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• How to treat Base* pointer as Derived<T>* pointer?

  - by dehmann

  I would like to store pointers to a Base class in a vector, but then use them as function arguments where they act as a specific class, see here: #include <iostream> #include <vector> class Base {}; template<class T> class Derived : public Base {}; void Foo(Derived<int>* d) { std::cerr << "Processing int" << std::endl; } void Foo(Derived<double>* d) { std::cerr << "Processing double" << std::endl; } int main() { std::vector<Base*> vec; vec.push_back(new Derived<int>()); vec.push_back(new Derived<double>()); Foo(vec[0]); Foo(vec[1]); delete vec[0]; delete vec[1]; return 0; } This doesn't compile: error: call of overloaded 'Foo(Base*&)' is ambiguous Is it possible to make it work? I need to process the elements of the vector differently, according to their int, double, etc. types.

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• Creating ostream manipulators for a specific class

  - by petersohn

  I have a class that is derived from ostream: class my_ostream: public std::ostream { // ... } I want to make a manipulator (for example do_something), that works specifically to this class, like this: my_ostream s; s << "some text" << do_something << "some more text"; I did the following: std::ostream &do_something(std::ostream &os) { my_ostream *s = dynamic_cast<my_ostream*>(&os); if (s != NULL) { // do something } return os; } This works, but is rather ugly. I tried the following: my_ostream &do_something(my_ostream &s) { // do something return s; } This doesn't work. I also tried another approach: class my_ostream: public std::ostream { // ... my_ostream &operator<<(const do_something & x) { // do something return *this; } } This still doesn't work.

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• Poco C++ library on OSX 10.8.2: Undefined symbols for architecture x86_64

  - by Arman

  I'm trying to use Poco C++ library to do the simple http requests in C++ on Mac OS X 10.8.2. I installed Poco, copy-pasted the http_request.cc code from this tutorial, ran it with 'g++ -o http_get http_get.cc -lPocoNet', but got: Undefined symbols for architecture x86_64: "Poco::StreamCopier::copyStream(std::basic_istream<char, std::char_traits<char> >&, std::basic_ostream<char, std::char_traits<char> >&, unsigned long)", referenced from: _main in ccKuZb1g.o "Poco::URI::URI(char const*)", referenced from: _main in ccKuZb1g.o "Poco::URI::~URI()", referenced from: _main in ccKuZb1g.o "Poco::URI::getPathAndQuery() const", referenced from: _main in ccKuZb1g.o "Poco::URI::getPort() const", referenced from: _main in ccKuZb1g.o "Poco::Exception::displayText() const", referenced from: _main in ccKuZb1g.o "typeinfo for Poco::Exception", referenced from: GCC_except_table1 in ccKuZb1g.o ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status Have been struggling with this for couple of hours. Any idea how to fix this? Thanks in advance!

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• Casting a container of shared_ptr

  - by Jamie Cook

  Hi all, I have a method void foo(list<shared_ptr<Base>>& myList); Which I'm trying to call with a two different types of lists, one of DerivedClass1 and one of DerivedClass2 list<shared_ptr<DerivedClass1>> myList1; foo(myList1); list<shared_ptr<DerivedClass2>> myList2; foo(myList2); However this obviously generates a compiler error error: a reference of type "std::list<boost::shared_ptr<Base>, std::allocator<boost::shared_ptr<Base>>> &" (not const-qualified) cannot be initialized with a value of type "std::list<boost::shared_ptr<DerivedClass1>, std::allocator<boost::shared_ptr<DerivedClass1>>>" Is there any easy way to cast a container of shared_ptr? Of alternate containers that can accomplish this?

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• Closure and nested lambdas in C++0x

  - by DanDan

  Using C++0x, how do I capture a variable when I have a lambda within a lambda? For example: std::vector<int> c1; int v = 10; <--- I want to capture this variable std::for_each( c1.begin(), c1.end(), [v](int num) <--- This is fine... { std::vector<int> c2; std::for_each( c2.begin(), c2.end(), [v](int num) <--- error on this line, how do I recapture v? { // Do something }); });

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• Cross-platform iteration of Unicode string

