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• Returning new base class when the parent class shared pointer is the return type

  - by Ben Dol

  Can you have a parent class shared pointer return type of a function and then return a new child class without it being a shared pointer? I'm not sure how shared pointers work in these situations, do they act like a regular pointer? Here is my example: BaseEventPtr Actions::getEvent(const std::string& nodeName) { if(asLowerCaseString(nodeName) == "action") return new ActionEvent(&m_interface); return nullptr; } ActionEvent is the subclass of BaseEvent in this situation. Cheers!

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• string manipulations in C

  - by Vivek27

  Following are some basic questions that I have with respect to strings in C. If string literals are stored in read-only data segment and cannot be changed after initialisation, then what is the difference between the following two initialisations. char *string = "Hello world"; const char *string = "Hello world"; When we dynamically allocate memory for strings, I see the following allocation is capable enough to hold a string of arbitary length.Though this allocation work, I undersand/beleive that it is always good practice to allocate the actual size of actual string rather than the size of data type.Please guide on proper usage of dynamic allocation for strings. char *string = (char *)malloc(sizeof(char));

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• parsing Two-dimensional array in c

  - by gitter78

  I'm trying to parse an array that looks like the one below: char *arr[][2] = { { "1", "Purple" }, { "2", "Blue" }, { "22", "Red" }, ... }; I was thinking having a loop as: char *func(char *a){ for(i = 0; i<sizeof(arr)/sizeof(arr[0]);i++){ if(strstr(a,arr[i][0])!=NULL) return arr[i][1]; } } char *out; const char *hello = "this is my 2 string"; out = func(hello); In this case, I'm trying to get the second value based on the first one: Purple, Blue Red, etc.. The question is how would go in parsing this and instead of printing out the value, return the value. UPDATE/FIXED: It has been fixed above. Thanks

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• Quick, Beginner C++ Overloading Question - Getting the compiler to perceive << is defined for a spec

  - by Francisco P.

  Hello everyone. I edited a post of mine so I coul I overloaded << for a class, Score (defined in score.h), in score.cpp. ostream& operator<< (ostream & os, const Score & right) { os << right.getPoints() << " " << right.scoreGetName(); return os; } (getPoints fetches an int attribute, getName a string one) I get this compiling error for a test in main(), contained in main.cpp binary '<<' : no operator found which takes a right-hand operand of type 'Score' (or there is no acceptable conversion) How come the compiler doesn't 'recognize' that overload as valid? (includes are proper) Thanks for your time.

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• how to copy char * into a string and vice-versa

  - by user295030

  If i pass a char * into a function. I want to then take that char * convert it to a std::string and once I get my result convert it back to char * from a std::string to show the result. I don't know how to do this for conversion ( I am not talking const char * but just char *) I am not sure how to manipulate the value of the pointer I send in. so steps i need to do take in a char * convert it into a string. take the result of that string and put it back in the form of a char * return the result such that the value should be available outside the function and not get destroyed. If possible can i see how it could be done via reference vs a pointer (whose address I pass in by value however I can still modify the value that pointer is pointing to. so even though the copy of the pointer address in the function gets destroyed i still see the changed value outside. thanks!

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• Does C++ have a static polymorphism implementation of interface that does not use vtable?

  - by gilbertc

  Does C++ have a proper implementation of interface that does not use vtable? for example class BaseInterface{ public: virtual void func() const = 0; } class BaseInterfaceImpl:public BaseInterface{ public: void func(){ std::cout<<"called."<<endl; } } BaseInterface* obj = new BaseInterfaceImpl(); obj->func(); the call to func at the last line goes to vtable to find the func ptr of BaseInterfaceImpl::func, but is there any C++ way to do that directly as the BaseInterfaceImpl is not subclassed from any other class besides the pure interface class BaseInterface? Thanks. Gil.

