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• help in the Donalds B. Johnson's algorithm, i cannot understand the pseudo code

  - by Pitelk

  Hi , does anyone know the Donald B. Johnson's algorithm which enumarates all the elementary circuits (cycles) in a Directed graph? link text I have the paper he had published in 1975 but I cannot understand the pseudo-code. My goal is to implement this algorithm in java. Some questions i have is for example what is the matrix Ak it refers to. In the pseudo code mentions that Ak:=adjacency structure of strong component K with least vertex in subgraph of G induced by {s,s+1,....n}; Does that mean i have to implement another algorithm that finds the Ak matrix? Another question is what the following means? begin logical f; Does also the line "logical procedure CIRCUIT (integer value v);" means that the circuit procedure returns a logical variable. In the pseudo code also has the line "CIRCUIT := f;" . Does this mean? It would be great if someone could translate this 1970's pseudocode to a more modern type of pseudo code so i can understand it in case you are interested to help but you cannot find the paper please email me at [email protected] and i will send you the paper. Thanks in advance

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• Error C2451: Illegal conditional expression of type 'UnaryOp<E1, Op>' in ostream - visual studio 9

  - by Steven Hill

  I am getting a repeated error with VS 9. The code compiles under GNU C++, but I want debug with the VS IDE. Any idea what could be causing this error. Error 13 error C2451: conditional expression of type 'UnaryOp' is illegal \Microsoft Visual Studio 9.0\VC\include\ostream 512 //unary constraint template class UnaryOp : public Constraint { public: const E1& e1; UnaryOp(const E1& _e1); bool Satisfiable() const; Bool SatisfiableAux() const; void Print (std::ostream& os) const; UnaryOp* clone () const; //operator bool () const { return true; } }; template std::ostream& operator<<(std::ostream& os, const UnaryOp& unop); UnaryOp code that uses ostream: template INLINE void UnaryOp::Print (std::ostream& os) const { os << *this; } template INLINE std::ostream& operator<<(std::ostream& os, const UnaryOp& unop) { return os << Op::name << unop.e1; } ostream line with error: _Myt& __CLR_OR_THIS_CALL put(_Elem _Ch) { // insert a character ios_base::iostate _State = ios_base::goodbit; const sentry _Ok(*this); 512 if (!_Ok) _State |= ios_base::badbit; else { // state okay, insert character _TRY_IO_BEGIN

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• slicing a 2d numpy array

  - by MedicalMath

  The following code: import numpy as p myarr=[[0,1],[0,6],[0,1],[0,6],[0,1],[0,6],[0,1],[0,6],[0,1],[0,6],[0,1],[0,6],[0,1],[0,6],[0,1],[0,6],[0,1],[0,6]] copy=p.array(myarr) p.mean(copy)[:,1] Is generating the following error message: Traceback (most recent call last): File "<pyshell#3>", line 1, in <module> p.mean(copy)[:,1] IndexError: 0-d arrays can only use a single () or a list of newaxes (and a single ...) as an index I looked up the syntax at this link and I seem to be using the correct syntax to slice. However, when I type copy[:,1] into the Python shell, it gives me the following output, which is clearly wrong, and is probably what is throwing the error: array([1, 6, 1, 6, 1, 6, 1, 6, 1, 6, 1, 6, 1, 6, 1, 6, 1, 6]) Can anyone show me how to fix my code so that I can extract the second column and then take the mean of the second column as intended in the original code above? EDIT: Thank you for your solutions. However, my posting was an oversimplification of my real problem. I used your solutions in my real code, and got a new error. Here is my real code with one of your solutions that I tried: filteredSignalArray=p.array(filteredSignalArray) logical=p.logical_and(EndTime-10.0<=matchingTimeArray,matchingTimeArray<=EndTime) finalStageTime=matchingTimeArray.compress(logical) finalStageFiltered=filteredSignalArray.compress(logical) for j in range(len(finalStageTime)): if j == 0: outputArray=[[finalStageTime[j],finalStageFiltered[j]]] else: outputArray+=[[finalStageTime[j],finalStageFiltered[j]]] print 'outputArray[:,1].mean() is: ',outputArray[:,1].mean() And here is the error message that is now being generated by the new code: File "mypath\myscript.py", line 1545, in WriteToOutput10SecondsBeforeTimeMarker print 'outputArray[:,1].mean() is: ',outputArray[:,1].mean() TypeError: list indices must be integers, not tuple Second EDIT: This is solved now that I added: outputArray=p.array(outputArray) above my code. I have been at this too many hours and need to take a break for a while if I am making these kinds of mistakes.