  - by kizzx2

  I want to iterate each character of a Unicode string, treating each surrogate pair and combining character sequence as a single unit (one grapheme). Example The text "??????" is comprised of the code points: U+0928, U+092E, U+0938, U+094D, U+0924, U+0947, of which, U+0938 and U+0947 are combining marks. static void Main(string[] args) { const string s = "??????"; Console.WriteLine(s.Length); // Ouptuts "6" var l = 0; var e = System.Globalization.StringInfo.GetTextElementEnumerator(s); while(e.MoveNext()) l++; Console.WriteLine(l); // Outputs "4" } So there we have it in .NET. We also have Win32's CharNextW() #include <Windows.h> #include <iostream> #include <string> int main() { const wchar_t * s = L"??????"; std::cout << std::wstring(s).length() << std::endl; // Gives "6" int l = 0; while(CharNextW(s) != s) { s = CharNextW(s); ++l; } std::cout << l << std::endl; // Gives "4" return 0; } Question Both ways I know of are specific to Microsoft. Are there portable ways to do it? I heard about ICU but I couldn't find something related quickly (UnicodeString(s).length() still gives 6). Would be an acceptable answer to point to the related function/module in ICU. C++ doesn't have a notion of Unicode, so a lightweight cross-platform library for dealing with these issues would make an acceptable answer.

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• Problem with "moveable-only types" in VC++ 2010

  - by Luc Touraille

  I recently installed Visual Studio 2010 Professional RC to try it out and test the few C++0x features that are implemented in VC++ 2010. I instantiated a std::vector of std::unique_ptr, without any problems. However, when I try to populate it by passing temporaries to push_back, the compiler complains that the copy constructor of unique_ptr is private. I tried inserting an lvalue by moving it, and it works just fine. #include <utility> #include <vector> int main() { typedef std::unique_ptr<int> int_ptr; int_ptr pi(new int(1)); std::vector<int_ptr> vec; vec.push_back(std::move(pi)); // OK vec.push_back(int_ptr(new int(2)); // compiler error } As it turns out, the problem is neither unique_ptr nor vector::push_back but the way VC++ resolves overloads when dealing with rvalues, as demonstrated by the following code: struct MoveOnly { MoveOnly() {} MoveOnly(MoveOnly && other) {} private: MoveOnly(const MoveOnly & other); }; void acceptRValue(MoveOnly && mo) {} int main() { acceptRValue(MoveOnly()); // Compiler error } The compiler complains that the copy constructor is not accessible. If I make it public, the program compiles (even though the copy constructor is not defined). Did I misunderstand something about rvalue references, or is it a (possibly known) bug in VC++ 2010 implementation of this feature?

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

  - by Fabrizio Silvestri

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

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• overloading new/delete problem

  - by hidayat

  This is my scenario, Im trying to overload new and delete globally. I have written my allocator class in a file called allocator.h. And what I am trying to achieve is that if a file is including this header file, my version of new and delete should be used. So in a header file "allocator.h" i have declared the two functions extern void* operator new(std::size_t size); extern void operator delete(void *p, std::size_t size); I the same header file I have a class that does all the allocator stuff, class SmallObjAllocator { ... }; I want to call this class from the new and delete functions and I would like the class to be static, so I have done this: template<unsigned dummy> struct My_SmallObjectAllocatorImpl { static SmallObjAllocator myAlloc; }; template<unsigned dummy> SmallObjAllocator My_SmallObjectAllocatorImpl<dummy>::myAlloc(DEFAULT_CHUNK_SIZE, MAX_OBJ_SIZE); typedef My_SmallObjectAllocatorImpl<0> My_SmallObjectAllocator; and in the cpp file it looks like this: allocator.cc void* operator new(std::size_t size) { std::cout << "using my new" << std::endl; if(size > MAX_OBJ_SIZE) return malloc(size); else return My_SmallObjectAllocator::myAlloc.allocate(size); } void operator delete(void *p, std::size_t size) { if(size > MAX_OBJ_SIZE) free(p); else My_SmallObjectAllocator::myAlloc.deallocate(p, size); } The problem is when I try to call the constructor for the class SmallObjAllocator which is a static object. For some reason the compiler are calling my overloaded function new when initializing it. So it then tries to use My_SmallObjectAllocator::myAlloc.deallocate(p, size); which is not defined so the program crashes. So why are the compiler calling new when I define a static object? and how can I solve it?

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• Segmentation Fault when trying to push a string to the back of a list.