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• VBScript Permission Denied on CopyFile

  - by Chris

  I'm running a VBScript in SQL Agent but I get a 'Permission Denied' on line 34 (the first copy attempt). I've run this script outside SQL Agent with no problems Function Main() Const SourceDrive As String = "X:\" Dim fso Dim Today Dim FileName Dim FromFile Dim FromDrive Dim ArchivePath Set fso = CreateObject("Scripting.FileSystemObject") Today = Format(Now, "yyyyMMdd") 'To add more sources just add them to the array list Dim Sources() As Variant Sources() = Array("Item1", _ "Item2") 'To add more targets just add them to the array list Dim Targets() As Variant Targets() = Array("C:\Users\myalias\Desktop\MyToFolder", _ "C:\Users\myalias\Desktop\MyToFolder2") For i = 0 To UBound(Sources) FileName = "WebSurveyAlertCallbacks_" & Sources(i) & "_" & Today & ".xls" FromDrive = fso.BuildPath(SourceDrive, Sources(i)) FromFile = fso.BuildPath(FromDrive, FileName) ArchivePath = fso.BuildPath(FromDrive, "Archive") If fso.FileExists(FromFile) Then For t = 0 To UBound(Targets) fso.CopyFile FromFile, fso.BuildPath(Targets(t), FileName), True Next fso.CopyFile FromFile, fso.BuildPath(ArchivePath, FileName), True fso.DeleteFile FromFile End If Next Set fso = Nothing Main = DTSTaskExecResult_Success End Function

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• How to translate such AS3 class into C#?

  - by Ole Jak

  So I try to create opensource C# project for slicing FLVs I began with translating of existing project called flvslicer Can any one please help me with translating one of their classes package org.bytearray.video.events { import flash.events.Event; import flash.utils.ByteArray; public final class MergedEvent extends Event { public var time:Number; public var stream:ByteArray; public static const COMPLETE:String = "mergeComplete"; public function MergedEvent(type:String, stream:ByteArray, duration:Number) { super(type, false, false); // base this.stream = stream; this.time = duration; } } }

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• Boost::asio bug in MSVC10 - Failing BOOST_WORKAROUND in ~buffer_debug_check() in buffer.hpp

  - by shaz

  A straight compilation of example http://www.boost.org/doc/libs/1_43_0/doc/html/boost_asio/tutorial/tutdaytime3/src.html results in a runtime null pointer exception. Stack trace points to the buffer_debug_check destructor which contains this comment: // MSVC's string iterator checking may crash in a std::string::iterator // object's destructor when the iterator points to an already-destroyed // std::string object, unless the iterator is cleared first. The test #if BOOST_WORKAROUND(BOOST_MSVC, = 1400) succeeds in MSVC10 and (but) results in a null pointer exception in c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xutility line 123 _Iterator_base12& operator=(const _Iterator_base12& _Right) { // assign an iterator if (_Myproxy != _Right._Myproxy) _Adopt(_Right._Myproxy->_Mycont); return (*this); } _Right._Myproxy is NULL

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• typedef resolution rule

  - by kumar_m_kiran

  Hi All, Can you Please tell me the resolution rule involved in resolving the meaning of the variable in a typedef. Any link related to the same will be very useful. Example #typedef string* pstring; const pstring parr; Here confusion arises whether const'ness is for pointer or the content. Now based on what thumb rule do can we start resolving the above interpretation of the pstring?. Smilarly, If I have a very complex typedef'ed variable, like #typedef void (func*)()(int), I should be able to resolve it using the thumb rule. Thanks in advance for your suggestions

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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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• std::string constructor corrupts pointer

  - by computergeek6

  I have an Entity class, which contains 3 pointers: m_rigidBody, m_entity, and m_parent. Somewhere in Entity::setModel(std::string model), it's crashing. Apparently, this is caused by bad data in m_entity. The weird thing is that I nulled it in the constructor and haven't touched it since then. I debugged it and put a watchpoint on it, and it comes up that the m_entity member is being changed in the constructor for std::string that's being called while converting a const char* into an std::string for the setModel call. I'm running on a Mac, if that helps (I think I remember some problem with std::string on the Mac). Any ideas about what's going on?

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• extraneous calls to copy-constructor and destructor

  - by eSKay

  [This question is a follow up to this question] class A { public: A() {cout<<"A Construction" <<endl;} A(A const& a){cout<<"A Copy Construction"<<endl;} ~A() {cout<<"A Destruction" <<endl;} }; int main() { { vector<A> t; t.push_back(A()); t.push_back(A()); // once more } } The output is: A Construction // 1 A Copy Construction // 1 A Destruction // 1 A Construction // 2 A Copy Construction // 2 A Copy Construction // WHY THIS? A Destruction // 2 A Destruction // deleting element from t A Destruction // deleting element from t A Destruction // WHY THIS?

