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• Syntax for finding structs in multisets - C++

  - by Sarah

  I can't seem to figure out the syntax for finding structs in containers. I have a multiset of Event structs. I'm trying to find one of these structs by searching on its key. I get the compiler error commented below. struct Event { public: bool operator < ( const Event & rhs ) const { return ( time < rhs.time ); } bool operator > ( const Event & rhs ) const { return ( time > rhs.time ); } bool operator == ( const Event & rhs ) const { return ( time == rhs.time ); } double time; int eventID; int hostID; int s; }; typedef std::multiset< Event, std::less< Event > > EventPQ; EventPQ currentEvents; double oldRecTime = 20.0; EventPQ::iterator ceItr = currentEvents.find( EventPQ::key_type( oldRecTime ) ); // no matching function call I've tried a few permutations to no avail. I thought defining the conditional equality operator was going to be enough.

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• Operator+ for a subtype of a template class.

  - by baol

  I have a template class that defines a subtype. I'm trying to define the binary operator+ as a template function, but the compiler cannot resolve the template version of the operator+. #include <iostream> template<typename other_type> struct c { c(other_type v) : cs(v) {} struct subtype { subtype(other_type v) : val(v) {} other_type val; } cs; }; template<typename other_type> typename c<other_type>::subtype operator+(const typename c<other_type>::subtype& left, const typename c<other_type>::subtype& right) { return typename c<other_type>::subtype(left.val + right.val); } // This one works // c<int>::subtype operator+(const c<int>::subtype& left, // const c<int>::subtype& right) // { return c<int>::subtype(left.val + right.val); } int main() { c<int> c1 = 1; c<int> c2 = 2; c<int>::subtype cs3 = c1.cs + c2.cs; std::cerr << cs3.val << std::endl; } I think the reason is because the compiler (g++4.3) cannot guess the template type so it's searching for operator+<int> instead of operator+. What's the reason for that? What elegant solution can you suggest?

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• Variable not accessible within and if statment

  - by Chris

  I have a variable which holds a score for a game. My variable is accessible and correct outside of an if statement but not inside as shown below cout << score << endl; //works if(!justFinished){ cout << score << endl; // doesn't work prints a large negative number endTime = time(NULL); ifstream highscoreFile; highscoreFile.open("highscores.txt"); if(highscoreFile.good()){ highscoreFile.close(); }else{ std::ofstream outfile ("highscores.txt"); cout << score << endl; outfile << score << std::endl; outfile.close(); } justFinished = true; } cout << score << endl;//works

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• why cannot use uncaught_exception in dtor?

  - by camino

  Hi , Herb Sutter in his article http://www.gotw.ca/gotw/047.htm pointed out that we cannot use uncaught_exception in desturctor function, // Why the wrong solution is wrong // U::~U() { try { T t; // do work } catch( ... ) { // clean up } } If a U object is destroyed due to stack unwinding during to exception propagation, T::~T will fail to use the "code that could throw" path even though it safely could. but I write a test program, and T::~T in fact didn't use the "code that could throw" #include <exception> #include <iostream> using namespace std; class T { public: ~T() { if( !std::uncaught_exception() ) { cout<<"can throw"<<endl; throw 1; } else { cout<<"cannot throw"<<endl; } } }; struct U { ~U() { try { T t; } catch( ... ) { } } }; void f() { U u; throw 2; } int main() { try { f(); } catch(...) {} } output is : cannot throw did I miss something? Thanks

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• How to call shared_ptr<boost::signal> from a vector in a loop?

