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• Operator issues with cout

  - by BSchlinker

  I have a simple package class which is overloaded so I can output package data simply with cout << packagename. I also have two data types, name which is a string and shipping cost with a double. protected: string name; string address; double weight; double shippingcost; ostream &operator<<( ostream &output, const Package &package ) { output << "Package Information ---------------"; output << "Recipient: " << package.name << endl; output << "Shipping Cost (including any applicable fees): " << package.shippingcost; The problem is occurring with the 4th line (output << "Recipient:...). I'm receiving the error "no operator "<<" matches these operands". However, line 5 is fine. I'm guessing this has to do with the data type being a string for the package name. Any ideas?

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• how do i call an overloaded action in .net mvc?

  - by Jeff Martin

  I have an overloaded action in my Controller: public ActionResult AssignList(int id) { ... } [AcceptVerbs((HttpVerbs.Get))] public ActionResult AssignList(int id, bool altList) { ... } I'd like to use the same partial view for both lists but it will potentially have a differently filtered list of Images. I am trying to call it from another view using RenderAction: <% Html.RenderAction("AssignList", "Image", new { id = Model.PotholeId, altList = true }); %> However I am getting the following error: The current request for action 'AssignList' on controller type 'ImageController' is ambiguous between the following action methods: System.Web.Mvc.ActionResult AssignList(Int32) on type UsiWeb.Controllers.ImageController System.Web.Mvc.ActionResult AssignList(Int32, Boolean) on type UsiWeb.Controllers.ImageController How can I call the specific overload?

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• C# 4: conflicting overloaded methods with optional parameters

  - by Thomas

  I have two overloaded methods, one with an optional parameter. void foo(string a) { } void foo(string a, int b = 0) { } now I call: foo("abc"); interestingly the first overload is called. why not the second overload with optional value set to zero? To be honest, I would have expect the compiler to bring an error, at least a warning to avoid unintentional execution of the wrong method. What's the reason for this behaviour? Why did the C# team define it that way? Thanks for your opinions!

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• Why friend overloaded operator is preferred to conversion operator in this case

  - by skydoor

  Hi I have a code like this, I think both the friend overloaded operator and conversion operator have the similar function. However, why does the friend overloaded operator is called in this case? What's the rules? Thanks so much! class A{ double i; public: A(int i):i(i) {} operator double () const { cout<<"conversion operator"<<endl;return i;} // a conversion operator friend bool operator>(int i, A a); // a friend funcion of operator > }; bool operator>(int i, A a ){ cout<<"Friend"<<endl; return i>a.i; } int main() { A aa(1); if (0 > aa){ return 1; } }

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• trying to make a simple grid-class, non-lvalue in assignment

  - by Tyrfing

  I'm implementing a simple C++ grid class. One Function it should support is accessed through round brackets, so that I can access the elements by writing mygrid(0,0). I overloaded the () operator and i am getting the error message: "non-lvalue in assignment". what I want to be able to do: //main cGrid<cA*> grid(5, 5); grid(0,0) = new cA(); excerpt of my implementation of the grid class: template class cGrid { private: T* data; int mWidth; int mHeight; public: cGrid(int width, int height) : mWidth(width), mHeight(height) { data = new T[width*height]; } ~cGrid() { delete data; } T operator ()(int x, int y) { if (x >= 0 && x <= mWidth) { if (y >= 0 && y <= mHeight) { return data[x + y * mWidth]; } } } const T &operator ()(int x, int y) const { if (x >= 0 && x <= mWidth) { if (y >= 0 && y <= mHeight) { return data[x + y * mWidth]; } } } The rest of the code deals with the implementation of an iterator and should not be releveant.

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• Python: Can subclasses overload inherited methods?

  - by Rosarch

  I'm making a shopping cart app in Google App Engine. I have many classes that derive from a base handler: class BaseHandler(webapp.RequestHandler): def get(self, CSIN=None): self.body(CSIN) Does this mean that the body() method of every descendant class needs to have the same argument? This is cumbersome. Only one descendant actually uses that argument. And what about when I add new args? Do I need to go through and change every class? class Detail(BaseHandler): def body(self, CSIN): class MainPage(BaseHandler): def body(self, CSIN=None): #@UnusedVariable class Cart(BaseHandler): def body(self, CSIN): #@UnusedVariable

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• Overload dereference operator

  - by zilgo

  I'm trying to overload the dereference operator, but compiling the following code results in the error 'initializing' : cannot convert from 'X' to 'int': struct X { void f() {} int operator*() const { return 5; } }; int main() { X* x = new X; int t = *x; delete x; return -898; } What am I doing wrong?

