std::string(typeid(*this).name()); } And yes, before anyone points it out, I realise that the output of typeinfo::name is implementation-specific. According to MSDN The type_info::name member function returns a const char* to a null-terminated string representing the human-readable name of the type. The memory pointed to is cached and should never be directly deallocated. However, when I exit my program in the debugger, any "new" use of typeinfo::name() shows up as a memory leak. If I output the information for 2 classes, I get 2 memory leaks, and so on. This hints that the cached data is never being freed. While this is not a major issue, it looks messy, and after a long debugging session it could easily hide genuine memory leaks. I have looked around and found some useful information (one SO answer gives some interesting information about how typeinfo may be implemented), but I'm wondering if this memory should normally be freed by the system, or if there is something i can do to "not notice" the leaks when debugging. I do have a back-up plan, which is to code the getMessageName method myself and not rely on typeinfo::name, but I'd like to know anyway if there's something I've missed.

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• C++ compiler unable to find function (namespace related)

  - by CS student

  I'm working in Visual Studio 2008 on a C++ programming assignment. We were supplied with files that define the following namespace hierarchy (the names are just for the sake of this post, I know "namespace XYZ-NAMESPACE" is redundant): (MAIN-NAMESPACE){ a bunch of functions/classes I need to implement... (EXCEPTIONS-NAMESPACE){ a bunch of exceptions } (POINTER-COLLECTIONS-NAMESPACE){ Set and LinkedList classes, plus iterators } } The MAIN-NAMESPACE contents are split between a bunch of files, and for some reason which I don't understand the operator<< for both Set and LinkedList is entirely outside of the MAIN-NAMESPACE (but within Set and LinkedList's header file). Here's the Set version: template<typename T> std::ostream& operator<<(std::ostream& os, const MAIN-NAMESPACE::POINTER-COLLECTIONS-NAMESPACE::Set<T>& set) Now here's the problem: I have the following data structure: Set A Set B Set C double num It's defined to be in a class within MAIN-NAMESPACE. When I create an instance of the class, and try to print one of the sets, it tells me that: error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'const MAIN-NAMESPACE::POINTER-COLLECTIONS-NAMESPACE::Set' (or there is no acceptable conversion) However, if I just write a main() function, and create Set A, fill it up, and use the operator- it works. Any idea what is the problem? (note: I tried any combination of using and include I could think of).

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• transfering a container of data to different classes

  - by user340699

  I am passing a vector of bids from Trader class to Simulator class.which class then passes it on to the auctioneer class.something seems messed up, can anyone spot it please. Below is part of the code: Error: 199 expected primary-expression before '&' token //Class of Origin of the vector. class Trader { private: int nextBidId; public: Trader(); ~Trader(){}; Bid getNextBid(); Bid getNextBid(int trdId, int qty, int price, char type); void loadRange( vector <Bid> & bids ) {} ; void loadRange(BidList &, int trdId, int qty, int price, char type, int size); }; //To be received by the Simulator class Simulator { vector <Bid> list; Trader trader; Auctioneer auctioneer; public: void run(); }; // Passing the vector into a function in simulator Simulator::accept_bids(bid_vector::const_iterator begin, bid_vector::const_iterator end){ vector<Bid>::iterator itr; } //Its journey should end with the Auctioneer. who displays the data class Auctioneer { public: vector <Bid>v2;// created a new vector to hold the objects void accept_bids(vector<Bid> & bids); void displayBids(){return bids} };

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• Eliminate full table scan due to BETWEEN (and GROUP BY)