  - by BTR

  I've got a working callback system that uses boost::signal. I'm extending it into a more flexible and efficient callback manager which uses a vector of shared_ptr's to my signals. I've been able to successfully create and add callbacks to the list, but I'm unclear as to how to actually execute the signals. ... // Signal aliases typedef boost::signal<void (float *, int32_t)> Callback; typedef std::shared_ptr<Callback> CallbackRef; // The callback list std::vector<CallbackRef> mCallbacks; // Adds a callback to the list template<typename T> void addCallback(void (T::* callbackFunction)(float * data, int32_t size), T * callbackObject) { CallbackRef mCallback = CallbackRef(new Callback()); mCallback->connect(boost::function<void (float *, int32_t)>(boost::bind(callbackFunction, callbackObject, _1, _2))); mCallbacks.push_back(mCallback); } // Pass the float array and its size to the callbacks void execute(float * data, int32_t size) { // Iterate through the callback list for (vector<CallbackRef>::iterator i = mCallbacks.begin(); i != mCallbacks.end(); ++i) { // What do I do here? // (* i)(data, size); // <-- Dereferencing doesn't work } } ... All of this code works. I'm just not sure how to run the call from within a shared_ptr from with a vector. Any help would be neat-o. Thanks, in advance.

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• Renaming a DOMNode in PHP

  - by python

  <?xml version='1.0' encoding='UTF-8' standalone='no'?> <Document xmlns='urn:iso:std:iso:20022:tech:xsd:pain.001.001.02'> <books> <book> <qty>12</qty> <title>C++</title> </book> <book> <qty>21</qty> <title>PHP</title> </book> </books> <books> <book> <qty>25</qty> <title>Java</title> </book> <book> <qty>32</qty> <title>Python</title> </book> <book> <qty>22</qty> <title>History</title> </book> </books> </Document> How Can I Rename ? <Document xmlns='urn:iso:std:iso:20022:tech:xsd:pain.001.001.02'> TO <Document>

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• Portable way of counting milliseconds in C++ ?

  - by ereOn

  Hi, Is there any portable (Windows & Linux) way of counting how many milliseconds elapsed between two calls ? Basically, I want to achieve the same functionnality than the StopWatch class of .NET. (for those who already used it) In a perfect world, I would have used boost::date_time but that's not an option here due to some silly rules I'm enforced to respect. For those who better read code, this is what I'd like to achieve. Timer timer; timer.start(); // Some instructions here timer.stop(); // Print out the elapsed time std::cout << "Elapsed time: " << timer.milliseconds() << "ms" << std::endl; So, if there is a portable (set of) function(s) that can help me implement the Timer class, what is it ? If there is no such function, what Windows & Linux API should I use to achieve this functionnality ? (using #ifdef WINDOWS-like macros) Thanks !

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• Operator+ for a subtype of a template classe.

  - by baol

  I have a template class that defines a subtype. I'm trying to define the binary operator+ as a template function, but the compiler cannot resolve the template version of the operator+. #include <iostream> template<typename other_type> struct c { c(other_type v) : cs(v) {} struct subtype { subtype(other_type v) : val(v) {} other_type val; } cs; }; template<typename other_type> typename c<other_type>::subtype operator+(const typename c<other_type>::subtype& left, const typename c<other_type>::subtype& right) { return typename c<other_type>::subtype(left.val + right.val); } // This one works // c<a>::subtype operator+(const c<a>::subtype& left, // const c<a>::subtype& right) // { return c<a>::subtype(left.val + right.val); } int main() { c<int> c1 = 1; c<int> c2 = 2; c<int>::subtype cs3 = c1.cs + c2.cs; std::cerr << cs3.val << std::endl; } I think the reason is because the compiler (g++4.3) cannot guess the template type so it's searching for operator+<int> instead of operator+. What's the reason for that? What elegant solution can you suggest?

