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  • Template Child Class Overriding a Parent Class's Virtual Function

    - by user334066
    The below code compiles with gcc v4.3.3 and the templated child class seems to be overriding a virtual function in the parent, but doesn't that break the rule that you cannot have a virtual template function? Or is something else happening that I don't understand? class BaseClass { public: virtual void Func(int var) { std::cout<<"Base int "<<var<<std::endl; } virtual void Func(double var) { std::cout<<"Base double "<<var<<std::endl; } }; template <class TT> class TemplateClass : public BaseClass { public: using BaseClass::Func; virtual void Func(TT var) { std::cout<<"Child TT "<<var<<std::endl; } }; int main(int argc, char **argv) { BaseClass a; TemplateClass<int> b; BaseClass *c = new TemplateClass<int>; int intVar = 3; double doubleVar = 5.5; a.Func(intVar); a.Func(doubleVar); b.Func(intVar); b.Func(doubleVar); c->Func(intVar); c->Func(doubleVar); delete c; } This then outputs: Base int 3 Base double 5.5 Child TT 3 Base double 5.5 Child TT 3 Base double 5.5 as I hoped, but I'm not sure why it works.

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  • Cannot get MEDIA_URL from Django widget's template

    - by Eric
    Hi folks, I am a new Djangoer, and figuring out how to build custom widget, my problem is cannot get the MEDIA_URL in my widget's template, while the form use MySelectWidget able to get the MEDIA_URL itself. # #plus_sign.html # <a href="" class="" id="id_{{ field }}"> <img src="{{ MEDIA_URL }}images/plus_sign.gif" width="10" height="10" alt="Add"/> </a> *^ cannot load the {{ MEDIA_URL}} to this widget's template, and therefore I can't load the .gif image properly. :(* # #custom_widgets.py # from django import forms class MySelectMultiple(forms.SelectMultiple): def render(self, name, *args, **kwargs): html = super(MySelectMultiple, self).render(name, *args, **kwargs) plus = render_to_string("plus_sign.html", {'field': name}) return html+plus # #forms.py # from django import forms from myapp.custom_widgets.py import MySelectMultiple class MyForm(forms.ModelForm): contacts = forms.ModelMultipleChoiceField(Contact.objects, required=False, widget=MySelectMultiple) # #views.py # def AddContacts(request): if request.method == 'POST': form = MyForm(request.POST) if form.is_valid(): cd = form.cleaned_data new = form.save() return HttpResponseRedirect('/addedContact/') else: form = MyForm() return render_to_response('shop/my_form.html', {'form': form}, context_instance=RequestContext(request)) # #my_form.html # {% extends "base.html" %} {% block content %} {{ form.contacts }} {% endblock %} Please let me know how can I load the widget's image properly. Thank you so much for all responses.

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  • Template meta-programming with member function pointers?

    - by wheaties
    Is it possible to use member function pointers with template meta-programming? Such as: class Connection{ public: string getName() const; string getAlias() const; //more stuff }; typedef string (Connection::*Con_Func)() const; template<Con_Func _Name> class Foo{ Connection m_Connect; public: void Foo(){ cout << m_Connect.(*_Name); } }; typedef Foo<&Connection::getName> NamedFoo; typedef Foo<&Connection::getAlias> AliasFoo; Granted, this is rather contrived but is it possible? (yes, there are probably much better ways but humor me.)

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  • Is it possible to render a template from middleware?

    - by pajton
    I have a middleware that does some processing. On certain conditions it raises an exception and the user sees my 500.html template - correctly responding to 500 http status. Now, on some exceptions I would like to render different template than default 500.html. Is it possible/how to achieve that?

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  • Why are there two implementations of std::sort (with and without a comparator) rather than one implementation with a default template parameter?