  - by user308012

  I am trying to write a logger class for my C++ calculator, but I'm experiencing a problem while trying to push a string into a list. I have tried researching this issue and have found some information on this, but nothing that seems to help with my problem. I am using a rather basic C++ compiler, with little debugging utilities and I've not used C++ in quite some time (even then it was only a small amount). My code: #ifndef _LOGGER_H_ #define _LOGGER_H_ #include <iostream> #include <list> #include <string> using std::cout; using std::cin; using std::endl; using std::list; using std::string; class Logger { private: list<string> *mEntries; public: Logger() { // Initialize the entries list mEntries = new list<string>(); } ~Logger() { // Release the list mEntries->clear(); delete mEntries; } // Public Methods void WriteEntry(string entry) { // *** BELOW LINE IS MARKED WITH THE ERROR *** mEntries->push_back(string(entryData)); } void DisplayEntries() { cout << endl << "**********************" << endl << "* Logger Entries *" << endl << "**********************" << endl << endl; for(list<string>::iterator it = mEntries->begin(); it != mEntries->end(); it++) { cout << *it << endl; } } }; #endif I am calling the WriteEntry method by simply passing in a string, like so: mLogger->WriteEntry("Testing"); Any advice on this would be greatly appreciated.

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• c++ template: 'is not derived from type'

  - by Allan

  I do not understand why this code is not valid: #include <vector> template <typename T> class A{ public: A() { v.clear(); } std::vector<A<T> *>::const_iterator begin(){ return v.begin(); } private: std::vector<A<T> *> v; }; When compiling it with gcc, it get the following error: test.cpp:8: error: type 'std::vector<A<T>*, std::allocator<A<T>*> >' is not derived from type 'A<T>' test.cpp:8: error: expected ';' before 'begin' test.cpp:12: error: expected `;' before 'private' What is wrong, and how to fix it??

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• How can I return an object into PHP userspace from my extension?

  - by John Factorial

  I have a C++ object, Graph, which contains a property named cat of type Category. I'm exposing the Graph object to PHP in an extension I'm writing in C++. As long as the Graph's methods return primitives like boolean or long, I can use the Zend RETURN_*() macros (e.g. RETURN_TRUE(); or RETURN_LONG(123);. But how can I make Graph-getCategory(); return a Category object for the PHP code to manipulate? I'm following the tutorial over at http://devzone.zend.com/article/4486, and here's the Graph code I have so far: #include "php_getgraph.h" zend_object_handlers graph_object_handlers; struct graph_object { zend_object std; Graph *graph; }; zend_class_entry *graph_ce; #define PHP_CLASSNAME "WFGraph" ZEND_BEGIN_ARG_INFO_EX(php_graph_one_arg, 0, 0, 1) ZEND_END_ARG_INFO() ZEND_BEGIN_ARG_INFO_EX(php_graph_two_args, 0, 0, 2) ZEND_END_ARG_INFO() void graph_free_storage(void *object TSRMLS_DC) { graph_object *obj = (graph_object*)object; delete obj-graph; zend_hash_destroy(obj-std.properties); FREE_HASHTABLE(obj-std.properties); efree(obj); } zend_object_value graph_create_handler(zend_class_entry *type TSRMLS_DC) { zval *tmp; zend_object_value retval; graph_object *obj = (graph_object*)emalloc(sizeof(graph_object)); memset(obj, 0, sizeof(graph_object)); obj-std.ce = type; ALLOC_HASHTABLE(obj-std.properties); zend_hash_init(obj-std.properties, 0, NULL, ZVAL_PTR_DTOR, 0); zend_hash_copy(obj-std.properties, &type-default_properties, (copy_ctor_func_t)zval_add_ref, (void*)&tmp, sizeof(zval*)); retval.handle = zend_objects_store_put(obj, NULL, graph_free_storage, NULL TSRMLS_CC); retval.handlers = &graph_object_handlers; return retval; } PHP_METHOD(Graph, __construct) { char *perspectives; int perspectives_len; Graph *graph = NULL; zval *object = getThis(); if(zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &perspectives, &perspectives_len) == FAILURE) { RETURN_NULL(); } graph = new Graph(perspectives); graph_object *obj = (graph_object*)zend_object_store_get_object(object TSRMLS_CC); obj-graph = graph; } PHP_METHOD(Graph, hasCategory) { long perspectiveId; Graph *graph; graph_object *obj = (graph_object*)zend_object_store_get_object(getThis() TSRMLS_CC); graph = obj-graph; if (graph == NULL) { RETURN_NULL(); } if(zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "l", &perspectiveId) == FAILURE) { RETURN_NULL(); } RETURN_BOOL(graph-hasCategory(perspectiveId)); } PHP_METHOD(Graph, getCategory) { // what to do here? RETURN_TRUE; } function_entry php_getgraph_functions[] = { PHP_ME(Graph,__construct,NULL,ZEND_ACC_PUBLIC|ZEND_ACC_CTOR) PHP_ME(Graph,hasCategory,php_graph_one_arg,ZEND_ACC_PUBLIC) PHP_ME(Graph,getCategory,php_graph_one_arg,ZEND_ACC_PUBLIC) { NULL, NULL, NULL } }; PHP_MINIT_FUNCTION(getgraph) { zend_class_entry ce; INIT_CLASS_ENTRY(ce, PHP_CLASSNAME, php_getgraph_functions); graph_ce = zend_register_internal_class(&ce TSRMLS_CC); graph_ce-create_object = graph_create_handler; memcpy(&graph_object_handlers, zend_get_std_object_handlers(), sizeof(zend_object_handlers)); graph_object_handlers.clone_obj = NULL; return SUCCESS; } zend_module_entry getgraph_module_entry = { #if ZEND_MODULE_API_NO = 20010901 STANDARD_MODULE_HEADER, #endif PHP_GETGRAPH_EXTNAME, NULL, /* Functions */ PHP_MINIT(getgraph), NULL, /* MSHUTDOWN */ NULL, /* RINIT */ NULL, /* RSHUTDOWN */ NULL, /* MINFO */ #if ZEND_MODULE_API_NO = 20010901 PHP_GETGRAPH_EXTVER, #endif STANDARD_MODULE_PROPERTIES }; #ifdef COMPILE_DL_GETGRAPH extern "C" { ZEND_GET_MODULE(getgraph) } #endif

