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• Is memory allocation in linux non-blocking?

  - by Mark

  I am curious to know if the allocating memory using a default new operator is a non-blocking operation. e.g. struct Node { int a,b; }; ... Node foo = new Node(); If multiple threads tried to create a new Node and if one of them was suspended by the OS in the middle of allocation, would it block other threads from making progress? The reason why I ask is because I had a concurrent data structure that created new nodes. I then modified the algorithm to recycle the nodes. The throughput performance of the two algorithms was virtually identical on a 24 core machine. However, I then created an interference program that ran on all the system cores in order to create as much OS pre-emption as possible. The throughput performance of the algorithm that created new nodes decreased by a factor of 5 relative the the algorithm that recycled nodes. I'm curious to know why this would occur. Thanks. *Edit : pointing me to the code for the c++ memory allocator for linux would be helpful as well. I tried looking before posting this question, but had trouble finding it.

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• LINQ to Entites - Left Outer Join - SQL 2000

  - by user255234

  Hi! I'm using Linq to Entities. I have the following query in my code, it includes left outer Join: var surgeonList = (from item in context.T1_STM_Surgeon.Include("T1_STM_SurgeonTitle") .Include("OTER").Include("OSLP") join reptable in context.OSLP on item.Rep equals reptable.SlpCode into surgRepresentative where item.ID == surgeonId select new { ID = item.ID, First = item.First, Last = item.Last, Rep = (surgRepresentative.FirstOrDefault() != null) ? surgRepresentative.FirstOrDefault().SlpName : "N/A", Reg = item.OTER.descript, PrimClinic = item.T1_STM_ClinicalCenter.Name, Titles = item.T1_STM_SurgeonTitle, Phone = item.Phone, Email = item.Email, Address1 = item.Address1, Address2 = item.Address2, City = item.City, State = item.State, Zip = item.Zip, Comments = item.Comments, Active = item.Active, DateEntered = item.DateEntered }) .ToList(); My DEV server has SQL 2008, so the code works just fine. When I moved this code to client's production server - they use SQL 2000, I started getting "Incorrect syntax near '(' ". I've tried changing the ProviderManifestToken to 2000 in my .edmx file, then I started getting "The execution of this query requires the APPLY operator, which is not supported in versions of SQL Server earlier than SQL Server 2005." I tied changing the token to 2005, the "Incorrect syntax near '(' " is back. Can anybody help me to find a workaround for this? Thank you very much in advance!

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• C++ CRTP(template pattern) question

  - by aaa

  following piece of code does not compile, the problem is in T::rank not be inaccessible (I think) or uninitialized in parent template. Can you tell me exactly what the problem is? is passing rank explicitly the only way? or is there a way to query tensor class directly? Thank you #include <boost/utility/enable_if.hpp> template<class T, // size_t N, class enable = void> struct tensor_operator; // template<class T, size_t N> template<class T> struct tensor_operator<T, typename boost::enable_if_c< T::rank == 4>::type > { tensor_operator(T &tensor) : tensor_(tensor) {} T& operator()(int i,int j,int k,int l) { return tensor_.layout.element_at(i, j, k, l); } T &tensor_; }; template<size_t N, typename T = double> // struct tensor : tensor_operator<tensor<N,T>, N> { struct tensor : tensor_operator<tensor<N,T> > { static const size_t rank = N; }; I know the workaround, however am interested in mechanics of template instantiation for self-education

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• How to use a class's type as the type argument for an inherited collection property in C#

  - by Edelweiss Peimann

  I am trying to create a representation of various types of card that inherit from a generic card class and which all contain references to their owning decks. I tried re-declaring them, as suggested here, but it still won't convert to the specific card type. The code I currently have is as such: public class Deck<T> : List<T> where T : Card { void Shuffle() { throw new NotImplementedException("Shuffle not yet implemented."); } } public class Card { public Deck<Card> OwningDeck { get; set; } } public class FooCard : Card { public Deck<FooCard> OwningDeck { get { return (Deck<FooCard>)base.OwningDeck; } set { OwningDeck = value; } } } The compile-time error I am getting: Error 2 Cannot convert type 'Game.Cards.Deck' to 'Game.Cards.Deck' And a warning suggesting I use a new operator to specify that the hiding is intentional. Would doing so be a violation of convention? Is there a better way? My question to stackoverflow is this: Can what I am trying to do be done elegantly in the .NET type system? If so, can some examples be provided?