    - by PolyVox
    In my code I'm adopting a design strategy which is similar to some standard library algorithms in that the exact behavior can be customized by a function object. The simplest example is std::sort, where a function object can control how the comparison is made between objects. I notice that the Visual C++ provides two implementations of std::sort, which naturally involves code duplication. I would have imagined that it was instead possible to have only one implementation, and provide a default comparator (using operator< ) as a default template parameter. What is the rational behind two separate versions? Would my suggestion make the interface more complex in some way? Or result in confusing error messages when the object does not provide operator Thanks, David

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  • Typedef equivalence in function arguments

    - by Warren Seine
    Hi guys, The question is kind of hard to ask without an example so here it is: #include <vector> struct O { }; struct C { template <typename T> void function1(void (C::*callback)(const O*)); template <typename T> void function2(void (C::*callback)(const typename T::value_type)); void print(const O*); }; int main() { C c; c.function1< std::vector<O*> >(&C::print); // Success. c.function2< std::vector<O*> >(&C::print); // Fail. } The error that I am given is: error: no matching function for call to ‘C::function2(void (C::*)(const O*))’. Basically, the only difference between calls is that in function2, I'm more generic since I use the typedef std::vector<O*>::value_type which should resolve to O*, hence similar to function1. I'm using G++ 4.2.1 (I know it's old), but Comeau confirms I'm wrong. Why does the compilation fail?

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  • Templated function with two type parameters fails compile when used with an error-checking macro

    - by SirPentor
    Because someone in our group hates exceptions (let's not discuss that here), we tend to use error-checking macros in our C++ projects. I have encountered an odd compilation failure when using a templated function with two type parameters. There are a few errors (below), but I think the root cause is a warning: warning C4002: too many actual parameters for macro 'BOOL_CHECK_BOOL_RETURN' Probably best explained in code: #include "stdafx.h" template<class A, class B> bool DoubleTemplated(B & value) { return true; } template<class A> bool SingleTemplated(A & value) { return true; } bool NotTemplated(bool & value) { return true; } #define BOOL_CHECK_BOOL_RETURN(expr) \ do \ { \ bool __b = (expr); \ if (!__b) \ { \ return false; \ } \ } while (false) \ bool call() { bool thing = true; // BOOL_CHECK_BOOL_RETURN(DoubleTemplated<int, bool>(thing)); // Above line doesn't compile. BOOL_CHECK_BOOL_RETURN((DoubleTemplated<int, bool>(thing))); // Above line compiles just fine. bool temp = DoubleTemplated<int, bool>(thing); // Above line compiles just fine. BOOL_CHECK_BOOL_RETURN(SingleTemplated<bool>(thing)); BOOL_CHECK_BOOL_RETURN(NotTemplated(thing)); return true; } int _tmain(int argc, _TCHAR* argv[]) { call(); return 0; } Here are the errors, when the offending line is not commented out: 1>------ Build started: Project: test, Configuration: Debug Win32 ------ 1>Compiling... 1>test.cpp 1>c:\junk\temp\test\test\test.cpp(38) : warning C4002: too many actual parameters for macro 'BOOL_CHECK_BOOL_RETURN' 1>c:\junk\temp\test\test\test.cpp(38) : error C2143: syntax error : missing ',' before ')' 1>c:\junk\temp\test\test\test.cpp(38) : error C2143: syntax error : missing ';' before '{' 1>c:\junk\temp\test\test\test.cpp(41) : error C2143: syntax error : missing ';' before '{' 1>c:\junk\temp\test\test\test.cpp(48) : error C2143: syntax error : missing ';' before '{' 1>c:\junk\temp\test\test\test.cpp(49) : error C2143: syntax error : missing ';' before '{' 1>c:\junk\temp\test\test\test.cpp(52) : error C2143: syntax error : missing ';' before '}' 1>c:\junk\temp\test\test\test.cpp(54) : error C2065: 'argv' : undeclared identifier 1>c:\junk\temp\test\test\test.cpp(54) : error C2059: syntax error : ']' 1>c:\junk\temp\test\test\test.cpp(55) : error C2143: syntax error : missing ';' before '{' 1>c:\junk\temp\test\test\test.cpp(58) : error C2143: syntax error : missing ';' before '}' 1>c:\junk\temp\test\test\test.cpp(60) : error C2143: syntax error : missing ';' before '}' 1>c:\junk\temp\test\test\test.cpp(60) : fatal error C1004: unexpected end-of-file found 1>Build log was saved at "file://c:\junk\temp\test\test\Debug\BuildLog.htm" 1>test - 12 error(s), 1 warning(s) ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ========== Any ideas? Thanks!

