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  • C++: Template functor cannot deduce reference type

    - by maciekp
    I've got a functor f, which takes a function func and a parameter t of the same type as func. I cannot pass g to f because of compilation error (no matching function for call to f(int&, void (&)(int&)) ). If g would take non-reference parameter g(int s), compilation finishes. Or if I manually specify template parameter f(i, g), compilation also finishes. template<typename T> void f(T t, void (*func)(T)) {} void g(int& s) {} int main(int, char*[]) { int i = 7; f(i, g); // compilation error here return 0; } How can I get deduction to work?

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  • Is there a way to serialize automatically enums as int?

    - by FireAphis
    Hello, Is there a way to serialize enums automatically as int? Every time I define a new enum and write std::stringstream stream; stream << myenum1; stream >> myenum2; the compiler complains that the operators << and are not defined. Do you know a way to tell the compiler to treat enums as plain int's? What makes the problem harder is that, actually, the serialization is inside a template. Something like this: template <typename T> void serialize(const T& value) { std::stringstream stream; stream << value; } So I cannot add any casts :( Maybe I can specialize it somehow? Thank you.

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  • Using C++ is a Linked-List implementation without using pointers possible or not?

    - by sonicoder
    My question is very simply, can one using C++, implment a link-list data structure without using pointers (next nodes)? To further qualify my question, I'm mean can one create a Linked-List data structure using only class instantiations. A common node definition might be like so: template<typename T> struct node { T t; node<T>* next; node<T>* prev; }; I'm aware of std::list etc, I'm just curious to know if its possible or not - and if so how? code examples will be greatly appreciated.

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  • Is the use of union in this matrix class completely safe?

    - by identitycrisisuk
    Unions aren't something I've used that often and after looking at a few other questions on them here it seems like there is almost always some kind of caveat where they might not work. Eg. structs possibly having unexpected padding or endian differences. Came across this in a math library I'm using though and I wondered if it is a totally safe usage. I assume that multidimensional arrays don't have any extra padding and since the type is the same for both definitions they are guaranteed to take up exactly the same amount of memory? template<typename T> class Matrix44T { ... union { T M[16]; T m[4][4]; } m; }; Are there any downsides to this setup? Would the order of definition make any difference to how this works?

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  • Get all descendants types of base class

    - by user1260827
    I have a base class called BaseEvent and several descendants classes: public class BaseEvent { // the some properties ... } [MapInheritance(MapInheritanceType.ParentTable)] public class Film : BaseEvent { // the some properties ... } [MapInheritance(MapInheritanceType.ParentTable)] public class Concert : BaseEvent { // the some properties ... } I have a code which create the BaseEvent instance at runtime: BaseEvent event = new BaseEvent(); //assign values for a properties ... baseEvent.XPObjectType = Database.XPObjectTypes.SingleOrDefault(t => t.TypeName == "MyApp.Module.BO.Events.BaseEvent"); Now, this event will be shows in BaseEvent list view. I want to do the following: when a user click Edit button then show in list view lookup field with all descendants types. And when user saves record change ObjectType to selected value. How can I do this? Thanks. PS. this is asp.net app.

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  • Looking at the C++ new[] cookie. How portable is this code?

    - by carleeto
    I came up with this as a quick solution to a debugging problem - I have the pointer variable and its type, I know it points to an array of objects allocated on the heap, but I don't know how many. So I wrote this function to look at the cookie that stores the number of bytes when memory is allocated on the heap. template< typename T > int num_allocated_items( T *p ) { return *((int*)p-4)/sizeof(T); } //test #include <iostream> int main( int argc, char *argv[] ) { using std::cout; using std::endl; typedef long double testtype; testtype *p = new testtype[ 45 ]; //prints 45 std::cout<<"num allocated = "<<num_allocated_items<testtype>(p)<<std::endl; delete[] p; return 0; } I'd like to know just how portable this code is.