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• C++ Suppress Automatic Initialization and Destruction

  - by Travis G

  How does one suppress the automatic initialization and destruction of a type? While it is wonderful that T buffer[100] automatically initializes all the elements of buffer, and destroys them when they fall out of scope, this is not the behavior I want. #include <iostream> static int created = 0, destroyed = 0; struct S { S() { ++created; } ~S() { ++destroyed; } }; template <typename T, size_t KCount> class Array { private: T m_buffer[KCount]; public: Array() { // some way to suppress the automatic initialization of m_buffer } ~Array() { // some way to suppress the automatic destruction of m_buffer } }; int main() { { Array<S, 100> arr; } std::cout << "Created:\t" << created << std::endl; std::cout << "Destroyed:\t" << destroyed << std::endl; return 0; } The output of this program is: Created: 100 Destroyed: 100 I would like it to be: Created: 0 Destroyed: 0 My only idea is to make m_buffer some trivially constructed and destructed type like char and then rely on operator[] to wrap the pointer math for me, although this seems like a horribly hacked solution. Another solution would be to use malloc and free, but that gives a level of indirection that I do not want.

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• Template trick to optimize out allocations

  - by anon

  I have: struct DoubleVec { std::vector<double> data; }; DoubleVec operator+(const DoubleVec& lhs, const DoubleVec& rhs) { DoubleVec ans(lhs.size()); for(int i = 0; i < lhs.size(); ++i) { ans[i] = lhs[i]] + rhs[i]; // assume lhs.size() == rhs.size() } return ans; } DoubleVec someFunc(DoubleVec a, DoubleVec b, DoubleVec c, DoubleVec d) { DoubleVec ans = a + b + c + d; } Now, in the above, the "a + b + c + d" will cause the creation of 3 temporary DoubleVec's -- is there a way to optimize this away with some type of template magic ... i.e. to optimize it down to something equivalent to: DoubleVec ans(a.size()); for(int i = 0; i < ans.size(); i++) ans[i] = a[i] + b[i] + c[i] + d[i]; You can assume all DoubleVec's have the same # of elements. The high level idea is to have do some type of templateied magic on "+", which "delays the computation" until the =, at which point it looks into itself, goes hmm ... I'm just adding thes numbers, and syntheizes a[i] + b[i] + c[i] + d[i] ... instead of all the temporaries. Thanks!

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• C# Type conversion between two similar Datatable objects

  - by Ali

  I have .NET project with sync framework and two separate Datasets for MS SQL and Compact SQL. in my base class I have a generic DataTable object. in my derived classed I assign Typed DataTable to the generic object based on whether the application is operating online or offline: example: if (online) _dataTable = new MSSQLDataSet.Customer; else _dataTable = new CompactSQLDataSet.Customer; Now every where in my code i have to check and do a cast based on the current network mode like this: public void changeCustomerID(int ID) { if (online) (MSSQLDataSet.CustomerDataTable)_dataTable)[i].CustomerID = value; else (CompactMSSQLDataSet.CustomerDataTable)_dataTable)[i].CustomerID = value; } but I don't think this is very efficient and I believe it can be done in a smarter way to only use one line of code by dynamically getting the Type of _dataTable on the run time. my problem is at the design time, in order to acess datatable porperties such as "CustomerID" it has to be casted to either MSSQLDataSet.CustomerDataTable or CompactMSSQLDataSet.CustomerDataTable. Is there a way to have a function or a operator to convert the _datatable to its runtime type but still be able to use it's design time properties which are the same between the two types? something like: ((aType)_dataTable)[i].CustomerID = value; //or GetRuntimeType(_dataTable)[i].CustomerID = value;

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• GUID or int entity key with SQL Compact/EF4?