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• Why does this cast to Base class in virtual function give a segmentation fault?

  - by dehmann

  I want to print out a derived class using the operator<<. When I print the derived class, I want to first print its base and then its own content. But I ran into some trouble (see segfault below): class Base { public: friend std::ostream& operator<<(std::ostream&, const Base&); virtual void Print(std::ostream& out) const { out << "BASE!"; } }; std::ostream& operator<<(std::ostream& out, const Base& b) { b.Print(out); return out; } class Derived : public Base { public: virtual void Print(std::ostream& out) const { out << "My base: "; //((const Base*)this)->Print(out); // infinite, calls this fct recursively //((Base*)this)->Print(out); // segfault (from infinite loop?) ((Base)*this).Print(out); // OK out << " ... and myself."; } }; int main(int argc, char** argv){ Derived d; std::cout << d; return 0; } Why can't I cast in one of these ways? ((const Base*)this)->Print(out); // infinite, calls this fct recursively ((Base*)this)->Print(out); // segfault (from infinite loop?)

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• Problem with std::map and std::pair

  - by Tom

  Hi everyone. I have a small program I want to execute to test something #include <map> #include <iostream> using namespace std; struct _pos{ float xi; float xf; bool operator<(_pos& other){ return this->xi < other.xi; } }; struct _val{ float f; }; int main() { map<_pos,_val> m; struct _pos k1 = {0,10}; struct _pos k2 = {10,15}; struct _val v1 = {5.5}; struct _val v2 = {12.3}; m.insert(std::pair<_pos,_val>(k1,v1)); m.insert(std::pair<_pos,_val>(k2,v2)); return 0; } The problem is that when I try to compile it, I get the following error $ g++ m2.cpp -o mtest In file included from /usr/include/c++/4.4/bits/stl_tree.h:64, from /usr/include/c++/4.4/map:60, from m2.cpp:1: /usr/include/c++/4.4/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = _pos]’: /usr/include/c++/4.4/bits/stl_tree.h:1170: instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = _pos, _Val = std::pair<const _pos, _val>, _KeyOfValue = std::_Select1st<std::pair<const _pos, _val> >, _Compare = std::less<_pos>, _Alloc = std::allocator<std::pair<const _pos, _val> >]’ /usr/include/c++/4.4/bits/stl_map.h:500: instantiated from ‘std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::pair<const _Key, _Tp>&) [with _Key = _pos, _Tp = _val, _Compare = std::less<_pos>, _Alloc = std::allocator<std::pair<const _pos, _val> >]’ m2.cpp:30: instantiated from here /usr/include/c++/4.4/bits/stl_function.h:230: error: no match for ‘operator<’ in ‘__x < __y’ m2.cpp:9: note: candidates are: bool _pos::operator<(_pos&) $ I thought that declaring the operator< on the key would solve the problem, but its still there. What could be wrong? Thanks in advance.

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• operator<< cannot output std::endl -- Fix?

  - by dehmann

  The following code gives an error when it's supposed to output just std::endl: #include <iostream> #include <sstream> struct MyStream { std::ostream* out_; MyStream(std::ostream* out) : out_(out) {} std::ostream& operator<<(const std::string& s) { (*out_) << s; return *out_; } }; template<class OutputStream> struct Foo { OutputStream* out_; Foo(OutputStream* out) : out_(out) {} void test() { (*out_) << "OK" << std::endl; (*out_) << std::endl; // ERROR } }; int main(int argc, char** argv){ MyStream out(&std::cout); Foo<MyStream> foo(&out); foo.test(); return EXIT_SUCCESS; } The error is: stream1.cpp:19: error: no match for 'operator<<' in '*((Foo<MyStream>*)this)->Foo<MyStream>::out_ << std::endl' stream1.cpp:7: note: candidates are: std::ostream& MyStream::operator<<(const std::string&) So it can output a string (see line above the error), but not just the std::endl, presumably because std::endl is not a string, but the operator<< definition asks for a string. Templating the operator<< didn't help: template<class T> std::ostream& operator<<(const T& s) { ... } How can I make the code work? Thanks!

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• Operator overload for [] operator

  - by viswanathan

  Why would you need to overload the [] operator? I have never come across a practical scenario where this was necessary. Can somebody tell me a practical use case for this.

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• Are free operator->* overloads evil?