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• Better way of enforcing this template?

  - by Dennis Ritchie

  Currently, I have a function template like this that converts a vector into a string (just a natural string, separating the elements with a comma): //the type T must be passable into std::to_string template<typename T> std::string vec_to_str(const std::vector<T> &vec); As you can see, this is only meant for vectors whose elements can be passed into the built-in std::to_string function (such as int, double, etc.) Is it considered a good practice to document with comments the allowed T? If not, what should I do? Is it possible to enforce this in a better way?

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• Circumvent c++ null-terminated string frustration

  - by ypnos

  I'm using boost::program_options and it suffers from the same as many other c++ libs, even std itself: It still uses C-style null-terminated strings, because nobody really likes the weak std::string. The method in question is: options_description_easy_init& operator()(const char* name, const value_semantic* s, const char* description); The typical use case is just fine: options.add_options() ("graphical", bool_switch(&isGraphical)->default_value(false), "Show any graphical output during runtime") However, I need the name of the option to be set dynamically. The reason is that in some cases I nead a custom prefix, which is added to the string by my function std::string key(const std::string& k): options.add_options() (key("graphical"), bool_switch(&isGraphical)->default_value(false), "Show any graphical output during runtime") This fails. I could now use c_str() on the std::string but that's evil -- I don't know how long program_options keeps the variable around and if my string is still alive when needed. I could also reserve memory in a buffer etc. and hand in that. The buffer is never freed and it sucks/is evil. Is there anything else I can do to circumvent the C-style string mess in this situation?

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• Why boost::recursive_mutex is not working as expected?