  - by David Veeneman

  This is a follow-up to an earlier question I posted on EF4 entity keys with SQL Compact. SQL Compact doesn't allow server-generated identity keys, so I am left with creating my own keys as objects are added to the ObjectContext. My first choice would be an integer key, and the previous answer linked to a blog post that shows an extension method that uses the Max operator with a selector expression to find the next available key: public static TResult NextId<TSource, TResult>(this ObjectSet<TSource> table, Expression<Func<TSource, TResult>> selector) where TSource : class { TResult lastId = table.Any() ? table.Max(selector) : default(TResult); if (lastId is int) { lastId = (TResult)(object)(((int)(object)lastId) + 1); } return lastId; } Here's my take on the extension method: It will work fine if the ObjectContext that I am working with has an unfiltered entity set. In that case, the ObjectContext will contain all rows from the data table, and I will get an accurate result. But if the entity set is the result of a query filter, the method will return the last entity key in the filtered entity set, which will not necessarily be the last key in the data table. So I think the extension method won't really work. At this point, the obvious solution seems to be to simply use a GUID as the entity key. That way, I only need to call Guid.NewGuid() method to set the ID property before I add a new entity to my ObjectContext. Here is my question: Is there a simple way of getting the last primary key in the data store from EF4 (without having to create a second ObjectContext for that purpose)? Any other reason not to take the easy way out and simply use a GUID? Thanks for your help.

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• Instrumenting a string

  - by George Polevoy

  Somewhere in C++ era i have crafted a library, which enabled string representation of the computation history. Having a math expression like: TScalar Compute(TScalar a, TScalar b, TScalar c) { return ( a + b ) * c; } I could render it's string representation: r = Compute(VerbalScalar("a", 1), VerbalScalar("b", 2), VerbalScalar("c", 3)); Assert.AreEqual(9, r.Value); Assert.AreEqual("(a+b)*c==(1+2)*3", r.History ); C++ operator overloading allowed for substitution of a simple type with a complex self-tracking entity with an internal tree representation of everything happening with the objects. Now i would like to have the same possibility for NET strings, only instead of variable names i would like to see a stack traces of all the places in code which affected a string. And i want it to work with existing code, and existing compiled assemblies. Also i want all this to hook into visual studio debugger, so i could set a breakpoint, and see everything that happened with a string. Which technology would allow this kind of things? I know it sound like an utopia, but I think visual studio code coverage tools actually do the same kind of job while instrumenting the assemblies.

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• useer degined Copy ctor, and copy-ctors further down the chain - compiler bug ? programers brainbug

  - by J.Colmsee

  Hi. i have a little problem, and I am not sure if it's a compiler bug, or stupidity on my side. I have this struct : struct BulletFXData { int time_next_fx_counter; int next_fx_steps; Particle particles[2];//this is the interesting one ParticleManager::ParticleId particle_id[2]; }; The member "Particle particles[2]" has a self-made kind of smart-ptr in it (resource-counted texture-class). this smart-pointer has a default constructor, that initializes to the ptr to 0 (but that is not important) I also have another struct, containing the BulletFXData struct : struct BulletFX { BulletFXData data; BulletFXRenderFunPtr render_fun_ptr; BulletFXUpdateFunPtr update_fun_ptr; BulletFXExplosionFunPtr explode_fun_ptr; BulletFXLifetimeOverFunPtr lifetime_over_fun_ptr; BulletFX( BulletFXData data, BulletFXRenderFunPtr render_fun_ptr, BulletFXUpdateFunPtr update_fun_ptr, BulletFXExplosionFunPtr explode_fun_ptr, BulletFXLifetimeOverFunPtr lifetime_over_fun_ptr) :data(data), render_fun_ptr(render_fun_ptr), update_fun_ptr(update_fun_ptr), explode_fun_ptr(explode_fun_ptr), lifetime_over_fun_ptr(lifetime_over_fun_ptr) { } /* //USER DEFINED copy-ctor. if it's defined things go crazy BulletFX(const BulletFX& rhs) :data(data),//this line of code seems to do a plain memory-copy without calling the right ctors render_fun_ptr(render_fun_ptr), update_fun_ptr(update_fun_ptr), explode_fun_ptr(explode_fun_ptr), lifetime_over_fun_ptr(lifetime_over_fun_ptr) { } */ }; If i use the user-defined copy-ctor my smart-pointer class goes crazy, and it seems that calling the CopyCtor / assignment operator aren't called as they should. So - does this all make sense ? it seems as if my own copy-ctor of struct BulletFX should do exactly what the compiler-generated would, but it seems to forget to call the right constructors down the chain. compiler bug ? me being stupid ? Sorry about the big code, some small example could have illustrated too. but often you guys ask for the real code, so well - here it is :D