  - by Potatoswatter

  I was perusing section 13.5 after refuting the notion that built-in operators do not participate in overload resolution, and noticed that there is no section on operator->*. It is just a generic binary operator. Its brethren, operator->, operator*, and operator[], are all required to be non-static member functions. This precludes definition of a free function overload to an operator commonly used to obtain a reference from an object. But the uncommon operator->* is left out. In particular, operator[] has many similarities. It is binary (they missed a golden opportunity to make it n-ary), and it accepts some kind of container on the left and some kind of locator on the right. Its special-rules section, 13.5.5, doesn't seem to have any actual effect except to outlaw free functions. (And that restriction even precludes support for commutativity!) So, for example, this is perfectly legal (in C++0x, remove obvious stuff to translate to C++03): #include <utility> #include <iostream> #include <type_traits> using namespace std; template< class F, class S > typename common_type< F,S >::type operator->*( pair<F,S> const &l, bool r ) { return r? l.second : l.first; } template< class T > T & operator->*( pair<T,T> &l, bool r ) { return r? l.second : l.first; } template< class T > T & operator->*( bool l, pair<T,T> &r ) { return l? r.second : r.first; } int main() { auto x = make_pair( 1, 2.3 ); cerr << x->*false << " " << x->*4 << endl; auto y = make_pair( 5, 6 ); y->*(0) = 7; y->*0->*y = 8; // evaluates to 7->*y = y.second cerr << y.first << " " << y.second << endl; } I can certainly imagine myself giving into temp[la]tation. For example, scaled indexes for vector: v->*matrix_width[2][5] = x; Did the standards committee forget to prevent this, was it considered too ugly to bother, or are there real-world use cases?

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• Working with operator[] and operator=

  - by calebthorne

  Given a simple class that overloads the '[ ]' operator: class A { public: int operator[](int p_index) { return a[p_index]; } private: int a[5]; }; I would like to accomplish the following: void main() { A Aobject; Aobject[0] = 1; // Problem here } How can I overload the assignment '=' operator in this case to work with the '[ ]' operator?

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• how to refer to the current struct in an overloaded operator?

  - by genesys

  Hi! I have a struct for which i want to define a relative order by defining < , , <= and = operators. actually in my order there won't be any equality, so if one struct is not smaller than another, it's automatically larger. I defined the first operator like this: struct MyStruct{ ... ... bool operator < (const MyStruct &b) const {return (somefancycomputation);} }; now i'd like to define the other operators based on this operator, such that <= will return the same as < and the other two will simply return the oposite. so for example for the operator i'd like to write something like bool operator > (const MyStruct &b) const {return !(self<b);} but i don't know how to refere to this 'self' since i can refere only to the fields inside the current struct. whole is in C++ hope my question was understandable :) thank you for the help!

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• Override number of parameters of pure virtual functions

  - by Jir

  I have implemented the following interface: template <typename T> class Variable { public: Variable (T v) : m_value (v) {} virtual void Callback () = 0; private: T m_value; }; A proper derived class would be defined like this: class Derived : public Variable<int> { public: Derived (int v) : Variable<int> (v) {} void Callback () {} }; However, I would like to derive classes where Callback accepts different parameters (eg: void Callback (int a, int b)). Is there a way to do it?

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• wrong operator() overload called

  - by user313202

  okay, I am writing a matrix class and have overloaded the function call operator twice. The core of the matrix is a 2D double array. I am using the MinGW GCC compiler called from a windows console. the first overload is meant to return a double from the array (for viewing an element). the second overload is meant to return a reference to a location in the array (for changing the data in that location. double operator()(int row, int col) const ; //allows view of element double &operator()(int row, int col); //allows assignment of element I am writing a testing routine and have discovered that the "viewing" overload never gets called. for some reason the compiler "defaults" to calling the overload that returns a reference when the following printf() statement is used. fprintf(outp, "%6.2f\t", testMatD(i,j)); I understand that I'm insulting the gods by writing my own matrix class without using vectors and testing with C I/O functions. I will be punished thoroughly in the afterlife, no need to do it here. Ultimately I'd like to know what is going on here and how to fix it. I'd prefer to use the cleaner looking operator overloads rather than member functions. Any ideas? -Cal the matrix class: irrelevant code omitted class Matrix { public: double getElement(int row, int col)const; //returns the element at row,col //operator overloads double operator()(int row, int col) const ; //allows view of element double &operator()(int row, int col); //allows assignment of element private: //data members double **array; //pointer to data array }; double Matrix::getElement(int row, int col)const{ //transform indices into true coordinates (from sorted coordinates //only row needs to be transformed (user can only sort by row) row = sortedArray[row]; result = array[usrZeroRow+row][usrZeroCol+col]; return result; } //operator overloads double Matrix::operator()(int row, int col) const { //this overload is used when viewing an element return getElement(row,col); } double &Matrix::operator()(int row, int col){ //this overload is used when placing an element return array[row+usrZeroRow][col+usrZeroCol]; } The testing program: irrelevant code omitted int main(void){ FILE *outp; outp = fopen("test_output.txt", "w+"); Matrix testMatD(5,7); //construct 5x7 matrix //some initializations omitted fprintf(outp, "%6.2f\t", testMatD(i,j)); //calls the wrong overload }