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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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  • Portfolio problem on a flash template

    - by Nikko
    Hi guys! I have another question about the same template as before. in my website. (www.nikstudio.cl) I need to show in the webpage's portfolio (menu "trabajos") show a few pictures. If you click the thumbnail picture you can see the full size version of that thumbnail. My problem are two: First the picture one and two are the same as five a six ( and i can't change it cause i don't find the place to do this.) And the second is when I copy and paste (in a new layer) of the movieclip "sprite 656" i get in the swf a new picture on the portfolio but i can't click it. Can you help me why is that??? pd: the full template is here. (the .fla an all the files) http://www.2shared.com/file/xbGOYnzC/TM20653ByWMForce.html thanks

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  • Why does a Silverlight application show a blank browser screen when created from exported template?

    - by Edward Tanguay
    I created a silverlight app (without website) named TestApp, with one TextBox: <UserControl x:Class="TestApp.MainPage" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480"> <Grid x:Name="LayoutRoot"> <TextBlock Text="this is a test"/> </Grid> </UserControl> I press F5 and see "this is a test" in my browser (firefox). I select File | Export Template | name it TestAppTemplate and save it. I create a new silverlight app based on the above template. The MainPage.xaml has the exact same XAML as above. I press F5 and see a blank screen in my browser. I look at the HTML source of both of these and they are identical. Everything I have compared in both projects is identical. What do I have to do so that a Silverlight application which is created from my exported template does not show a blank screen? (creating a WPF application from an exported template like this works fine)

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  • Multiset without Compare?

    - by nimcap
    I want to use multiset to count some custom defined keys. The keys are not comparable numerically, comparing two keys does not mean anything, but their equality can be checked. I see that multiset template wants a Compare to order the multiset. The order is not important to me, only the counts are important. If I omit Compare completely what happens? Does multiset work without any problems for my custom keys?

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  • word Application.AddIns.Add throws 'Word cannot open this document template'

    - by Vinay B R
    Hi, I have a template document with a simple macro to insert a file into a document. When i try to load this template file using Application.Addins.Add i am getting an error saying 'Word cannot open this document template'. wordApplication.AddIns.Add( %template file path%, ref trueObj ); This works fine on some machines. Also is there any way to make sure that we load the template file as a global Template always.

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  • Why size_t arguments in template declaration need to be const?

    - by ArunSaha
    I can have std::bitset< 10 > bitsetA; or const size_t LengthB = 20; std::bitset< LengthB > bitsetB; without any problem. But, if the length is not const size_t LengthC = 30; std::bitset< LengthC > bitsetC; // Line 30, say I face the following compilation error 'LengthC' cannot appear in a constant-expression template argument 1 is invalid What is the reason for that? What would be the problem, for compiler and for user code, if line 30 was to be accepted? Is it because LengthC might have some alias?

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  • Member function overloading/template specialization issue

    - by Ferruccio
    I've been trying to call the overloaded table::scan_index(std::string, ...) member function without success. For the sake of clarity, I have stripped out all non-relevant code. I have a class called table which has an overloaded/templated member function named scan_index() in order to handle strings as a special case. class table : boost::noncopyable { public: template <typename T> void scan_index(T val, std::function<bool (uint recno, T val)> callback) { // code } void scan_index(std::string val, std::function<bool (uint recno, std::string val)> callback) { // code } }; Then there is a hitlist class which has a number of templated member functions which call table::scan_index(T, ...) class hitlist { public: template <typename T> void eq(uint fieldno, T value) { table* index_table = db.get_index_table(fieldno); // code index_table->scan_index<T>(value, [&](uint recno, T n)->bool { // code }); } }; And, finally, the code which kicks it all off: hitlist hl; // code hl.eq<std::string>(*fieldno, p1.to_string()); The problem is that instead of calling table::scan_index(std::string, ...), it calls the templated version. I have tried using both overloading (as shown above) and a specialized function template (below), but nothing seems to work. After staring at this code for a few hours, I feel like I'm missing something obvious. Any ideas? template <> void scan_index<std::string>(std::string val, std::function<bool (uint recno, std::string val)> callback) { // code }

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  • What does "static" mean in the context of declaring global template functions?

    - by smf68
    I know what static means in the context of declaring global non-template functions (see e.g. What is a "static" function?), which is useful if you write a helper function in a header that is included from several different locations and want to avoid "duplicate definition" errors. So my question is: What does static mean in the context of declaring global template functions? Please note that I'm specifically asking about global, non-member template functions that do not belong to a class. In other words, what is the difference between the following two: template <typename T> void foo(T t) { /* implementation of foo here */ } template <typename T> static void bar(T t) { /* implementation of bar here */ }

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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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  • How do I translate a ISO 8601 datetime string into a Python datetime object?