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• Simple matrix example using C++ template class

  - by skyeagle

  I am trying to write a trivial Matrix class, using C++ templates in an attempt to brush up my C++, and also to explain something to a fellow coder. This is what I have som far: template class<T> class Matrix { public: Matrix(const unsigned int rows, const unsigned int cols); Matrix(const Matrix& m); Matrix& operator=(const Matrix& m); ~Matrix(); unsigned int getNumRows() const; unsigned int getNumCols() const; template <class T> T getCellValue(unsigned int row, unsigned col) const; template <class T> void setCellValue(unsigned int row, unsigned col, T value) const; private: // Note: intentionally NOT using smart pointers here ... T * m_values; }; template<class T> inline T Matrix::getCellValue(unsigned int row, unsigned col) const { } template<class T> inline void Matrix::setCellValue(unsigned int row, unsigned col, T value) { } I'm stuck on the ctor, since I need to allocate a new[] T, it seems like it needs to be a template method - however, I'm not sure I have come accross a templated ctor before. How can I implemnt the ctor?

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• How to write a custom predicate for multi_index_containder with composite_key?

  - by Titan

  I googled and searched in the boost's man, but didn't find any examples. May be it's a stupid question...anyway. So we have the famous phonebook from the man: typedef multi_index_container< phonebook_entry, indexed_by< ordered_non_unique< composite_key< phonebook_entry, member<phonebook_entry,std::string,&phonebook_entry::family_name>, member<phonebook_entry,std::string,&phonebook_entry::given_name> >, composite_key_compare< std::less<std::string>, // family names sorted as by default std::greater<std::string> // given names reversed > >, ordered_unique< member<phonebook_entry,std::string,&phonebook_entry::phone_number> > > > phonebook; phonebook pb; ... // look for all Whites std::pair<phonebook::iterator,phonebook::iterator> p= pb.equal_range(boost::make_tuple("White"), my_custom_comp()); How should my_custom_comp() look like? I mean it's clear for me then it takes boost::multi_index::composite_key_result<CompositeKey> as an argumen (due to compilation errors :) ), but what is CompositeKey in that particular case? struct my_custom_comp { bool operator()( ?? boost::multi_index::composite_key_result<CompositeKey> ?? ) const { return blah_blah_blah; } }; Thanks in advance.

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• Modify passed, nested dict/list

  - by Gerenuk

  I was thinking of writing a function to normalize some data. A simple approach is def normalize(l, aggregate=sum, norm_by=operator.truediv): aggregated=aggregate(l) for i in range(len(l)): l[i]=norm_by(l[i], aggregated) l=[1,2,3,4] normalize(l) l -> [0.1, 0.2, 0.3, 0.4] However for nested lists and dicts where I want to normalize over an inner index this doesnt work. I mean I'd like to get l=[[1,100],[2,100],[3,100],[4,100]] normalize(l, ?? ) l -> [[0.1,100],[0.2,100],[0.3,100],[0.4,100]] Any ideas how I could implement such a normalize function? Maybe it would be crazy cool to write normalize(l[...][0]) Is it possible to make this work?? Or any other ideas? Also not only lists but also dict could be nested. Hmm... EDIT: I just found out that numpy offers such a syntax (for lists however). Anyone know how I would implement the ellipsis trick myself?

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• Help with this compile error