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• What am i doing wrong

  - by Erik Sapir

  I have the following code. I need B class to have a min priority queue of AToTime objects. AToTime have operator, and yet i receive error telling me than there is no operator matching the operands... #include <queue> #include <functional> using namespace std; class B{ //public functions public: B(); virtual ~B(); //private members private: log4cxx::LoggerPtr m_logger; class AToTime { //public functions public: AToTime(const ACE_Time_Value& time, const APtr a) : m_time(time), m_a(a){} bool operator >(const AToTime& other) { return m_time > other.m_time; } //public members - no point using any private members here public: ACE_Time_Value m_time; APtr m_a; }; priority_queue<AToTime, vector<AToTime>, greater<AToTime> > m_myMinHeap; };

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• friending istream operator with class

  - by user1388172

  hello i'm trying to overload my operator >> to my class but i ecnouter an error in eclipse. code: friend istream& operator>>(const istream& is, const RAngle& ra){ return is >> ra.x >> ra.y; } code2: friend istream& operator>>(const istream& is, const RAngle& ra) { is >> ra.x; is >> ra.y; return is } Both crash and i don't know why, please help. EDIT: ra.x & ra.y are both 2 private ints of my class; Full error: error: ..\/rightangle.h: In function 'std::istream& operator>>(std::istream&, const RAngle&)': ..\/rightangle.h:65:12: error: ambiguous overload for 'operator>>' in 'is >> ra.RAngle::x' ..\/rightangle.h:65:12: note: candidates are: c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:122:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__istream_type& (*)(std::basic_istream<_CharT, _Traits>::__istream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:122:7: note: no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&) {aka std::basic_istream<char>& (*)(std::basic_istream<char>&)}' c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:126:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__ios_type& (*)(std::basic_istream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>, std::basic_istream<_CharT, _Traits>::__ios_type = std::basic_ios<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:126:7: note: no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}' c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:133:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:133:7: note: no known conversion for argument 1 from 'const int' to 'std::ios_base& (*)(std::ios_base&)' c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:241:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:241:7: note: no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}' ..\/rightangle.h:66:12: error: ambiguous overload for 'operator>>' in 'is >> ra.RAngle::y' ..\/rightangle.h:66:12: note: candidates are: c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:122:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__istream_type& (*)(std::basic_istream<_CharT, _Traits>::__istream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:122:7: note: no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__istream_type& (*)(std::basic_istream<char>::__istream_type&) {aka std::basic_istream<char>& (*)(std::basic_istream<char>&)}' c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:126:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__ios_type& (*)(std::basic_istream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>, std::basic_istream<_CharT, _Traits>::__ios_type = std::basic_ios<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:126:7: note: no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__ios_type& (*)(std::basic_istream<char>::__ios_type&) {aka std::basic_ios<char>& (*)(std::basic_ios<char>&)}' c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:133:7: note: std::basic_istream<_CharT, _Traits>::__istream_type& std::basic_istream<_CharT, _Traits>::operator>>(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__istream_type = std::basic_istream<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:133:7: note: no known conversion for argument 1 from 'const int' to 'std::ios_base& (*)(std::ios_base&)' c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:241:7: note: std::basic_istream<_CharT, _Traits>& std::basic_istream<_CharT, _Traits>::operator>>(std::basic_istream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_istream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>] <near match> c:\mingw\bin\../lib/gcc/mingw32/4.6.1/include/c++/istream:241:7: note: no known conversion for argument 1 from 'const int' to 'std::basic_istream<char>::__streambuf_type* {aka std::basic_streambuf<char>*}''

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• Defining < for STL sort algorithm - operator overload, functor or standalone function?

  - by Andy

  I have a stl::list containing Widget class objects. They need to be sorted according to two members in the Widget class. For the sorting to work, I need to define a less-than comparator comparing two Widget objects. There seems to be a myriad of ways to do it. From what I can gather, one can either: a. Define a comparison operator overload in the class: bool Widget::operator< (const Widget &rhs) const b. Define a standalone function taking two Widgets: bool operator<(const Widget& lhs, const Widget& rhs); And then make the Widget class a friend of it: class Widget { // Various class definitions ... friend bool operator<(const Widget& lhs, const Widget& rhs); }; c. Define a functor and then include it as a parameter when calling the sort function: class Widget_Less : public binary_function<Widget, Widget, bool> { bool operator()(const Widget &lhs, const Widget& rhs) const; }; Does anybody know which method is better? In particular I am interested to know if I should do 1 or 2. I searched the book Effective STL by Scott Meyer but unfortunately it does not have anything to say about this. Thank you for your reply.