  - by Kjir

  I have a custom class that uses boost mutexes and locks like this (only relevant parts): template<class T> class FFTBuf { public: FFTBuf(); [...] void lock(); void unlock(); private: T *_dst; int _siglen; int _processed_sums; int _expected_sums; int _assigned_sources; bool _written; boost::recursive_mutex _mut; boost::unique_lock<boost::recursive_mutex> _lock; }; template<class T> FFTBuf<T>::FFTBuf() : _dst(NULL), _siglen(0), _expected_sums(1), _processed_sums(0), _assigned_sources(0), _written(false), _lock(_mut, boost::defer_lock_t()) { } template<class T> void FFTBuf<T>::lock() { std::cerr << "Locking" << std::endl; _lock.lock(); std::cerr << "Locked" << std::endl; } template<class T> void FFTBuf<T>::unlock() { std::cerr << "Unlocking" << std::endl; _lock.unlock(); } If I try to lock more than once the object from the same thread, I get an exception (lock_error): #include "fft_buf.hpp" int main( void ) { FFTBuf<int> b( 256 ); b.lock(); b.lock(); b.unlock(); b.unlock(); return 0; } This is the output: sb@dex $ ./src/test Locking Locked Locking terminate called after throwing an instance of 'boost::lock_error' what(): boost::lock_error zsh: abort ./src/test Why is this happening? Am I understanding some concept incorrectly?

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• Qt and finding partial matches in a QList

  - by ExpatEgghead

  I have a struct viz: struct NameKey { std::string fullName; std::string probeName; std::string format; std::string source; } which are held in a QList: QList<NameKey> keyList; what I need to do is find an occurence in keyList of a partial match where the search is for a NameKey that only has two members filled. All the keyList entries are full NameKey's. My current implementation is , well, boring in the extreme with too many if's and conditions. So, If I have a DataKey with a fullName and a format I need to find all the occurences in keyList which match. Any useful Qt/boost things available?

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• Vector iterators in for loops, return statements, warning, c++

  - by Crystal

  Had 3 questions regarding a hw assignment for C++. The goal was to create a simple palindrome method. Here is my template for that: #ifndef PALINDROME_H #define PALINDROME_H #include <vector> #include <iostream> #include <cmath> template <class T> static bool palindrome(const std::vector<T> &input) { std::vector<T>::const_iterator it = input.begin(); std::vector<T>::const_reverse_iterator rit = input.rbegin(); for (int i = 0; i < input.size()/2; i++, it++, rit++) { if (!(*it == *rit)) { return false; } } return true; } template <class T> static void showVector(const std::vector<T> &input) { for (std::vector<T>::const_iterator it = input.begin(); it != input.end(); it++) { std::cout << *it << " "; } } #endif Regarding the above code, can you have more than one iterator declared in the first part of the for loop? I tried defining both the "it" and "rit" in the palindrome() method, and I kept on getting an error about needing a "," before rit. But when I cut and paste outside the for loop, no errors from the compiler. (I'm using VS 2008). Second question, I pretty much just brain farted on this one. But is the way I have my return statements in the palindrome() method ok? In my head, I think it works like, once the *it and *rit do not equal each other, then the function returns false, and the method exits at this point. Otherwise if it goes all the way through the for loop, then it returns true at the end. I totally brain farted on how return statements work in if blocks and I tried looking up a good example in my book and I couldn't find one. Finally, I get this warnings: \palindrome.h(14) : warning C4018: '<' : signed/unsigned mismatch Now is that because I run my for loop until (i < input.size()/2) and the compiler is telling me that input can be negative? Thanks!

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• Looking at the C++ new[] cookie. How portable is this code?

  - by carleeto

  I came up with this as a quick solution to a debugging problem - I have the pointer variable and its type, I know it points to an array of objects allocated on the heap, but I don't know how many. So I wrote this function to look at the cookie that stores the number of bytes when memory is allocated on the heap. template< typename T > int num_allocated_items( T *p ) { return *((int*)p-4)/sizeof(T); } //test #include <iostream> int main( int argc, char *argv[] ) { using std::cout; using std::endl; typedef long double testtype; testtype *p = new testtype[ 45 ]; //prints 45 std::cout<<"num allocated = "<<num_allocated_items<testtype>(p)<<std::endl; delete[] p; return 0; } I'd like to know just how portable this code is.

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• What's the best way to return a subset of a list

  - by Pikrass

  I have a list of tasks. A task is defined by a name, a due date and a duration. My TaskManager class handles a std::list<Task> sorted by due date. It has to provide a way to get the tasks due for a specific date. How would you implement that ? I think a good way (from API point of view) would be to provide a std::list<Task>::iterator pair. So I would have a TaskManager::begin(date) method. Do you think this method should get the iterator by iterating from the start of the list until it finds the first task due on that date, or by getting it from a std::map<date, std::list<Task>::iterator> (but then we have to keep it up-to-date when adding or removing tasks) ? And then, how could I implement the TaskManager::end(date) method ?