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• QMap inheritance with QMapIterator

  - by gregseth

  Hi, I made a personnal class which inherits QMap: class CfgMgr : public QMap<QString, CfgSet*> {...} I'm trying to iterate over all its elements like that: CfgMgr* m_pDefaults = new CfgMgr; // .../... QMapIterator<QString, CfgSet*> ics(*m_pDefaults); while (ics.hasNext()) { // doing my stuff } And I get the compile error: Can't convert parameter 1 from 'CfgMgr' to 'const QMap< Key,T &' with [ Key=QString, T=CfgSet * ] I tried with a dynamic_cast: QMapIterator<QString, CfgSet*> ics( *dynamic_cast< QMap<QString,CfgSet*>* >(m_pDefaults) ); it compiles, but always returns NULL. What's wrong? How can I solve this?

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• undefined reference to static member variable

  - by Max

  Hi. I have this class that has a static member. it is also a base class for several other classes in my program. Here's its header file: #ifndef YARL_OBJECT_HPP #define YARL_OBJECT_HPP namespace yarlObject { class YarlObject { // Member Variables private: static int nextID; // keeps track of the next ID number to be used int ID; // the identifier for a specific object // Member Functions public: YarlObject(): ID(++nextID) {} virtual ~YarlObject() {} int getID() const {return ID;} }; } #endif and here's its implementation file. #include "YarlObject.hpp" namespace yarlObject { int YarlObject::nextID = 0; } I'm using g++, and it returns three undefined reference to 'yarlObject::YarlObject::nextID linker errors. If I change the ++nextID phrase in the constructor to just nextID, then I only get one error, and if I change it to 1, then it links correctly. I imagine it's something simple, but what's going on?

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• Why do I get corrupt output on my file?

  - by Martin

  I have a simple program which I have compiled in both MinGW and Visual C++ 2008 Express, and both give an output file larger than 88200. When I set s = 0, both programs work as expected. What am I doing wrong? #include <fstream> using namespace std; int main(int argc, char *argv[]) { int i; short s; fstream f; f.open("test.raw", ios_base::out); for(i = 0; i < 44100; i++) { s = i & 0xFFFF; // PROBLEM? f.write(reinterpret_cast<const char *>(&s), sizeof(s)); } f.close(); return 0; }

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• Why we used double pointer in objective-C or C language?

  - by Rajendra Bhole

  Hi, I confused when i want to take single pointer and when should i take double pointer? In following structure what exactly did? struct objc_class { Class isa; Class super_class; const char *name; long version; long info; long instance_size; struct objc_ivar_list *ivars; struct objc_method_list **methodLists; struct objc_cache *cache; struct objc_protocol_list *protocols; }; Why we use the "**methodLists" double pointer?

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• Python extension building with boost

  - by user1544053

  Hey guys I'm fairly new to boost c/c++ library. I downloaded boost library and build the library. I created a very simple python library in c++ using boost interface (actually it is example code given in the documentation). I built it into a dll file. In the documentation it reads that this dll is exposed to python and they just show the import function in python and include the created library. I don't understand how to expose that dll to python and load the library inside in tradition ('import') manner. In case if you wanna look at the code then here it is: #include <boost/python.hpp> char const* greet() { return "hello, world"; } BOOST_PYTHON_MODULE(hello_ext) { using namespace boost::python; def("greet", greet); } Please help I really want to build applications with c/c++ and python. I simply want to use hello_ext as: >>>import hello_ext >>>print hello_ext.greet() Thank you.

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• Statically initialize anonymous union in C++

  - by wpfwannabe

  I am trying to statically initialize the following structure in Visual Studio 2010: struct Data { int x; union { char ch; const Data* data; }; }; The following is fails with error C2440: 'initializing' : cannot convert from 'Data *' to 'char'. static Data d1; static Data d = {1, &d1}; I have found references to some ways this can be initialized properly but none of them work in VS2010. Any ideas?

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• How to I count key collisions when using boost::unordered_map?