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• C++ Vector of vectors is messing with me

  - by xbonez

  If I put this code in a .cpp file and run it, it runs just fine: #include <iostream> #include <vector> #include <string> using namespace std; typedef vector<int> row; typedef vector<row> myMatrix; void main() { //cout << endl << "test" << endl; myMatrix mat(2,2); mat[0][1] = 2; cout << endl << mat[0][1] << endl; } But, if I make a .h and a .cpp file with the .h file like this, it gives me boatloads of errors. #ifndef _grid_ #define _grid_ #include<iostream> #include<vector> #include<string> using namespace std; typedef vector<int> row; typedef vector<row> myMatrix; class grid { public: grid(); ~grid(); int getElement(unsigned int ri, unsigned int ci); bool setElement(unsigned int ri, unsigned int ci, unsigned int value); private: myMatrix sudoku_(9,9); }; #endif These are some of the errors I get: warning C4091: 'typedef ' : ignored on left of 'int' when no variable is declared error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

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• Binary Search Tree, cannot do traversal

  - by ihm

  Please see BST codes below. It only outputs "5". what did I do wrong? #include <iostream> class bst { public: bst(const int& numb) : root(new node(numb)) {} void insert(const int& numb) { root->insert(new node(numb), root); } void inorder() { root->inorder(root); } private: class node { public: node(const int& numb) : left(NULL), right(NULL) { value = numb; } void insert(node* insertion, node* position) { if (position == NULL) position = insertion; else if (insertion->value > position->value) insert(insertion, position->right); else if (insertion->value < position->value) insert(insertion, position->left); } void inorder(node* tree) { if (tree == NULL) return; inorder(tree->left); std::cout << tree->value << std::endl; inorder(tree->right); } private: node* left; node* right; int value; }; node* root; }; int main() { bst tree(5); tree.insert(4); tree.insert(2); tree.insert(10); tree.insert(14); tree.inorder(); return 0; }

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• Could this C cast to avoid a signed/unsigned comparison make any sense?

  - by sharptooth

  I'm reviewing a C++ project and see effectively the following: std::vector<SomeType> objects; //then later int size = (int)objects.size(); for( int i = 0; i < size; ++i ) { process( objects[i] ); } Here's what I see. std::vector::size() returns size_t that can be of some size not related to the size of int. Even if sizeof(int) == sizeof(size_t) int is signed and can't hold all possible values of size_t. So the code above could only process the lower part of a very long vector and contains a bug. That said I'm curious of why the author might have written this? My only guess is that first he omitted the (int) cast and the compiler emitted something like Visual C++ C4018 warning: warning C4018: '<' : signed/unsigned mismatch so the author though that the best way to avoid the compiler warning would be to simply cast the size_t to int thus making the compiler shut up. Is there any other possible sane reason for that C cast?

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• C++ - Where to throw exception?

  - by HardCoder1986

  Hello! I have some kind of an ideological question, so: Suppose I have some templated function template <typename Stream> void Foo(Stream& stream, Object& object) { ... } which does something with this object and the stream (for example, serializes that object to the stream or something like that). Let's say I also add some plain wrappers like (and let's say the number of these wrappers equals 2 or 3): void FooToFile(const std::string& filename, Object& object) { std::ifstream stream(filename.c_str()); Foo(stream, object); } So, my question is: Where in this case (ideologically) should I throw the exception if my stream is bad? Should I do this in each wrapper or just move that check to my Foo, so that it's body would look like if (!foo.good()) throw (something); // Perform ordinary actions I understand that this may be not the most important part of coding and these solutions are actually equal, but I just wan't to know "the proper" way to implement this. Thank you.

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• C++ reinterpret cast ?

  - by Ian

  I would like to cast one object of the class PointsList to another object Points3DList (and vice versa) where: template <class T> class PointsList { protected: std::vector <Point <T> *> points; //Only illustration, not possible with templaes }; and template <class T> class Points3DList { protected: std::vector <Point3D<T> *> points; //Only illustration, not possible with templaes }; Between Point and Point3D there is no relationship (inheritance nor composition)... template <class T> class Point { protected: T x; T y; }; template <class T> class Point3D { protected: T x; T y; T z; }; What do you think about conversion Points3DList <T> *pl3D = new Points3DList <T> (); ... PointsList <T> *pl = reinterpret_cast < PointList <T> * > ( pl3D ); where pl3D represents pointer to Points3DList object.. Can reinterpret_cast be used in this case or it is better to create a conversion function? Data model in this case can not be changed...