  - by Dave Jarvis

  Description According to the explain command, there is a range that is causing a query to perform a full table scan (160k rows). How do I keep the range condition and reduce the scanning? I expect the culprit to be: Y.YEAR BETWEEN 1900 AND 2009 AND Code Here is the code that has the range condition (the STATION_DISTRICT is likely superfluous). SELECT COUNT(1) as MEASUREMENTS, AVG(D.AMOUNT) as AMOUNT, Y.YEAR as YEAR, MAKEDATE(Y.YEAR,1) as AMOUNT_DATE FROM CITY C, STATION S, STATION_DISTRICT SD, YEAR_REF Y FORCE INDEX(YEAR_IDX), MONTH_REF M, DAILY D WHERE -- For a specific city ... -- C.ID = 10663 AND -- Find all the stations within a specific unit radius ... -- 6371.009 * SQRT( POW(RADIANS(C.LATITUDE_DECIMAL - S.LATITUDE_DECIMAL), 2) + (COS(RADIANS(C.LATITUDE_DECIMAL + S.LATITUDE_DECIMAL) / 2) * POW(RADIANS(C.LONGITUDE_DECIMAL - S.LONGITUDE_DECIMAL), 2)) ) <= 50 AND -- Get the station district identification for the matching station. -- S.STATION_DISTRICT_ID = SD.ID AND -- Gather all known years for that station ... -- Y.STATION_DISTRICT_ID = SD.ID AND -- The data before 1900 is shaky; insufficient after 2009. -- Y.YEAR BETWEEN 1900 AND 2009 AND -- Filtered by all known months ... -- M.YEAR_REF_ID = Y.ID AND -- Whittled down by category ... -- M.CATEGORY_ID = '003' AND -- Into the valid daily climate data. -- M.ID = D.MONTH_REF_ID AND D.DAILY_FLAG_ID <> 'M' GROUP BY Y.YEAR Update The SQL is performing a full table scan, which results in MySQL performing a "copy to tmp table", as shown here: +----+-------------+-------+--------+-----------------------------------+--------------+---------+-------------------------------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+-----------------------------------+--------------+---------+-------------------------------+--------+-------------+ | 1 | SIMPLE | C | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 1 | SIMPLE | Y | range | YEAR_IDX | YEAR_IDX | 4 | NULL | 160422 | Using where | | 1 | SIMPLE | SD | eq_ref | PRIMARY | PRIMARY | 4 | climate.Y.STATION_DISTRICT_ID | 1 | Using index | | 1 | SIMPLE | S | eq_ref | PRIMARY | PRIMARY | 4 | climate.SD.ID | 1 | Using where | | 1 | SIMPLE | M | ref | PRIMARY,YEAR_REF_IDX,CATEGORY_IDX | YEAR_REF_IDX | 8 | climate.Y.ID | 54 | Using where | | 1 | SIMPLE | D | ref | INDEX | INDEX | 8 | climate.M.ID | 11 | Using where | +----+-------------+-------+--------+-----------------------------------+--------------+---------+-------------------------------+--------+-------------+ Related http://dev.mysql.com/doc/refman/5.0/en/how-to-avoid-table-scan.html http://dev.mysql.com/doc/refman/5.0/en/where-optimizations.html http://stackoverflow.com/questions/557425/optimize-sql-that-uses-between-clause Thank you!

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• Is the C++ compiler optimizer allowed to break my destructor ability to be called multiple times?

  - by sharptooth

  We once had an interview with a very experienced C++ developer who couldn't answer the following question: is it necessary to call the base class destructor from the derived class destructor in C++? Obviously the answer is no, C++ will call the base class destructor automagically anyway. But what if we attempt to do the call? As I see it the result will depend on whether the base class destructor can be called twice without invoking erroneous behavior. For example in this case: class BaseSafe { public: ~BaseSafe() { } private: int data; }; class DerivedSafe { public: ~DerivedSafe() { BaseSafe::~BaseSafe(); } }; everything will be fine - the BaseSafe destructor can be called twice safely and the program will run allright. But in this case: class BaseUnsafe { public: BaseUnsafe() { buffer = new char[100]; } ~BaseUnsafe () { delete[] buffer; } private: char* buffer; }; class DerivedUnsafe { public: ~DerivedUnsafe () { BaseUnsafe::~BaseUnsafe(); } }; the explicic call will run fine, but then the implicit (automagic) call to the destructor will trigger double-delete and undefined behavior. Looks like it is easy to avoid the UB in the second case. Just set buffer to null pointer after delete[]. But will this help? I mean the destructor is expected to only be run once on a fully constructed object, so the optimizer could decide that setting buffer to null pointer makes no sense and eliminate that code exposing the program to double-delete. Is the compiler allowed to do that?