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  • struct with template variables in c++

    - by monkeyking
    I'm playing around with template, so I'm not trying to reinvent the std::vector, I'm trying to get a grasp of templateting in c++. Can I do the following template <typename T> typedef struct{ size_t x; T *ary; }array; What I'm trying to do is a basic templated version of typedef struct{ size_t x; int *ary; }iArray; It looks like its working if I use a class instead of struct, so is it not possible with typedef structs? thanks

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  • Create Class objects based on type signature

    - by Andreas_D
    Class.forName(boolean.class.getName()); This doesn't work in Java - the virtual machine slaps you with a ClassNotFoundException. I was in need for something like that because I wanted to reflect methods based on Strings that included the method signatures, like public void doSomething(boolean yesWeCan, java.lang.String[] presidents); At the end I came up with a custom 'ClassFactory' which translates the type Strings to class objects. This factory includes a lot of handlers for primitive and array type values. The handler for array type objects is something like: if (isArrayOfObjects) { return Class.forName("L["+typeName.replace("[]", "")+";"); } My question is - have I missed something in the Java 1.5+ API that might do the trick?

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  • How to use the boost lexical_cast library for just for checking input

    - by Inverse
    I use the boost lexical_cast library for parsing text data into numeric values quite often. In several situations however, I only need to check if values are numeric; I don't actually need or use the conversion. So, I was thinking about writing a simple function to test if a string is a double: template<typename T> bool is_double(const T& s) { try { boost::lexical_cast<double>(s); return true; } catch (...) { return false; } } My question is, are there any optimizing compilers that would drop out the lexical_cast here since I never actually use the value? Is there a better technique to use the lexical_cast library to perform input checking?

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  • How to reliably specialize template with intptr_t in 32 and 64 bit environments?

    - by vava
    I have a template I want to specialize with two int types, one of them plain old int and another one is intptr_t. On 64 bit platform they have different sizes and I can do that with ease but on 32 bit both types are the same and compiler throws an error about redefinition. What can I do to fix it except for disabling one of definitions off with preprocessor? Some code as an example: template<typename T> type * convert(); template<> type * convert<int>() { return getProperIntType(sizeof(int)); } template<> type * convert<intptr_t>() { return getProperIntType(sizeof(intptr_t)); } //this template can be specialized with non-integral types as well, // so I can't just use sizeof() as template parameter. template<> type * convert<void>() { return getProperVoidType(); }

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  • Why my combobox count is 0?

    - by just_name
    Why the count of items in my ComboBox is always 0 although the datasource of this combobox has data !! <div align="right" dir="rtl"> <asp:Label ID="lbl_contactListName" runat="server" Text="Menu Name :" CssClass="span"></asp:Label> <telerik:RadComboBox ID="ddl_contactList" runat="server" AutoPostBack="True" CausesValidation="False" CollapseDelay="0" Culture="ar-EG" ExpandDelay="0" Filter="StartsWith" ItemsPerRequest="10" MarkFirstMatch="true" Skin="Outlook" EnableAutomaticLoadOnDemand="True" EmptyMessage="-New Menu-" ShowMoreResultsBox="True" OnSelectedIndexChanged="ddl_contactList_SelectedIndexChanged" EnableVirtualScrolling="True" DataTextField="list_desc" DataValueField="list_code" DataSourceID="ObjectDataSource1" EnableViewState="true" Width="300px"> </telerik:RadComboBox> </div> <asp:ObjectDataSource ID="ObjectDataSource1" runat="server" SelectMethod="GetContactListsByDep" TypeName="SendSMS_EmailModule.ContactList"> <SelectParameters> <asp:SessionParameter Name="year" SessionField="year" Type="Int32" /> <asp:SessionParameter Name="main_code" SessionField="main_code" Type="Int32" /> </SelectParameters> </asp:ObjectDataSource>

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  • Adding a child node to a JSON node dynamically

    - by Sai
    I have to create a nested multi level json depending on the resultset that I get from MYSQL. I created a json object initially. Now I want to add child nodes to the already child nodes in the object. d = collections.OrderedDict() jsonobj = {"test": dict(updated_at="today", ID="ID", ads=[])} for rows1 in rs: jsonobj['list']["ads"].append(dict(unit = "1", type ="ad_type", id ="123", updated_at="today", x_id="111", x_name="test")) cur.execute("SELECT * from f_test") rs1 = cur.fetchall() for rows2 in rs1: propertiesObj = [] d["name"]="propName" d["type"]="TypeName" d["value"]="Value1" propertiesObj.append(d) jsonobj['play_list']["ads"].append() Here in the above line I want to add another child node to [play_list].[ads] which is a array list again. the output should look like the following [list].[ads].[preferences].