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• Boost bind with asio::placeholders::error

  - by Leandro

  Why it doesn't work? --- boost_bind.cc --- #include #include #include void func1 (const int& i) { } void func2 (const ::asio::error_code& e) { } int main () { ::boost::function f1 = ::boost::bind (&func1, 1); // it doesn't work! ::boost::function f2 = ::boost::bind (&func2, ::asio::placeholders::error); return 0; } This is the error: while_true@localhost:~ g++ -lpthread boost_bind.cc -o boost_bind In file included from boost_bind.cc:2: /usr/include/boost/bind.hpp: In member function ‘void boost::_bi::list1::operator()(boost::_bi::type, F&, A&, int) [with F = void (*)(const asio::error_code&), A = boost::_bi::list0, A1 = boost::arg (*)()]’: /usr/include/boost/bind/bind_template.hpp:20: instantiated from ‘typename boost::_bi::result_traits::type boost::_bi::bind_t::operator()() [with R = void, F = void (*)(const asio::error_code&), L = boost::_bi::list1 (*)()]’ /usr/include/boost/function/function_template.hpp:152: instantiated from ‘static void boost::detail::function::void_function_obj_invoker0::invoke(boost::detail::function::function_buffer&) [with FunctionObj = boost::_bi::bind_t (*)() , R = void]’ /usr/include/boost/function/function_template.hpp:904: instantiated from ‘void boost::function0::assign_to(Functor) [with Functor = boost::_bi::bind_t (*)() , R = void]’ /usr/include/boost/function/function_template.hpp:720: instantiated from ‘boost::function0::function0(Functor, typename boost::enable_if_c::type) [with Functor = boost::_bi::bind_t (*)() , R = void]’ /usr/include/boost/function/function_template.hpp:1040: instantiated from ‘boost::function::function(Functor, typename boost::enable_if_c::type) [with Functor = boost::_bi::bind_t (*)() , R = void]’ boost_bind.cc:14: instantiated from here /usr/include/boost/bind.hpp:232: error: no match for ‘operator[]’ in ‘a[boost::_bi::storage1 (*)()::a1_ [with int I = 1]]’ while_true@localhost:~

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• Has Twisted changed its dependencies?

  - by cdecker

  Hi all, I'm currently working on a Python/Twisted project which is to be distributed and tested on Planetlab. For some reason my code was working on friday and now that I wanted to test a minor change it refuses to work at all: Traceback (most recent call last): File "acn_a4/src/node.py", line 6, in <module> from twisted.internet.protocol import DatagramProtocol File "/usr/lib/python2.5/site-packages/Twisted-10.0.0-py2.5-linux-i686.egg/twisted/__init__.py", line 18, in <module> from twisted.python import compat File "/usr/lib/python2.5/site-packages/Twisted-10.0.0-py2.5-linux-i686.egg/twisted/python/compat.py", line 146, in <module> import operator File "/home/cdecker/dev/acn/acn_a4/src/operator.py", line 7, in <module> File "/home/cdecker/acn_a4/src/node.py", line 6, in <module> from twisted.internet.protocol import DatagramProtocol File "/usr/lib/python2.5/site-packages/Twisted-10.0.0-py2.5-linux-i686.egg/twisted/internet/protocol.py", line 20, in <module> from twisted.python import log, failure, components File "/usr/lib/python2.5/site-packages/Twisted-10.0.0-py2.5-linux-i686.egg/twisted/python/log.py", line 19, in <module> from twisted.python import util, context, reflect File "/usr/lib/python2.5/site-packages/Twisted-10.0.0-py2.5-linux-i686.egg/twisted/python/util.py", line 5, in <module> import os, sys, hmac, errno, new, inspect, warnings File "/usr/lib/python2.5/inspect.py", line 32, in <module> from operator import attrgetter ImportError: cannot import name attrgetter And since I'm pretty new to python I have no idea what could have caused this problem. All suggestions are welcome :-)

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• How to print a number with a space as thousand separator?