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• Currently using View, Should I use a hard table instead?

  - by 1001010101

  I am currently debating whether my table, mapping_uGroups_uProducts, which is a view formed by the following table: CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `db`.`mapping_uGroups_uProducts` AS select distinct `X`.`upID` AS `upID`,`Z`.`ugID` AS `ugID` from ((`db`.`mapping_uProducts_Products` `X` join `db`.`productsInfo` `Y` on((`X`.`pID` = `Y`.`pID`))) join `db`.`mapping_uGroups_Groups` `Z` on((`Y`.`gID` = `Z`.`gID`))); My current query is: SELECT upID FROM uProductsInfo \ JOIN fs_uProducts USING (upID) column \ JOIN mapping_uGroups_uProducts USING (upID) -- could be faster if we use hard table and index \ JOIN mapping_fs_key USING (fsKeyID) \ WHERE fsName="OVERALL" \ AND ugID=1 \ ORDER BY score DESC \ LIMIT 0,30; which is pretty slow. (for 30 results, it requires about 10 secondes). I think the reason for my query being so slow is definitely due to the fact that that particular query relies on a VIEW which has no index to speed things up. +----+-------------+----------------+--------+----------------+---------+---------+---------------------------------------+-------+---------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------------+--------+----------------+---------+---------+---------------------------------------+-------+---------------------------------+ | 1 | PRIMARY | mapping_fs_key | const | PRIMARY,fsName | fsName | 386 | const | 1 | Using temporary; Using filesort | | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 19706 | Using where | | 1 | PRIMARY | uProductsInfo | eq_ref | PRIMARY | PRIMARY | 4 | mapping_uGroups_uProducts.upID | 1 | Using index | | 1 | PRIMARY | fs_uProducts | ref | upID | upID | 4 | db.uProductsInfo.upID | 221 | Using where | | 2 | DERIVED | X | ALL | PRIMARY | NULL | NULL | NULL | 40772 | Using temporary | | 2 | DERIVED | Y | eq_ref | PRIMARY | PRIMARY | 4 | db.X.pID | 1 | Distinct | | 2 | DERIVED | Z | ref | PRIMARY | PRIMARY | 4 | db.Y.gID | 2 | Using index; Distinct | +----+-------------+----------------+--------+----------------+---------+---------+---------------------------------------+-------+---------------------------------+ 7 rows in set (0.48 sec) The explain here looks pretty cryptic, and I don't know whether I should drop view and write a script to just insert everything in the view to a hard table. ( obviously, it will lose the flexibility of the view since the mapping changes quite frequently). Does anyone have any idea to how I can optimize my schema better?

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• c# finding matching words in table column using Linq2Sql

  - by David Liddle

  I am trying to use Linq2Sql to return all rows that contain values from a list of strings. The linq2sql class object has a string property that contains words separated by spaces. public class MyObject { public string MyProperty { get; set; } } Example MyProperty values are: MyObject1.MyProperty = "text1 text2 text3 text4" MyObject2.MyProperty = "text2" For example, using a string collection, I pass the below list var list = new List<>() { "text2", "text4" } This would return both items in my example above as they both contain "text2" value. I attempted the following using the below code however, because of my extension method the Linq2Sql cannot be evaluated. public static IQueryable<MyObject> WithProperty(this IQueryable<MyProperty> qry, IList<string> p) { return from t in qry where t.MyProperty.Contains(p, ' ') select t; } I also wrote an extension method public static bool Contains(this string str, IList<string> list, char seperator) { if (String.IsNullOrEmpty(str) || list == null) return false; var splitStr = str.Split(new char[] { seperator }, StringSplitOptions.RemoveEmptyEntries); foreach (string s in splitStr) foreach (string l in list) if (String.Compare(s, l, true) == 0) return true; return false; } Any help or ideas on how I could achieve this?

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