  - by Scott

  I just picked up an old project and I'm not sure what the following error could mean. g++ -o BufferedReader.o -c -g -Wall -std=c++0x -I/usr/include/xmms2 -Ijsoncpp/include/json/ -fopenmp -I/usr/include/ImageMagick -I/usr/include/xmms2 -I/usr/include/libvisual-0.4 -D_GNU_SOURCE=1 -D_REENTRANT -I/usr/include/SDL -DQT_CORE_LIB -DQT_GUI_LIB -DQT_SCRIPT_LIB -DQT_SHARED -I/usr/include/QtCore -I/usr/include/QtGui -I/usr/include/QtScript BufferedReader.cpp In file included from BufferedReader.cpp:23: /usr/include/string.h:36:42: error: missing binary operator before token "(" In file included from /usr/lib/gcc/i686-redhat-linux/4.4.3/../../../../include/c++/4.4.3/cwchar:47, from /usr/lib/gcc/i686-redhat-linux/4.4.3/../../../../include/c++/4.4.3/bits/postypes.h:42, from /usr/lib/gcc/i686-redhat-linux/4.4.3/../../../../include/c++/4.4.3/iosfwd:42, from /usr/lib/gcc/i686-redhat-linux/4.4.3/../../../../include/c++/4.4.3/ios:39, from /usr/lib/gcc/i686-redhat-linux/4.4.3/../../../../include/c++/4.4.3/istream:40, from /usr/lib/gcc/i686-redhat-linux/4.4.3/../../../../include/c++/4.4.3/sstream:39, from BufferedReader.cpp:24: At line 24 of BufferedReader.cpp is #include <string.h>. I've tried it with just <string> but get the same thing. Any clue? Here's the snippet of code from string.h /* Tell the caller that we provide correct C++ prototypes. */ #if defined __cplusplus && __GNUC_PREREQ (4, 4) //line 36 # define __CORRECT_ISO_CPP_STRING_H_PROTO #endif Does that mean __GNUC_PREREQ isn't defined?

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• Compiling GWT 2.6.1 at Java 7 source level

  - by Neeko

  I've recently updated my GWT project to 2.6.1, and started to make use of Java 7 syntax since 2.6 now supports Java 7. However, when I attempt to compile, I'm receiving compiler errors such as [ERROR] Line 42: '<>' operator is not allowed for source level below 1.7 How do I specify the GWT compiler to target 1.7? I was under the impression that it would do that by default, but I guess not. I've attempted cleaning the project, including deleting the gwt-unitCache directory but to no avail. Here is my Ant compile target. <target name="compile" depends="prepare"> <javac includeantruntime="false" debug="on" debuglevel="lines,vars,source" srcdir="${src.dir}" destdir="${build.dir}"> <classpath refid="project.classpath"/> </javac> </target> <target name="gwt-compile" depends="compile"> <java failonerror="true" fork="true" classname="com.google.gwt.dev.Compiler"> <classpath> <!-- src dir is added to ensure the module.xml file(s) are on the classpath --> <pathelement location="${src.dir}"/> <pathelement location="${build.dir}"/> <path refid="project.classpath"/> </classpath> <jvmarg value="-Xmx256M"/> <arg value="${gwt.module.name}"/> </java> </target>

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• Databinding expression for retrieving value of related collection using LINQ

  - by joshb

  I have a GridView that is bound to a LINQDataSource control that is returning a collection of customers. Within my DataGrid I need to display the home phone number of a customer, if they have one. The phone numbers of a customer are stored in a separate table with a foreign key pointing to the customer table. The following binding expression gets me the first phone number for a customer: <asp:TemplateField HeaderText="LastName" SortExpression="LastName"> <ItemTemplate> <asp:Label ID="PhoneLabel" runat="server" Text='<%# Eval("Phones[0].PhoneNumber") %>'></asp:Label> </ItemTemplate> </asp:TemplateField> I need to figure out how to get the home phone number specifically (filter based on phone type) and handle the scenario where the customer does not have a home phone in the database. Right now it's throwing an out of range exception if the customer does not have any phone numbers. I've tried using the Where operator with a lambda expression to filter the phone type but it doesn't work: <%# Eval("Phones.Where(p => p.PhoneTypeId == 2).PhoneNumber") %> Solutions or links to any good articles on the subject would be much appreciated.