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• c++ expected constant expression

  - by cpp

  #include <iostream> #include <fstream> #include <cmath> #include <math.h> #include <iomanip> using std::ifstream; using namespace std; int main (void) { int count=0; float sum=0; float maximum=-1000000; float sumOfX; float sumOfY; int size; int negativeY=0; int positiveX=0; int negativeX=0; ifstream points; //the points to be imported from file //points.open( "data.dat"); //points>>size; //cout<<size<<endl; size=100; float x[size][2]; while (count<size) { points>>(x[count][0]); //cout<<"x= "<<(x[count][0])<<" ";//read in x value points>>(x[count][1]); //cout<<"y= "<<(x[count][1])<<endl;//read in y value count++; } This program is giving me expected constant expression error on the line where I declare float x[size][2]. why?

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• Acessing a struct member, using a pointer to a vector of structs. Error:base operand of '->' has non-pointer type

  - by Matt Munson

  #include <iostream> #include <vector> using namespace std; struct s_Astruct { vector <int> z; }; int main () { vector <s_Astruct> v_a; for(int q=0;q<10;q++) { v_a.push_back(s_Astruct()); for(int w =0;w<5;w++) v_a[q].z.push_back(8); } vector <s_Astruct> * p_v_a = & v_a; cout << p_v_a[0]->z[4]; //error: base operand of '->' has non-pointer type //'__gnu_debug_def::vector<s_Astruct, std::allocator<s_Astruct> >' } There seems to be some issue with this sort of operation that I don't understand. In the code that I'm working on I actually have things like p_class-vector[]-vector[]-int; and I'm getting a similar error.

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• Common protected data member in base class?

  - by EXP0

  I have a base class and several derived classes. The derived classes use some common data, can I just put those common data as protected member of the base class? I know the protected member breaks encapsulation sometimes, so I wonder if there is any good approach. Here is a specific example: class Base{ public: virtual void foo() = 0; void printData(); protected: std::vector<std::string> mData; } class Dr1 : public Base{ public: virtual void foo(); //could change mData } class Dr2 : public Base{ public: virtual void foo(); //could change mData } If I put mData into Dr1 and Dr2 as private member, then I need to put it in both of them, and I can not have printData() in Base since printData() need access to mData unless I make printData() virtual and have identical function in both Dr1 and Dr2, which doesn't make much sense to me. Is there a better way to approach this without using protected member? Thank you.

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• Equvalent c++0x program withought using boost threads..

  - by Eternal Learner

  I have the below simple program using boost threads, what would be the changes needed to do the same in c++0X #include<iostream> #include<boost/thread/thread.hpp> boost::mutex mutex; struct count { count(int i): id(i){} void operator()() { boost::mutex::scoped_lock lk(mutex); for(int i = 0 ; i < 10000 ; i++) { std::cout<<"Thread "<<id<<"has been called "<<i<<" Times"<<std::endl; } } private: int id; }; int main() { boost::thread thr1(count(1)); boost::thread thr2(count(2)); boost::thread thr3(count(3)); thr1.join(); thr2.join(); thr3.join(); return 0; }

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• C++: Case statement within while loop?

  - by Jason

  I just started C++ but have some prior knowledge to other languages (vb awhile back unfortunately), but have an odd predicament. I disliked using so many IF statements and wanted to use switch/cases as it seemed cleaner, and I wanted to get in the practice.. But.. Lets say I have the following scenario (theorietical code): while(1) { //Loop can be conditional or 1, I use it alot, for example in my game char something; std::cout << "Enter something\n -->"; std::cin >> something; //Switch to read "something" switch(something) { case 'a': cout << "You entered A, which is correct"; break; case 'b': cout << "..."; break; } } And that's my problem. Lets say I wanted to exit the WHILE loop, It'd require two break statements? This obviously looks wrong: case 'a': cout << "You entered A, which is correct"; break; break; So can I only do an IF statement on the 'a' to use break;? Am I missing something really simple? This would solve a lot of my problems that I have right now.