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• C++ associative array with arbitrary types for values

  - by Gerald Kaszuba

  What is the best way to have an associative array with arbitrary value types for each key in C++? Currently my plan is to create a "value" class with member variables of the types I will be expecting. For example: class Value { int iValue; Value(int v) { iValue = v; } std::string sValue; Value(std::string v) { sValue = v; } SomeClass *cValue; Value(SomeClass *v) { cValue = c; } }; std::map<std::string, Value> table; A downside with this is you have to know the type when accessing the "Value". i.e.: table["something"] = Value(5); SomeClass *s = table["something"].cValue; // broken pointer Also the more types that are put in Value, the more bloated the array will be. Any better suggestions?

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• How to speed-up a simple method (preferably without changing interfaces or data structures)?

  - by baol

  I have some data structures: all_unordered_m is a big vector containing all the strings I need (all different) ordered_m is a small vector containing the indexes of a subset of the strings (all different) in the former vector position_m maps the indexes of objects from the first vector to their position in the second one. The string_after(index, reverse) method returns the string referenced by ordered_m after all_unordered_m[index]. ordered_m is considered circular, and is explored in natural or reverse order depending on the second parameter. The code is something like the following: struct ordered_subset { // [...] std::vector<std::string>& all_unordered_m; // size = n >> 1 std::vector<size_t> ordered_m; // size << n std::tr1::unordered_map<size_t, size_t> position_m; const std::string& string_after(size_t index, bool reverse) const { size_t pos = position_m.find(index)->second; if(reverse) pos = (pos == 0 ? orderd_m.size() - 1 : pos - 1); else pos = (pos == ordered.size() - 1 ? 0 : pos + 1); return all_unordered_m[ordered_m[pos]]; } }; Given that: I do need all of the data-structures for other purposes; I cannot change them because I need to access the strings: by their id in the all_unordered_m; by their index inside the various ordered_m; I need to know the position of a string (identified by it's position in the first vector) inside ordered_m vector; I cannot change the string_after interface without changing most of the program. How can I speed up the string_after method that is called billions of times and is eating up about 10% of the execution time?

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• How to speed-up a simple method? (possibily without changing interfaces or data structures)

  - by baol

  Hello. I have some data structures: all_unordered_mordered_m is a big vector containing all the strings I need (all different) ordered_m is a small vector containing the indexes of a subset of the strings (all different) in the former vector position_m maps the indexes of objects from the first vector to their position in the second one. The string_after(index, reverse) method returns the string referenced by ordered_m after all_unordered_m[index]. ordered_m is considered circular, and is explored in natural or reverse order depending on the second parameter. The code is something like the following: struct ordered_subset { // [...] std::vector<std::string>& all_unordered_m; // size = n >> 1 std::vector<size_t> ordered_m; // size << n std::map<size_t, size_t> position_m; // positions of strings in ordered_m const std::string& string_after(size_t index, bool reverse) const { size_t pos = position_m.find(index)->second; if(reverse) pos = (pos == 0 ? orderd_m.size() - 1 : pos - 1); else pos = (pos == ordered.size() - 1 ? 0 : pos + 1); return all_unordered_m[ordered_m[pos]]; } }; Given that: I do need all of the data-structures for other purposes; I cannot change them because I need to access the strings: by their id in the all_unordered_m; by their index inside the various ordered_m; I need to know the position of a string (identified by it's position in the first vector) inside ordered_m vector; I cannot change the string_after interface without changing most of the program. How can I speed up the string_after method that is called billions of times and is eating up about 10% of the execution time?

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• Porting Perl to C++ `print "\x{2501}" x 12;`