  - by Nikhil

  I have a data structure with 15 unsigned longs, I have defined a hash function using hash_combine as follows: friend std::size_t hash_value(const TUPLE15& given) { std::size_t seed = 0; boost::hash_combine(seed, val1); boost::hash_combine(seed, val2); ... return seed; } I insert a large number of values into a boost::unordered_map but the performance is not good enough. Probably, I could do better with an alternative hashing function. To confirm this, I need to check how many collisions I am getting. How do I do this?

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• Constructor/Destructor involving a class and a struct

  - by Bogdan Maier

  I am working on a program and need to make an array of objects, specifically I have a 31x1 array where each position is an object, (each object is basically built out of 6 ints). Here is what I have but something is wrong and i could use some help thank you. 31x1 struct header" const int days=31; struct Arr{ int days; int *M; }; typedef Arr* Array; 31x1 matrix constructor: void constr(){ int *M; M = new Expe[31]; // Expe is the class class header: class Expe { private: //0-HouseKeeping, 1-Food, 2-Transport, 3-Clothing, 4-TelNet, 5-others int *obj; } Class object constructor: Expe::Expe() { this->obj=new int[6]; } help please... because i`m pretty lost.

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• Static member object of a class in the same class

  - by Luv

  Suppose we have a class as class Egg { static Egg e; int i; Egg(int ii):i(ii) {} Egg(const Egg &); //Prevents copy-constructor to be called public: static Egg* instance() {return &e} }; Egg Egg::e(47); This code guarantees that we cannot create any object, but could use only the static object. But how could we declare static object of the same class in the class. And also one thing more since e is a static object, and static objects can call only static member functions, so how could the constructor been called here for static object e, also its constructors are private.

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• Does C++ have a proper implementation of interface that does not use vtable?

  - by gilbertc

  Does C++ have a proper implementation of interface that does not use vtable? for example class BaseInterface{ public: virtual void func() const = 0; } class BaseInterfaceImpl:public BaseInterface{ public: void func(){ std::cout<<"called."<<endl; } } BaseInterface* obj = new BaseInterfaceImpl(); obj->func(); the call to func at the last line goes to vtable to find the func ptr of BaseInterfaceImpl::func, but is there any C++ way to do that directly as the BaseInterfaceImpl is not subclassed from any other class besides the pure interface class BaseInterface? Thanks. Gil.

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• How to look up an NHibernate entity's table mapping from the type of the entity?

  - by snicker

  Once I've mapped my domain in NHibernate, how can I reverse lookup those mappings somewhere else in my code? Example: The entity Pony is mapped to a table named "AAZF1203" for some reason. (Stupid legacy database table names!) I want to find out that table name from the NH mappings using only the typeof(Pony) because I have to write a query elsewhere. How can I make the following test pass? private const string LegacyPonyTableName = "AAZF1203"; [Test] public void MakeSureThatThePonyEntityIsMappedToCorrectTable() { string ponyTable = GetNHibernateTableMappingFor(typeof(Pony)); Assert.AreEqual(LegacyPonyTableName, ponyTable); } In other words, what does the GetNHibernateTableMappingFor(Type t) need to look like?

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• Problem with number/type of arguments passed to an overloaded c++ constructor wrapped with swig.

  - by MiKo

  I am trying to wrap a c++ class (let's call it "Spam") written by someone else with swig to expose it to Python. After solving several problems, I am able to import the module in python, but when I try to create an object of such class I obtain the following error: foo = Spam.Spam('abc',3) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "Spam.py", line 96, in __init__ this = _Spam.new_Spam(*args) NotImplementedError: Wrong number of arguments for overloaded function 'new_Spam'. Possible C/C++ prototypes are: Spam(unsigned char *,unsigned long,bool,unsigned int,SSTree::io_action,char const *) Spam(unsigned char *,unsigned long,bool,unsigned int,SSTree::io_action) Spam(unsigned char *,unsigned long,bool,unsigned int) Spam(unsigned char *,unsigned long,bool) Spam(unsigned char *,unsigned long) Googling around, I realized that the error is probably caused by the type of the arguments and not by the number (which is quite confusing), but I still cannot identify. I suspect the problem lies in passing a string as the first argument, but have no idea on how to fix it (keep in mind that I know almost no c/c++).

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