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• C++ STL Map vs Vector speed

  - by sub

  In the interpreter for my experimental programming language I have a symbol table. Each symbol consists of a name and a value (the value can be e.g.: of type string, int, function, etc.). At first I represented the table with a vector and iterated through the symbols checking if the given symbol name fitted. Then I though using a map, in my case map<string,symbol>, would be better than iterating through the vector all the time but: It's a bit hard to explain this part but I'll try. If a variable is retrieved the first time in a program in my language, of course its position in the symbol table has to be found (using vector now). If I would iterate through the vector every time the line gets executed (think of a loop), it would be terribly slow (as it currently is, nearly as slow as microsoft's batch). So I could use a map to retrieve the variable: SymbolTable[ myVar.Name ] But think of the following: If the variable, still using vector, is found the first time, I can store its exact integer position in the vector with it. That means: The next time it is needed, my interpreter knows that it has been "cached" and doesn't search the symbol table for it but does something like SymbolTable.at( myVar.CachedPosition ). Now my (rather hard?) question: Should I use a vector for the symbol table together with caching the position of the variable in the vector? Should I rather use a map? Why? How fast is the [] operator? Should I use something completely different?

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• SFINAE + sizeof = detect if expression compiles

  - by FredOverflow

  I just found out how to check if operator<< is provided for a type. template<class T> T& lvalue_of_type(); template<class T> T rvalue_of_type(); template<class T> struct is_printable { template<class U> static char test(char(*)[sizeof( lvalue_of_type<std::ostream>() << rvalue_of_type<U>() )]); template<class U> static long test(...); enum { value = 1 == sizeof test<T>(0) }; typedef boost::integral_constant<bool, value> type; }; Is this trick well-known, or have I just won the metaprogramming Nobel prize? ;) EDIT: I made the code simpler to understand and easier to adapt with two global function template declarations lvalue_of_type and rvalue_of_type.

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• How do you calculate div and mod of floating point numbers?

  - by boost

  In Perl, the % operator seems to assume integers. For instance: sub foo { my $n1 = shift; my $n2 = shift; print "perl's mod=" . $n1 % $n2, "\n"; my $res = $n1 / $n2; my $t = int($res); print "my div=$t", "\n"; $res = $res - $t; $res = $res * $n2; print "my mod=" . $res . "\n\n"; } foo( 3044.952963, 7.1 ); foo( 3044.952963, -7.1 ); foo( -3044.952963, 7.1 ); foo( -3044.952963, -7.1 ); gives perl's mod=6 my div=428 my mod=6.15296300000033 perl's mod=-1 my div=-428 my mod=6.15296300000033 perl's mod=1 my div=-428 my mod=-6.15296300000033 perl's mod=-6 my div=428 my mod=-6.15296300000033 Now as you can see, I've come up with a "solution" already for calculating div and mod. However, what I don't understand is what effect the sign of each argument should have on the result. Wouldn't the div always be positive, being the number of times n2 fits into n1? How's the arithmetic supposed to work in this situation?

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• which is better: a lying copy constructor or a non-standard one?

  - by PaulH

  I have a C++ class that contains a non-copyable handle. The class, however, must have a copy constructor. So, I've implemented one that transfers ownership of the handle to the new object (as below) class Foo { public: Foo() : h_( INVALID_HANDLE_VALUE ) { }; // transfer the handle to the new instance Foo( const Foo& other ) : h_( other.Detach() ) { }; ~Foo() { if( INVALID_HANDLE_VALUE != h_ ) CloseHandle( h_ ); }; // other interesting functions... private: /// disallow assignment const Foo& operator=( const Foo& ); HANDLE Detach() const { HANDLE h = h_; h_ = INVALID_HANDLE_VALUE; return h; }; /// a non-copyable handle mutable HANDLE h_; }; // class Foo My problem is that the standard copy constructor takes a const-reference and I'm modifying that reference. So, I'd like to know which is better (and why): a non-standard copy constructor: Foo( Foo& other ); a copy-constructor that 'lies': Foo( const Foo& other ); Thanks, PaulH

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• How can variadic char template arguments from user defined literals be converted back into numeric types?