  - by dygi

  I've got a simple class Currency with overloaded operator<<. I don't know how can i separate the number with spaces every 3 digits, so it looks like: "1 234 567 ISK". #include <cstdlib> #include <iostream> using namespace std; class Currency { int val; char curr[4]; public: Currency(int _val, const char * _curr) { val = _val; strcpy(curr, _curr); } friend ostream & operator<< (ostream & out, const Currency & c); }; ostream & operator<< (ostream & out, const Currency & c) { out << c.val<< " " << c.curr; return out; } int main(int argc, char *argv[]) { Currency c(2354123, "ISK"); cout << c; } What interests me, is somehow the easiest solution for this particular situation.

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• video calling (center)

  - by rrejc

  We are starting to develop a new application and I'm searching for information/tips/guides on application architecture. Application should: read the data from an external (USB) device send the data to the remote server (through internet) receive the data from the remote server perform a video call with to the calling (support) center receive a video call call from the calling (support) center support touch screens In addition: some of the data should also be visible through the web page. So I was thinking about: On the server side: use the database (probably MS SQL) use ORM (nHibernate) to map the data from the DB to the domain objects create a layer with business logic in C# create a web (WCF) services (for client application) create an asp.net mvc application (for item 7.) to enable data view through the browser On the client side I would use WPF 4 application which will communicate with external device and the wcf services on the server. So far so good. Now the problem begins. I have no idea how to create a video call (outgoing or incoming) part of the application. I believe that there is no problem to communicate with microphone, speaker, camera with WPF/C#. But how to communicate with the call center? What protocol and encoding should be used? I think that I will need to create some kind of server which will: have a list of operators in the calling center and track which operator is occupied and which operator is free have a list of connected end users receive incoming calls from end users and delegate call to free operator delegate calls from calling center to the end user Any info, link, anything on where to start would be much appreciated. Many thanks!

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• Coredump in Multithreading Application in RHEL-5

  - by Chinnu

  I am working on multi-threading application it is dumping frequently.I could not able to analyaze the core.The core is showing like this Core was generated by 'thread-process'. Program terminated with signal 6, Aborted. #0 0x00000038f4e30045 in raise () from /lib64/libc.so.6 (gdb) where #0 0x00000038f4e30045 in raise () from /lib64/libc.so.6 #1 0x00000038f4e31ae0 in abort () from /lib64/libc.so.6 #2 0x00000038f4e681bb in __libc_message () from /lib64/libc.so.6 #3 0x00000038f4e72b96 in free () from /lib64/libc.so.6 #4 0x000000000046a137 in std::string::substr () #5 0x000000000042c549 in std::operator<< <char, std::char_traits<char>, std::allocator<char> > () #6 0x000000000042cc1d in std::operator<< <char, std::char_traits<char>, std::allocator<char> > () #7 0x000000000046b069 in std::string::substr () #8 0x000000000046c866 in std::string::substr () #9 0x0000000000431062 in std::operator<< <char, std::char_traits<char>, std::allocator<char> > () #10 0x00000038f5a062e7 in start_thread () from /lib64/libpthread.so.0 #11 0x00000038f4ece3bd in clone () from /lib64/libc.so.6

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• rails: has_many :through validation?

  - by ramonrails

  Rails 2.1.0 (Cannot upgrade for now due to several constraints) I am trying to achieve this. Any hints? A project has many users through join model A user has many projects through join model Admin class inherits User class. It also has some Admin specific stuff. Admin like inheritance for Supervisor and Operator Project has one Admin, One supervisor and many operators. Now I want to 1. submit data for project, admin, supervisor and operator in a single project form 2. validate all and show errors on the project form. Project has_many :projects_users ; has_many :users, :through => :projects_users User has_many :projects_users ; has_many :projects, :through => :projects_users ProjectsUser = :id integer, :user_id :integer, :project_id :integer, :user_type :string ProjectUser belongs_to :project, belongs_to :user Admin < User # User has 'type:string' column for STI Supervisor < User Operator < User Is the approach correct? Any and all suggestions are welcome.