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  • variadic constructors

    - by FredOverflow
    Are variadic constructors supposed to hide the implicitly generated ones, i.e. the default constructor and the copy constructor? struct Foo { template<typename... Args> Foo(Args&&... x) { std::cout << "inside the variadic constructor\n"; } }; int main() { Foo a; Foo b(a); } Somehow I was expecting this to print nothing after reading this answer, but it prints inside the variadic constructor twice on g++ 4.5.0 :( Is this behavior correct?

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  • How do I map nested generics in NHibernate

    - by Gluip
    In NHibernate you can map generics like this <class name="Units.Parameter`1[System.Int32], Units" table="parameter_int" > </class> But how can I map a class like this? Set<T> where T is a Parameter<int> like this Set<Parameter<int>> My mapping hbm.xml looking like this fails <class name="Set`1[[Units.Parameter`1[System.Int32], Units]],Units" table="settable"/> I simplified my mappings a little to get my point accross very clearly. Basically I want NHibernate to map generic class which has has generic type parameter. Want I understand from googling around is that NHibernate is not able to parse the name to the correct type in TypeNameParser.Parse() which result in the following error when adding the mapping to the configuration System.ArgumentException: Exception of type 'System.ArgumentException' was thrown. Parameter name: typeName@31 Anybody found a way around this limitation?

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  • boost variant static_visitor problem picking correct function

    - by Steve
    I'm sure I'm having a problem with template resolution here, but I'm not sure why I'm having the problem. I have a static visitor I'm passing to boost variant where i've had to do template specialization for certain cases. The case for everything except for MyClass should throw in the static_visitor below. Unfortunately, when the visitor is applied to pull a MyClass out, it selects the most generic case rather than the exact match. I would type each case explicitly, but that will be rather long. So, why is the compiler resolving the most generic case over the exact match, and is there anyway to fix it template<> class CastVisitor<MyClass>:public boost::static_visitor<MyClass> { public: template<typename U> MyClass operator()(const U & i) const { throw std::exception("Unable to cast"); } MyClass operator()(const MyClass& i) { return i; } };

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  • Bug in variadic function template specialization with MSVC?

    - by Andrei Tita
    Using the Visual Studio Nov 2012 CTP, which supports variadic templates (among other things). The following code: template<int, typename... Args> void myfunc(Args... args) { } template<> void myfunc<1, float>(float) { } produces the following errors: error C2785: 'void myfunc(Args...)' and 'void myfunc(float)' have different return types error C2912: explicit specialization 'void myfunc(float)' is not a specialization of a function template (yeah, the first one is pretty funny) So my questions are: 1) Am I writing legal C++11 here? 2) If yes, is there a way to find out if this is a known bug in MSVC before submitting it?

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  • How would one call std::forward on all arguments in a variadic function?

    - by Noah Roberts
    I was just writing a generic object factory and using the boost preprocessor meta-library to make a variadic template (using 2010 and it doesn't support them). My function uses rval references and std::forward to do perfect forwarding and it got me thinking...when C++0X comes out and I had a standard compiler I would do this with real variadic templates. How though, would I call std::forward on the arguments? template < typename ... Params void f(Params ... params) // how do I say these are rvalue reference? { y(std::forward(...params)); //? - I doubt this would work. } Only way I can think of would require manual unpacking of ...params and I'm not quite there yet either. Is there a quicker syntax that would work?

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  • How do I tell if the ObjectDataSource OnSelected event was called for the selectmethod or selectcou

    - by itchi
    I have an object datasource that looks like this: <asp:ObjectDataSource ID="obdsList" runat="server" EnablePaging="True" SelectCountMethod="GetCountByID" SortParameterName="sortExpression" OldValuesParameterFormatString="original_{0}" SelectMethod="GetByID" TypeName="Services.Users" onselected="obdsList_Selected"> <SelectParameters> <asp:QueryStringParameter Name="ID" QueryStringField="ID" Type="Int32" /> </SelectParameters> </asp:ObjectDataSource> And a onselected event like this: protected void obdsList_Selected(object sender, ObjectDataSourceStatusEventArgs e) { } However, the event method is being called twice.. once with my returned list, and once with the returned Int32 count. If I want to cast e.ReturnValue to the return List how do I differentiate between the count and select methods? I can do a e.ReturnValue.GetType().ToString() but that seems like a hack.