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• Returning a lua table on SWIG call

  - by Tom J Nowell

  I have a class with a methodcalled GetEnemiesLua. I have bound this class to lua using SWIG, and I can call this method using my lua code. I am trying to get the method to return a lua table of objects. Here is my current code: void CSpringGame::GetEnemiesLua(){ std::vector<springai::Unit*> enemies = callback->GetEnemyUnits(); if( enemies.empty()){ lua_pushnil(ai->L); return; } else{ lua_newtable(ai->L); int top = lua_gettop(ai->L); int index = 1; for (std::vector<springai::Unit*>::iterator it = enemies.begin(); it != enemies.end(); ++it) { //key lua_pushinteger(ai->L,index);//lua_pushstring(L, key); //value CSpringUnit* unit = new CSpringUnit(callback,*it,this); ai->PushIUnit(unit); lua_settable(ai->L, -3); ++index; } ::lua_pushvalue(ai->L,-1); } } PushIUnit is as follows: void CTestAI::PushIUnit(IUnit* unit){ SWIG_NewPointerObj(L,unit,SWIGTYPE_p_IUnit,1); } To test this I have the following code: t = game:GetEnemiesLua() if t == nil then game:SendToConsole("t is nil! ") end The result is always 't is nil', despite this being incorrect. I have put breakpoints in the code and it is indeed going over the loop, rather than doing lua_pushnil. So how do I make my method return a table when called via lua?

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• What influences running time of reading a bunch of images?

  - by remi

  I have a program where I read a handful of tiny images (50000 images of size 32x32). I read them using OpenCV imread function, in a program like this: std::vector<std::string> imageList; // is initialized with full path to the 50K images for(string s : imageList) { cv::Mat m = cv::imread(s); } Sometimes, it will read the images in a few seconds. Sometimes, it takes a few minutes to do so. I run this program in GDB, with a breakpoint further away than the loop for reading images so it's not because I'm stuck in a breakpoint. The same "erratic" behaviour happens when I run the program out of GDB. The same "erratic" behaviour happens with program compiled with/without optimisation The same "erratic" behaviour happens while I have or not other programs running in background The images are always at the same place in the hard drive of my machine. I run the program on a Linux Suse distrib, compiled with gcc. So I am wondering what could affect the time of reading the images that much?

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• C++: Switch statement within while loop?

  - by Jason

  I just started C++ but have some prior knowledge to other languages (vb awhile back unfortunately), but have an odd predicament. I disliked using so many IF statements and wanted to use switch/cases as it seemed cleaner, and I wanted to get in the practice.. But.. Lets say I have the following scenario (theorietical code): while(1) { //Loop can be conditional or 1, I use it alot, for example in my game char something; std::cout << "Enter something\n -->"; std::cin >> something; //Switch to read "something" switch(something) { case 'a': cout << "You entered A, which is correct"; break; case 'b': cout << "..."; break; } } And that's my problem. Lets say I wanted to exit the WHILE loop, It'd require two break statements? This obviously looks wrong: case 'a': cout << "You entered A, which is correct"; break; break; So can I only do an IF statement on the 'a' to use break;? Am I missing something really simple? This would solve a lot of my problems that I have right now.

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• What is causing this template-related compile error? (c++)

  - by Setien

  When I try to compile this: #include <map> #include <string> template <class T> class ZUniquePool { typedef std::map< int, T* > ZObjectMap; ZObjectMap m_objects; public: T * Get( int id ) { ZObjectMap::const_iterator it = m_objects.find( id ); if( it == m_objects.end() ) { T * p = new T; m_objects[ id ] = p; return p; } return m_objects[ id ]; } }; int main( int argc, char * args ) { ZUniquePool< std::string > pool; return 0; } I get this: main.cpp: In member function ‘T* ZUniquePool<T>::Get(int)’: main.cpp:12: error: expected `;' before ‘it’ main.cpp:13: error: ‘it’ was not declared in this scope I'm using GCC 4.2.1 on Mac OS X. It works in VS2008. I'm wondering whether it might be a variation of this problem: http://stackoverflow.com/questions/1364837/why-doesnt-this-c-template-code-compile But as my error output is only partially similar, and my code works in VS2008, I am not sure. Can anyone shed some light on what I am doing wrong?