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• How to pass multiple different records (not class due to delphi limitations) to a function?

  - by mingo

  Hi to all. I have a number of records I cannot convert to classes due to Delphi limitation (all of them uses class operators to implement comparisons). But I have to pass to store them in a class not knowing which record type I'm using. Something like this: type R1 = record begin x :Mytype; class operator Equal(a,b:R1) end; type R2 = record begin y :Mytype; class operator Equal(a,b:R2) end; type Rn = record begin z :Mytype; class operator Equal(a,b:Rn) end; type TC = class begin x : TObject; y : Mytype; function payload (n:TObject) end; function TC.payload(n:TObject) begin x := n; end; program: c : TC; x : R1; y : R2; ... c := TC.Create(): n:=TOBject(x); c.payload(n); Now, Delphi do not accept typecast from record to TObject, and I cannot make them classes due to Delphi limitation. Anyone knows a way to pass different records to a function and recognize their type when needed, as we do with class: if x is TMyClass then TMyClass(x) ... ???

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• function objects versus function pointers

  - by kumar_m_kiran

  Hi All, I have two questions related to function objects and function pointers, Question : 1 When I read the different uses sort algorithm of STL, I see that the third parameter can be a function objects, below is an example class State { public: //... int population() const; float aveTempF() const; //... }; struct PopLess : public std::binary_function<State,State,bool> { bool operator ()( const State &a, const State &b ) const { return popLess( a, b ); } }; sort( union, union+50, PopLess() ); Question : Now, How does the statement, sort(union, union+50,PopLess()) work? PopLess() must be resolved into something like PopLess tempObject.operator() which would be same as executing the operator () function on a temporary object. I see this as, passing the return value of overloaded operation i.e bool (as in my example) to sort algorithm. So then, How does sort function resolve the third parameter in this case? Question : 2 Question Do we derive any particular advantage of using function objects versus function pointer? If we use below function pointer will it derive any disavantage? inline bool popLess( const State &a, const State &b ) { return a.population() < b.population(); } std::sort( union, union+50, popLess ); // sort by population PS : Both the above references(including example) are from book "C++ Common Knowledge: Essential Intermediate Programming" by "Stephen C. Dewhurst". I was unable to decode the topic content, thus have posted for help. Thanks in advance for your help.

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• How `is_base_of` works?

  - by Alexey Malistov

  Why the following code works? typedef char (&yes)[1]; typedef char (&no)[2]; template <typename B, typename D> struct Host { operator B*() const; operator D*(); }; template <typename B, typename D> struct is_base_of { template <typename T> static yes check(D*, T); static no check(B*, int); static const bool value = sizeof(check(Host<B,D>(), int())) == sizeof(yes); }; //Test sample class Base {}; class Derived : private Base {}; //Exspression is true. int test[is_base_of<Base,Derived>::value && !is_base_of<Derived,Base>::value]; Note that B is private base. Note that operator B*() is const. How does this work? Why this works? Why static yes check(D*, T); is better than static yes check(B*, int); ?

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• Does this language feature already exists?

  - by Pindatjuh

  I'm currently developing a new language for programming in a continuous environment (compare it to electrical engineering), and I've got some ideas on a certain language construction. Let me explain the feature by explanation and then by definition; x = a | b; Where x is a variable and a and b are other variables (or static values). if(x == a) { // all references to "x" are essentially references to "a". } if(x == b) { // same but with "b" } if(x != a) { // ... } if(x == a | b) { // guaranteed that "x" is '"a" | "b"'; interacting with "x" // will interact with both "a" and "b". } // etc. In the above, all code-blocks are executed, but the "scope" changes in each block how x is interpreted. In the first block, x is guaranteed to be a: thus interacting with x inside that block will interact on a. The second and the third code-block are only equal in this situation (because not b only remains a). The last block guarantees that x is at least a or b. Further more; | is not the "bitwise or operator", but I've called it the "and/or"-operator. It's definition is: "|" = "and" | "or" (On my blog, http://cplang.wordpress.com/2009/12/19/binop-and-or/, is more (mathematical) background information on this operator. It's loosely based on sets.) I do not know if this construction already exists, so that's my question: does this language feature already exists?