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• What is the correct less operator for simple class with two int members?

  - by Frank

  What is the correct operator< for the following class? struct Foo { int x; int y; friend bool operator<(const Foo&, const Foo&); }; Thanks!

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• How can i obtain in C# a specific operator's function?

  - by Silviu

  Is it possible to obtain the function behind a C# operator? For example in F# you can do let add = (+);; val add : (int -> int -> int) Is it possible in c# to do something like this: Func<int, int, int> add = (+); or Func<int, int, int> add = Int32.(+) ?

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• SqlParameter contructor compiler overload choice

  - by Ash

  When creating a SqlParameter (.NET3.5) or OdbcParameter I often use the SqlParameter(string parameterName, Object value) constructor overload to set the value in one statement. When I tried passing a literal 0 as the value paramter I was initially caught by the C# compiler choosing the (string, OdbcType) overload instead of (string, Object). MSDN actually warns about this gotcha in the remarks section, but the explanation confuses me. Why does the C# compiler decide that a literal 0 parameter should be converted to OdbcType rather than Object? The warning also says to use Convert.ToInt32(0) to force the Object overload to be used. It confusingly says that this converts the 0 to an "Object type". But isn't 0 already an "Object type"? The Types of Literal Values section of this page seems to say literals are always typed and so inherit from System.Object. This behavior doesn't seem very intuitive given my current understanding? Is this something to do with Contra-variance or Co-variance maybe?

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• Can operator<< in derived class call another operator<< in base class in c++?

  - by ivory

  In my code, Manager is derived from Employee and each of them have an operator<< override. class Employee{ protected: int salary; int rank; public: int getSalary()const{return salary;} int getRank()const{return rank;} Employee(int s, int r):salary(s), rank(r){}; }; ostream& operator<< (ostream& out, Employee& e){ out << "Salary: " << e.getSalary() << " Rank: " << e.getRank() << endl; return out; } class Manager: public Employee{ public: Manager(int s, int r): Employee(s, r){}; }; ostream& operator<< (ostream& out, Manager& m){ out << "Manager: "; cout << (Employee)m << endl; //can not compile, how to call function of Employee? return out; } I hoped cout << (Employee)m << endl; would call ostream& operator<< (ostream& out, Employee& e), but it failed.

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• Java method keyword "final" and its use

  - by Lukas Eder

  When I create complex type hierarchies (several levels, several types per level), I like to use the final keyword on methods implementing some interface declaration. An example: interface Garble { int zork(); } interface Gnarf extends Garble { /** * This is the same as calling {@link #zblah(0)} */ int zblah(); int zblah(int defaultZblah); } And then abstract class AbstractGarble implements Garble { @Override public final int zork() { ... } } abstract class AbstractGnarf extends AbstractGarble implements Gnarf { // Here I absolutely want to fix the default behaviour of zblah // No Gnarf shouldn't be allowed to set 1 as the default, for instance @Override public final int zblah() { return zblah(0); } // This method is not implemented here, but in a subclass @Override public abstract int zblah(int defaultZblah); } I do this for several reasons: It helps me develop the type hierarchy. When I add a class to the hierarchy, it is very clear, what methods I have to implement, and what methods I may not override (in case I forgot the details about the hierarchy) I think overriding concrete stuff is bad according to design principles and patterns, such as the template method pattern. I don't want other developers or my users do it. So the final keyword works perfectly for me. My question is: Why is it used so rarely in the wild? Can you show me some examples / reasons where final (in a similar case to mine) would be very bad?

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• How do I call overloaded Java methods in Clojure.

  - by Pat Wallace

  For this example Java class: package foo; public class TestInterop { public String test(int i) { return "Test(int)"; } public String test(Object i) { return "Test(Object)"; } } When I start Clojure and try to call the test(int) method, the test(Object) method is called instead, because Clojure automatically boxes the integer into a java.lang.Integer object. How do I force Clojure to call the test(int) method? user=> (.test (new foo.TestInterop) 10) "Test(Object)" I want to call methods like Component.add(Component comp, int index) in AWT, but instead keep calling add(Component comp, Object constraints), so the buttons on my toolbar always appear in the wrong order.

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