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• Passing a string to a function in C++

  - by Chef Flambe

  I want to pass a string like "Celcius" into a function that I have but I keep getting errors tossed back at me from the Function. System::Console::WriteLine' : none of the 19 overloads could convert all the argument types I figure I just have something simple wrong. Can someone point out my mistake please? Using MS Visual C++ 2010 I've posted the offending code. The other functions (not posted) work fine. void PrintResult( double result, std::string sType ); // Print result and string // to the console //============================================================================================= // start of main //============================================================================================= void main( void ) { ConsoleKeyInfo CFM; // Program Title and Description ProgramDescription(); // Menu Selection and calls to data retrieval/calculation/result Print CFM=ChooseFromMenu(); switch(CFM.KeyChar) // ************************************************************ { //* case '1' : PrintResult(F2C(GetTemperature()),"Celsius"); //* break; //* //* case '2' : PrintResult(C2F(GetTemperature()),"Fahrenheit"); //* break; //* //* default : Console::Write("\n\nSwitch : Case !!!FAILURE!!!"); //* } //************************************************************ system("pause"); return; } //Function void PrintResult( double result, std::string sType ) { Console::WriteLine("\n\nThe converted temperature is {0:F2} degrees {1}\n\n",result,sType); return; }

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• error: 'void Base::output()' is protected within this context

  - by Bill

  I'm confused about the errors generated by the following code. In Derived::doStuff, I can access Base::output directly by calling it. Why can't I create a pointer to output() in the same context that I can call output()? (I thought protected / private governed whether you could use a name in a specific context, but apparently that is incomplete?) Is my fix of writing callback(this, &Derived::output); instead of callback(this, Base::output) the correct solution? #include <iostream> using std::cout; using std::endl; template <typename T, typename U> void callback(T obj, U func) { ((obj)->*(func))(); } class Base { protected: void output() { cout << "Base::output" << endl; } }; class Derived : public Base { public: void doStuff() { // call it directly: output(); Base::output(); // create a pointer to it: // void (Base::*basePointer)() = &Base::output; // error: 'void Base::output()' is protected within this context void (Derived::*derivedPointer)() = &Derived::output; // call a function passing the pointer: // callback(this, &Base::output); // error: 'void Base::output()' is protected within this context callback(this, &Derived::output); } }; int main() { Derived d; d.doStuff(); }

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• Is re-throwing an exception legal in a nested 'try'?

  - by Alexander Gessler

  Is the following well-defined in C++, or not? I am forced to 'convert' exceptions to return codes (the API in question is used by many C users, so I need to make sure all C++ exceptions are caught & handled before control is returned to the caller). enum ErrorCode {…}; ErrorCode dispatcher() { try { throw; } catch (std::bad_alloc&) { return ErrorCode_OutOfMemory; } catch (std::logic_error&) { return ErrorCode_LogicError; } catch (myownstdexcderivedclass&) { return ErrorCode_42; } catch(...) { return ErrorCode_UnknownWeWillAllDie; } } ErrorCode apifunc() { try { // foo() might throw anything foo(); } catch(...) { // dispatcher rethrows the exception and does fine-grained handling return dispatcher(); } return ErrorCode_Fine; } ErrorCode apifunc2() { try { // bar() might throw anything bar(); } catch(...) { return dispatcher(); } return ErrorCode_Fine; } I hope the sample shows my intention. My guess is that this is undefined behaviour, but I'm not sure. Please provide quotes from the standard, if applicable. Alternative approaches are appreciated as well. Thanks!

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