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• How might I wrap the FindXFile-style APIs to the STL-style Iterator Pattern in C++?

  - by BillyONeal

  Hello everyone :) I'm working on wrapping up the ugly innards of the FindFirstFile/FindNextFile loop (though my question applies to other similar APIs, such as RegEnumKeyEx or RegEnumValue, etc.) inside iterators that work in a manner similar to the Standard Template Library's istream_iterators. I have two problems here. The first is with the termination condition of most "foreach" style loops. STL style iterators typically use operator!= inside the exit condition of the for, i.e. std::vector<int> test; for(std::vector<int>::iterator it = test.begin(); it != test.end(); it++) { //Do stuff } My problem is I'm unsure how to implement operator!= with such a directory enumeration, because I do not know when the enumeration is complete until I've actually finished with it. I have sort of a hack together solution in place now that enumerates the entire directory at once, where each iterator simply tracks a reference counted vector, but this seems like a kludge which can be done a better way. The second problem I have is that there are multiple pieces of data returned by the FindXFile APIs. For that reason, there's no obvious way to overload operator* as required for iterator semantics. When I overload that item, do I return the file name? The size? The modified date? How might I convey the multiple pieces of data to which such an iterator must refer to later in an ideomatic way? I've tried ripping off the C# style MoveNext design but I'm concerned about not following the standard idioms here. class SomeIterator { public: bool next(); //Advances the iterator and returns true if successful, false if the iterator is at the end. std::wstring fileName() const; //other kinds of data.... }; EDIT: And the caller would look like: SomeIterator x = ??; //Construct somehow while(x.next()) { //Do stuff } Thanks! Billy3

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• What noncluster index would be better to create on SQL Server?

  - by Junior Mayhé

  Here I am studying nonclustered indexes on SQL Server Management Studio. I've created a table with more than 1 million records. This table has a primary key. SELECT CustomerName FROM Customers Which leads the execution plan to show me: I/O cost = 3.45646 Operator cost = 4.57715 For the first attempt to improve performance, I've created a nonclustered index for this table: CREATE NONCLUSTERED INDEX [IX_CustomerID_CustomerName] ON [dbo].[Customers] ( [CustomerId] ASC, [CustomerName] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] GO With this first try, I've executed the select statement and the execution plan shows me: I/O cost = 2.79942 Operator cost = 3.92001 Now the second try, I've deleted this nonclustered index in order to create a new one. CREATE NONCLUSTERED INDEX [IX_CategoryName] ON [dbo].[Categories] ( [CategoryId] ASC ) INCLUDE ( [CategoryName]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] GO With this second try, I've executed the select statement and the execution plan shows me the same result: I/O cost = 2.79942 Operator cost = 3.92001 Am I doing something wrong or this is expected? Shall I use the first nonclustered index with two fields, or the second nonclustered with one field (CategoryID) including the second field (CategoryName)?

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• Link error for user defined class type template parameter

  - by isurulucky

  Hi, I implemented a Simple STL map in C++. Factored out the comparison as a type as I was instructed to, then implemented the comparison as shown below: template <typename T> int KeyCompare<T>::operator () (T tKey1, T tKey2) { if(tKey1 < tKey2) return -1; else if(tKey1 > tKey2) return 1; else return 0; } here, tKey1 and tKet2 are the two keys I'm comparing. This worked well for all the basic data types and string. I added a template specialization to compare keys of a user defined type named Test and added a specialization as follows: int KeyCompare<Test>::operator () (Test tKey1, Test tKey2) { if(tKey1.a < tKey2.a) return -1; else if(tKey1.a > tKey2.a) return 1; else return 0; } when I run this, I get a linking error saying SimpleMap.obj : error LNK2005: "public: int __thiscall KeyCompare::operator()(class Test,class Test)" (??R?$KeyCompare@VTest@@@@QAEHVTest@@0@Z) already defined in MapTest.obj SimpleMap.obj : error LNK2005: "public: __thiscall KeyCompare::~KeyCompare(void)" (??1?$KeyCompare@VTest@@@@QAE@XZ) already defined in MapTest.obj SimpleMap.obj : error LNK2005: "public: __thiscall KeyCompare::KeyCompare(void)" (??0?$KeyCompare@VTester@@@@QAE@XZ) already defined in MapTest.obj MapTest.cpp is the test harness class in which I wrote the test case. I have used include guards as well, to stop multiple inclusions. Any idea what the matter is?? Thank you very much!!