    - by Andrey Fedorov
    I'm getting a datetime string in a format like "2009-05-28T16:15:00" (this is ISO 8601, I believe) one hack-ish option seems to be to parse the string using time.strptime and passing the first 6 elements of the touple into the datetime constructor, like: datetime.datetime(*time.strptime("2007-03-04T21:08:12", "%Y-%m-%dT%H:%M:%S")[:6]) I haven't been able to find a "cleaner" way of doing this, is there one?

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  • C++ Function Template With Flexible Return Type

    - by Ignatius Reza
    Let's say that we have a function like so template <class T, class T2> T getMin(T a, T2 b) { if(a < b) return a; return b; } if we call the function like so int a, b; long c; a = getMin(b, c); if c is < a, then the value of c will be type casted to int. Is it possible to make the return type flexible so that it would return an int, or long, or any other type considered smaller by "<" without being type casted?

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  • template static classes across dynamic linked libraries

    - by user322274
    Hello, I have a templated class with a static value, like this: template <class TYPE> class A{ static TYPE value; }; in the code of a dll I assign the static value: code of DLL_1 A<float>::value = 2.0; I wish the value to be shared by all the dlls I'm using, i.e. I want that: code of DLL_2 printf("value on DLL_2 %f",A<float>::value); print out "2.0" any clues? thx

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  • C++ Template Class Constructor with Variable Arguments

    - by david
    Is it possible to create a template function that takes a variable number of arguments, for example, in this Vector< T, C class constructor: template < typename T, uint C > Vector< T, C >::Vector( T, ... ) { assert( C > 0 ); va_list arg_list; va_start( arg_list, C ); for( uint i = 0; i < C; i++ ) { m_data[ i ] = va_arg( arg_list, T ); } va_end( arg_list ); } This almost works, but if someone calls Vector< double, 3 ( 1, 1, 1 ), only the first argument has the correct value. I suspect that the first parameter is correct because it is cast to a double during the function call, and that the others are interpreted as ints and then the bits are stuffed into a double. Calling Vector< double, 3 ( 1.0, 1.0, 1.0 ) gives the desired results. Is there a preferred way to do something like this?

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  • Providing *implicit* conversion operator for template specialization

    - by Neil G
    I have a templated sparse_vector<T> class, and I am also using Boost UBLAS. How would I provide implicit conversions between sparse_vector<double> and boost::numeric::ublas::compressed_vector<double>? I would also like to provide similar conversions between std::vector<double> and boost::numeric::ublas::vector<double>. (I am using gcc 4.4 with C++0x enabled.)

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  • [C++] Multiple inheritance from template class

    - by Tom P.
    Hello, I'm having issues with multiple inheritance from different instantiations of the same template class. Specifically, I'm trying to do this: template <class T> class Base { public: Base() : obj(NULL) { } virtual ~Base() { if( obj != NULL ) delete obj; } template <class T> T* createBase() { obj = new T(); return obj; } protected: T* obj; }; class Something { // ... }; class SomethingElse { // ... }; class Derived : public Base<Something>, public Base<SomethingElse> { }; int main() { Derived* d = new Derived(); Something* smth1 = d->createBase<Something>(); SomethingElse* smth2 = d->createBase<SomethingElse>(); delete d; return 0; } When I try to compile the above code, I get the following errors: 1>[...](41) : error C2440: '=' : cannot convert from 'SomethingElse *' to 'Something *' 1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast 1> [...](71) : see reference to function template instantiation 'T *Base<Something>::createBase<SomethingElse>(void)' being compiled 1> with 1> [ 1> T=SomethingElse 1> ] 1>[...](43) : error C2440: 'return' : cannot convert from 'Something *' to 'SomethingElse *' 1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast The issue seems to be ambiguity due to member obj being inherited from both Base< Something and Base< SomethingElse , and I can work around it by disambiguating my calls to createBase: Something* smth1 = d->Base<Something>::createBase<Something>(); SomethingElse* smth2 = d->Base<SomethingElse>::createBase<SomethingElse>(); However, this solution is dreadfully impractical, syntactically speaking, and I'd prefer something more elegant. Moreover, I'm puzzled by the first error message. It seems to imply that there is an instantiation createBase< SomethingElse in Base< Something , but how is that even possible? Any information or advice regarding this issue would be much appreciated.

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