  - by Pubby

  This question is being asked because of this one. C++11 allows you to define literals like this for numeric literals: template<char...> OutputType operator "" _suffix(); Which means that 503_suffix would become <'5','0','3'> This is nice, although it isn't very useful in the form it's in. How can I transform this back into a numeric type? This would turn <'5','0','3'> into a constexpr 503. Additionally, it must also work on floating point literals. <'5','.','3> would turn into int 5 or float 5.3 A partial solution was found in the previous question, but it doesn't work on non-integers: template <typename t> constexpr t pow(t base, int exp) { return (exp > 0) ? base * pow(base, exp-1) : 1; }; template <char...> struct literal; template <> struct literal<> { static const unsigned int to_int = 0; }; template <char c, char ...cv> struct literal<c, cv...> { static const unsigned int to_int = (c - '0') * pow(10, sizeof...(cv)) + literal<cv...>::to_int; }; // use: literal<...>::to_int // literal<'1','.','5'>::to_int doesn't work // literal<'1','.','5'>::to_float not implemented

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• Circumvent c++ null-terminated string frustration

  - by ypnos

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

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• C++ destructor called on array index - why

  - by tge

  The following code (from Apache Tuscany SDO C++) occasionally (actually very rarely) causes subsequent crashes and I don't understand what's going on. The following statement is in DataObjectImpl.cpp (see stack below): PropertyImpl* DataObjectImpl::getPropertyImpl(unsigned int index) { ... 904 PropertyList props = getType().getProperties(); 905 if (index < props.size()) 906 { 907 return (PropertyImpl*)&props[index]; ... causes the following stack (all omitted frames above and below look plausible): Note: #11 libtuscany_sdo.dll!std::vector<>::~vector<> [c:\program files\microsoft visual studio 9.0\vc\include\vector:559] Note: #12 libtuscany_sdo.dll!commonj::sdo::PropertyList::~PropertyList [y:\external\tuscany\src\runtime\core\src\commonj\sdo\propertylist.cpp:60] Note: #13 libtuscany_sdo.dll!commonj::sdo::DataObjectImpl::getPropertyImpl [y:\external\tuscany\src\runtime\core\src\commonj\sdo\dataobjectimpl.cpp:907] Note: #14 libtuscany_sdo.dll!commonj::sdo::DataObjectImpl::getSDOValue [y:\external\tuscany\src\runtime\core\src\commonj\sdo\dataobjectimpl.cpp:3845] The actual question is - why is the destructor of PropertyList called?? As stated, the stack looks OK otherwise, also the vector destructor, as PropertyList has a member std::vector<PropertyImplPtr plist; and the array index operator of PropertyList just calls the array index of the plist member. And, even more puzzling (to me), why this happens only occasionally ... Many thx!!

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• C++ variable to const expression

  - by user1344784

  template <typename Real> class A{ }; template <typename Real> class B{ }; //... a few dozen more similar template classes class Computer{ public slots: void setFrom(int from){ from_ = from; } void setTo(int to){ to_ = to; } private: template <int F, int T> void compute(){ using boost::fusion::vector; using boost::fusion::at_c; vector<A<float>, B<float>, ...> v; at_c<from_>(v).operator()(at_c<to_>(v)); //error; needs to be const-expression. }; This question isn't about Qt, but there is a line of Qt code in my example. The setFrom() and setTo() are functions that are called based on user selection via the GUI widget. The root of my problem is that 'from' and 'to' are variables. In my compute member function I need to pick a type (A, B, etc.) based on the values of 'from' and 'to'. The only way I know how to do what I need to do is to use switch statements, but that's extremely tedious in my case and I would like to avoid. Is there anyway to convert the error line to a constant-expression?

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• optimize output value using a class and public member

  - by wiso

  Suppose you have a function, and you call it a lot of times, every time the function return a big object. I've optimized the problem using a functor that return void, and store the returning value in a public member: #include <vector> const int N = 100; std::vector<double> fun(const std::vector<double> & v, const int n) { std::vector<double> output = v; output[n] *= output[n]; return output; } class F { public: F() : output(N) {}; std::vector<double> output; void operator()(const std::vector<double> & v, const int n) { output = v; output[n] *= n; } }; int main() { std::vector<double> start(N,10.); std::vector<double> end(N); double a; // first solution for (unsigned long int i = 0; i != 10000000; ++i) a = fun(start, 2)[3]; // second solution F f; for (unsigned long int i = 0; i != 10000000; ++i) { f(start, 2); a = f.output[3]; } } Yes, I can use inline or optimize in an other way this problem, but here I want to stress on this problem: with the functor I declare and construct the output variable output only one time, using the function I do that every time it is called. The second solution is two time faster than the first with g++ -O1 or g++ -O2. What do you think about it, is it an ugly optimization?

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