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• How to use boost::fusion::transform on heterogeneous containers?

  - by Kyle

  Boost.org's example given for fusion::transform is as follows: struct triple { typedef int result_type; int operator()(int t) const { return t * 3; }; }; // ... assert(transform(make_vector(1,2,3), triple()) == make_vector(3,6,9)); Yet I'm not "getting it." The vector in their example contains elements all of the same type, but a major point of using fusion is containers of heterogeneous types. What if they had used make_vector(1, 'a', "howdy") instead? int operator()(int t) would need to become template<typename T> T& operator()(T& const t) But how would I write the result_type? template<typename T> typedef T& result_type certainly isn't valid syntax, and it wouldn't make sense even if it was, because it's not tied to the function.

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• C++ Implicit Conversion Operators

  - by Imbue

  I'm trying to find a nice inheritance solution in C++. I have a Rectangle class and a Square class. The Square class can't publicly inherit from Rectangle, because it cannot completely fulfill the rectangle's requirements. For example, a Rectangle can have it's width and height each set separately, and this of course is impossible with a Square. So, my dilemma. Square obviously will share a lot of code with Rectangle; they are quite similar. For examlpe, if I have a function like: bool IsPointInRectangle(const Rectangle& rect); it should work for a square too. In fact, I have a ton of such functions. So in making my Square class, I figured I would use private inheritance with a publicly accessible Rectangle conversion operator. So my square class looks like: class Square : private Rectangle { public: operator const Rectangle&() const; }; However, when I try to pass a Square to the IsPointInRectangle function, my compiler just complains that "Rectangle is an inaccessible base" in that context. I expect it to notice the Rectangle operator and use that instead. Is what I'm trying to do even possible? If this can't work I'm probably going to refactor part of Rectangle into MutableRectangle class. Thanks.

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• template warnings and error help, (gcc)

  - by sil3nt

  Hi there, I'm working on an container class template (for int,bool,strings etc), and I've been stuck with this error cont.h:56: error: expected initializer before '&' token for this section template <typename T> const Container & Container<T>::operator=(const Container<T> & rightCont){ what exactly have I done wrong there?. Also not sure what this warning message means. cont.h:13: warning: friend declaration `bool operator==(const Container<T>&, const Container<T>&)' declares a non-template function cont.h:13: warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning at this position template <typename T> class Container{ friend bool operator==(const Container<T> &rhs,const Container<T> &lhs); public:

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• iOS layout; I'm not getting it

  - by Tbee

  Well, "not getting it" is too harsh; I've got it working in for what for me is a logical setup, but it does not seem to be what iOS deems logical. So I'm not getting something. Suppose I've got an app that shows two pieces of information; a date and a table. According to the MVC approach I've got three MVC at work here, one for the date, one for the table and one that takes both these MCVs and makes it into a screen, wiring them up. The master MVC knows how/where it wants to layout the two sub MVC's. Each detail MVC only takes care of its own childeren within the bounds that were specified by the master MVC. Something like: - (void)loadView { MVC* mvc1 = [[MVC1 alloc] initwithFrame:...] [self.view addSubview:mvc1.view]; MVC* mvc2 = [[MVC2 alloc] initwithFrame:...] [self.view addSubview:mvc2.view]; } If the above is logical (which is it for me) then I would expect any MVC class to have a constructor "initWithFrame". But an MVC does not, only view have this. Why? How would one correctly layout nested MVCs? (Naturally I do not have just these two, but the detail MVCs have sub MVCs again.)