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  • template specilization using member enums

    - by Altan
    struct Bar { enum { Special = 4 }; }; template<class T, int K> struct Foo {}; template<class T> struct Foo<T::Special> {}; Usage: Foo<Bar> aa; fails to compile using gcc 4.1.2 It complains about the usage of T::Special for partial specilization of Foo. If Special was a class the solution would be to a typename in front of it. Is there something equivalent to it for enums (or integers)? Thanks, Altan

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  • lambda+for_each+delete on STL containers

    - by rubenvb
    I'm trying to get a simple delete every pointer in my vector/list/... function written with an ultra cool lambda function. Mind you, I don't know c**p about those things :) template <typename T> void delete_clear(T const& cont) { for_each(T.begin(), T.end(), [](???){ ???->delete() } ); T.clear(); } I have no clue what to fill in for the ???'s. Any help is greatly appreciated!

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  • doubt in - Function Objects - c++

    - by Eternal Learner
    I have a class class fobj{ public: fobj(int i):id(i) {} void operator()() { std::cout<<"Prints"<<std::endl; } private: int id; }; template<typename T> void func(T type) { type(); } My Doubt is if I invoke func like Method 1: func(fobj(1); the message I wanted to print is printed. I was always thinking I needed to do something like Method 2: fobj Iobj(1); // create an instance of the fobj class func(Iobj); //call func by passing Iobj(which is a function object) How does Method 1 work? I mean what exactly happens? and how is a call made to the operator() in class fobj ?

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  • Declare variables that depend on unknown type in template functions.

    - by rem
    Suppose I'm writing a template function foo that has type parameter T. It gets an object of type T that must have method bar(). And inside foo I want to create a vector of objects of type returned by bar. In GNU C++ I can write something like that: template<typename T> void foo(T x) { std::vector<__typeof(x.bar())> v; v.push_back(x.bar()); v.push_back(x.bar()); v.push_back(x.bar()); std::cout << v.size() << std::endl; } How to do the same thing in Microsoft Visual C++? Is there some way to write this code that works in both GNU C++ and Visual C++?

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  • C++0x: how to get variadic template parameters without reference?

    - by Sydius
    Given the following contrived (and yes, terrible) example: template<typename... Args> void something(Args... args) { std::tuple<Args...> tuple; // not initializing for sake of example std::get<0>(tuple) = 5; } It works if you call it like so: int x = 10; something<int>(x); However, it does not work if you call it like this: int x = 10; something<int&>(x); Because of the assignment to 5. Assuming that I cannot, for whatever reason, initialize the tuple when it is defined, how might I get this to work when specifying the type as a reference? Specifically, I would like the tuple to be std::tuple<int> even when Args... is int&.

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  • template function error..

    - by sil3nt
    Hi there, I have function which takes in an parameter of a class called "Triple", and am returning the averge of 3 values of type float. template <typename ElemT> float average(Triple ElemT<float> &arg){ float pos1 = arg.getElem(1); float pos2 = arg.getElem(2); float pos3 = arg.getElem(3); return ( (pos1+pos2+po3) /3 ); } when i try compiling this i get q2b.cpp:32: error: template declaration of `float average' q2b.cpp:32: error: missing template arguments before "ElemT" not quite sure what this means.

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  • Can the template parameters of a constructor be explicitly specified?

    - by sth
    A constructor of a class can be a template function. At the point where such a constructor is called, the compiler usually looks at the arguments given to the constructor and determines the used template parameters from them. Is there also some syntax to specify the template parameters explicitly? A contrived example: struct A { template<typename T> A() {} }; Is there a way to instantiate this class? What is the syntax to explicitly specify the constructor's template parameters? My use case would be a problem were the compiler doesn't seem to find the correct templated constructor. Explicitly specifying the template parameters would probably generate more useful error messages or even resolve the problem.

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