  - by jippie

  I am porting a program from Perl to C++ as a learning objective. I arrived at a routine that draws a table with commands like the following: Perl: print "\x{2501}" x 12; And it draws 12 times a '?' ("box drawings heavy horizontal"). Now I figured out part of the problem already: Perl: \x{}, \x00 Hexadecimal escape sequence; C++: \unnnn To print a single Unicode character: C++: printf( "\u250f\n" ); But does C++ have a smart equivalent for the 'x' operator or would it come down to a for loop? UPDATE Let me include the full source code I am trying to compile with the proposed solution. The compiler does throw an errors: g++ -Wall -Werror project.cpp -o project project.cpp: In function ‘int main(int, char**)’: project.cpp:38:3: error: ‘string’ is not a member of ‘std’ project.cpp:38:15: error: expected ‘;’ before ‘s’ project.cpp:39:3: error: ‘cout’ is not a member of ‘std’ project.cpp:39:16: error: ‘s’ was not declared in this scope #include <stdlib.h> #include <stdint.h> #include <stdio.h> #include <string.h> int main ( int argc, char *argv[] ) { if ( argc != 2 ) { fprintf( stderr , "usage: %s matrix\n", argv[0] ); exit( 2 ); } else { //std::string s(12, "\u250f" ); std::string s(12, "u" ); std::cout << s; } }

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• How to throw meaningful data in exceptions ?

  - by ZeroCool

  I would like to throw exceptions with meaningful explanation ! I thought of using variable arguments worked fine but lately I figured out it's impossible to extend the class in order to implement other exception types. So I figured out using an std::ostreamstring as a an internal buffer to deal with formatting ... but doesn't compile! I guess it has something to deal with copy constructors. Anyhow here is my piece of code: class Exception: public std::exception { public: Exception(const char *fmt, ...); Exception(const char *fname, const char *funcname, int line, const char *fmt, ...); //std::ostringstream &Get() { return os_ ; } ~Exception() throw(); virtual const char *what() const throw(); protected: char err_msg_[ERRBUFSIZ]; //std::ostringstream os_; }; The variable argumens constructor can't be inherited from! this is why I thought of std::ostringstream! Any advice on how to implement such approach ?

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• Counting number of times an item occurs in a linked list

  - by HanaCHaN92

  Here is the assignment: Here's the assignment: Implement a method countValue() that counts the number of times an item occurs in a linked list. Remember to use the STL list. int countValue(list front, const int item); Generate 20 random numbers in the range of 0 to 4, and insert each number in the linked list. Output the list by using a method which you would call writeLinkedList which you would add to the ListP.cpp. In a loop, call the method countValue() , and display the number of occurrences of each value from 0 to 4 in the list. Remember that all the above is to be included in the file ListP.ccp Run: 2 3 4 0 1 0 2 4 2 3 3 4 3 3 3 0 0 2 0 2 0 : 5, 1 : 1, 2 : 5, 3 : 6, 4 : 3 and here is what I have so far: #include<iostream> #include<list> #include<tchar.h> int countValue(list<int> front, const int item); using namespace std; int _tmain(int argc, _TCHAR* argv[]){ list<int> front; int listCount; cout << "Enter the size of the list: "; cin >> listCount; for (int i = 1; i <= listCount; i++) front.insert(rand()%5); cout << "Original List of Values: " << endl; //writeLinkedList(front, " "); cout << endl; for(int j=0;j<5;++j) cout << countValue (front,j) << endl; cout << endl; return 0; } int countValue(list<int> front, const int item) { int count0; int count1; int count2; int count3; int count4; list<int> *List; for(list<int>::iterator i = front.begin(); i != front.end(); i++) { if(List->item == 0) { count0++; } if(List->item == 1) { count1++; } if(List->item == 2) { count2++; } if(List->item == 3) { count2++; }if(List->item == 4) { count4++; } } } And here are the errors: error C2065: 'list' : undeclared identifier line 5 error C2062: type 'int' unexpected line 5 error C2661: 'std::list<_Ty>::insert' : no overloaded function takes 1 arguments line 16 error C3861: 'countValue': identifier not found line 21 IntelliSense: no instance of overloaded function "std::list<_Ty, _Ax>::insert [with _Ty=int, _Ax=std::allocator<int>]" matches the argument list line 16 IntelliSense: too few arguments in function call line 16 error C2039: 'item': is not a member of 'std::list<_Ty>' lines 34, 38, 42, 46, 49 IntelliSense: declaration is incompatible with "int countValue" (declared at line 5) line 25 IntelliSense: class "std::list<int, std:: allocator<int>>" has no member "item" lines 34, 38, 42, 46, 49 I just want to know what I've done wrong and how to fix it and also if someone could help me figure out if I'm doing the countValue function wrong or not based on the instructions I would really appreciate it. I've read the chapter in our textbook several times, looked up tutorials on youtube and on Dream in Code, and still I can not figure this out. All helpful information is appreciated!

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