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• Calling Base Class Functions with Inherited Type

  - by Kein Mitleid

  I can't describe exactly what I want to say but I want to use base class functions with an inherited type. Like I want to declare "Coord3D operator + (Coord3D);" in one class, but if I use it with Vector3D operands, I want it to return Vector3D type instead of Coord3D. With this line of code below, I add two Vector3D's and get a Coord3D in return, as told to me by the typeid().name() function. How do I reorganize my classes so that I get a Vector3D on return? #include <iostream> #include <typeinfo> using namespace std; class Coord3D { public: float x, y, z; Coord3D (float = 0.0f, float = 0.0f, float = 0.0f); Coord3D operator + (Coord3D &); }; Coord3D::Coord3D (float a, float b, float c) { x = a; y = b; z = c; } Coord3D Coord3D::operator+ (Coord3D &param) { Coord3D temp; temp.x = x + param.x; temp.y = y + param.y; temp.z = z + param.z; return temp; } class Vector3D: public Coord3D { public: Vector3D (float a = 0.0f, float b = 0.0f, float c = 0.0f) : Coord3D (a, b, c) {}; }; int main () { Vector3D a (3, 4, 5); Vector3D b (6, 7, 8); cout << typeid(a + b).name(); return 0; }

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• Why `is_base_of` works with private inheritance?

  - by Alexey Malistov

  Why the following code works? typedef char (&yes)[1]; typedef char (&no)[2]; template <typename B, typename D> struct Host { operator B*() const; operator D*(); }; template <typename B, typename D> struct is_base_of { template <typename T> static yes check(D*, T); static no check(B*, int); static const bool value = sizeof(check(Host<B,D>(), int())) == sizeof(yes); }; //Test sample class B {}; class D : private B {}; //Exspression is true. int test[is_base_of<B,D>::value && !is_base_of<D,B>::value]; Note that B is private base. Note that operator B*() is const. How does this work? Why this works? Why static yes check(D*, T); is better than static yes check(B*, int); ?

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• Parsing: How to make error recovery in grammars like " a* b*"?

  - by Lavir the Whiolet

  Let we have a grammar like this: Program ::= a* b* where "*" is considered to be greedy. I usually implement "*" operator naively: Try to apply the expression under "*" to input one more time. If it has been applied successfully then we are still under current "*"-expression; try to apply the expression under "*" one more time. Otherwise we have reached next grammar expression; put characters parsed by expression under "*" back into input and proceed with next expression. But if there are errors in input in any of "a*" or "b*" part such a parser will "think" that in position of error both "a*" and "b*" have finished ("let's try "a"... Fail! OK, it looks like we have to proceed to "b*". Let's try "b"... Fail! OK, it looks like the string should have been finished...). For example, for string "daaaabbbbbbc" it will "say": "The string must end at position 1, delete superflous characters: daaaabbbbbbc". In short, greedy "*" operator becomes lazy if there are errors in input. How to make "*" operator to recover from errors nicely?

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• C++ Returning Pointers/References

  - by m00st

  I have a fairly good understanding of the dereferencing operator, the address of operator, and pointers in general. I however get confused when I see stuff such as this: int* returnA() { int *j = &a; return j; } int* returnB() { return &b; } int& returnC() { return c; } int& returnC2() { int *d = &c; return *d; } In returnA() I'm asking to return a pointer; just to clarify this works because j is a pointer? In returnB() I'm asking to return a pointer; since a pointer points to an address, the reason why returnB() works is because I'm returning &b? In returnC() I'm asking for an address of int to be returned. When I return c is the & operator automatically "appended" c? In returnC2() I'm asking again for an address of int to be returned. Does *d work because pointers point to an address? Assume a, b, c are initialized as integers. Can someone validate if I am correct with all four of my questions?

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• code is not compiling

  - by user323422

  template< class Type ,int Size = 3> class cStack { Type *m_array; int m_Top; int m_Size; public:cStack(); friend std::ostream& operator <<(std::ostream &, const cStack<Type,Size> &); }; template< class Type ,int Size > std::ostream& operator << ( std::ostream &os, const cStack<Type,Size> &s) { for( int i=0; i<=s.GetTop();i++) { os << s.m_array[i]; } return os; } on compilin it showing following error error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class cStack<int,3> const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$cStack@H$02@@@Z) referenced in function _main

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