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  • How to use the same template for different query sets?

    - by knuckfubuck
    I'm new to Django and setting up my first site. I have a Share model and a template called share_list.html that uses an object_list like this: {% for object in object_list %} I setup haystack using their tutorial and the search template looks like this: {% for result in page.object_list %} I would like to modify the search.html template to have an include of the share_list so I don't have to repeat myself. How can I make it use the same object_list?

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  • C++. How to define template parameter of type T for class A when class T needs a type A template parameter?

    - by jaybny
    Executor class has template of type P and it takes a P object in constructor. Algo class has a template E and also has a static variable of type E. Processor class has template T and a collection of Ts. Question how can I define Executor< Processor<Algo> > and Algo<Executor> ? Is this possible? I see no way to defining this, its kind of an "infinite recursive template argument" See code. template <class T> class Processor { map<string,T> ts; void Process(string str, int i) { ts[str].Do(i); } } template <class P> class Executor { Proc &p; Executor(P &p) : Proc(p) {} void Foo(string str, int i) { p.Process(str,i); } Execute(string str) { } } template <class E> class Algo { static E e; void Do(int i) {} void Foo() { e.Execute("xxx"); } } main () { typedef Processor<Algo> PALGO; // invalid typedef Executor<PALGO> EPALGO; typedef Algo<EPALGO> AEPALGO; Executor<PALGO> executor(PALGO()); AEPALGO::E = executor; }

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  • template pass by const reference

    - by 7vies
    Hi, I've looked over a few similar questions, but I'm still confused. I'm trying to figure out how to explicitly (not by compiler optimization etc) and C++03-compatible avoid copying of an object when passing it to a template function. Here is my test code: #include <iostream> using namespace std; struct C { C() { cout << "C()" << endl; } C(const C&) { cout << "C(C)" << endl; } ~C() { cout << "~C()" << endl; } }; template<class T> void f(T) { cout << "f<T>" << endl; } template<> void f(C c) { cout << "f<C>" << endl; } // (1) template<> void f(const C& c) { cout << "f<C&>" << endl; } // (2) int main() { C c; f(c); return 0; } (1) accepts the object of type C, and makes a copy. Here is the output: C() C(C) f<C> ~C() ~C() So I've tried to specialize with a const C& parameter (2) to avoid this, but this simply doesn't work (apparently the reason is explained in this question). Well, I could "pass by pointer", but that's kind of ugly. So is there some trick that would allow to do that somehow nicely? EDIT: Oh, probably I wasn't clear. I already have a templated function template<class T> void f(T) {...} But now I want to specialize this function to accept a const& to another object: template<> void f(const SpecificObject&) {...} But it only gets called if I define it as template<> void f(SpecificObject) {...}

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  • Problems with passing an anonymous temporary function-object to a templatized constructor.

    - by Akanksh
    I am trying to attach a function-object to be called on destruction of a templatized class. However, I can not seem to be able to pass the function-object as a temporary. The warning I get is (if the comment the line xi.data = 5;): warning C4930: 'X<T> xi2(writer (__cdecl *)(void))': prototyped function not called (was a variable definition intended?) with [ T=int ] and if I try to use the constructed object, I get a compilation error saying: error C2228: left of '.data' must have class/struct/union I apologize for the lengthy piece of code, but I think all the components need to be visible to assess the situation. template<typename T> struct Base { virtual void run( T& ){} virtual ~Base(){} }; template<typename T, typename D> struct Derived : public Base<T> { virtual void run( T& t ) { D d; d(t); } }; template<typename T> struct X { template<typename R> X(const R& r) { std::cout << "X(R)" << std::endl; ptr = new Derived<T,R>(); } X():ptr(0) { std::cout << "X()" << std::endl; } ~X() { if(ptr) { ptr->run(data); delete ptr; } else { std::cout << "no ptr" << std::endl; } } Base<T>* ptr; T data; }; struct writer { template<typename T> void operator()( const T& i ) { std::cout << "T : " << i << std::endl; } }; int main() { { writer w; X<int> xi2(w); //X<int> xi2(writer()); //This does not work! xi2.data = 15; } return 0; }; The reason I am trying this out is so that I can "somehow" attach function-objects types with the objects without keeping an instance of the function-object itself within the class. Thus when I create an object of class X, I do not have to keep an object of class writer within it, but only a pointer to Base<T> (I'm not sure if I need the <T> here, but for now its there). The problem is that I seem to have to create an object of writer and then pass it to the constructor of X rather than call it like X<int> xi(writer(); I might be missing something completely stupid and obvious here, any suggestions?

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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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  • 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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  • 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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  • Constructor initializer list: code from the C++ Primer, chapter 16

    - by Alexandros Gezerlis
    Toward the end of Chapter 16 of the "C++ Primer" I encountered the following code (I've removed a bunch of lines): class Sales_item { public: // default constructor: unbound handle Sales_item(): h() { } private: Handle<Item_base> h; // use-counted handle }; My problem is with the Sales_item(): h() { } line. For the sake of completeness, let me also quote the parts of the Handle class template that I think are relevant to my question (I think I don't need to show the Item_base class): template <class T> class Handle { public: // unbound handle Handle(T *p = 0): ptr(p), use(new size_t(1)) { } private: T* ptr; // shared object size_t *use; // count of how many Handles point to *ptr }; I would have expected something like either: a) Sales_item(): h(0) { } which is a convention the authors have used repeatedly in earlier chapters, or b) Handle<Item_base>() if the intention was to invoke the default constructor of the Handle class. Instead, what the book has is Sales_item(): h() { }. My gut reaction is that this is a typo, since h() looks suspiciously similar to a function declaration. On the other hand, I just tried compiling under g++ and running the example code that uses this class and it seems to be working correctly. Any thoughts?

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  • How do i return a template class from a template function?

    - by LoudNPossiblyRight
    It looks logical but for some reason when i uncomment the last cout line, this code does not compile. How do i return a template class? What do i have to do to this code to make it work? #include<iostream> using namespace std; template <int x> class someclass{ public: int size; int intarr[x]; someclass():size(x){} }; template<int x, int y> int somefunc(someclass<x> A, someclass<y> B){ return ( A.size > B.size ? A.size : B.size); } template<int x, int y, int z> someclass<x> anotherfunc(someclass<y> A, someclass<z> B){ return ( A.size > B.size ? A : B); } int main(){ someclass<5> A; someclass<10> B; cout << "SIZE = " << somefunc(A,B) << endl; //cout << "SIZE = " << (anotherfunc(A,B)).size << endl; //THIS DOES NOT COMPILE return 0; }

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  • Cannot overload function

    - by anio
    So I've got a templatized class and I want to overload the behavior of a function when I have specific type, say char. For all other types, let them do their own thing. However, c++ won't let me overload the function. Why can't I overload this function? I really really do not want to do template specialization, because then I've got duplicate the entire class. Here is a toy example demonstrating the problem: http://codepad.org/eTgLG932 The same code posted here for your reading pleasure: #include <iostream> #include <cstdlib> #include <string> struct Bar { std::string blah() { return "blah"; } }; template <typename T> struct Foo { public: std::string doX() { return m_getY(my_t); } private: std::string m_getY(char* p_msg) { return std::string(p_msg); } std::string m_getY(T* p_msg) { return p_msg->blah(); } T my_t; }; int main(int, char**) { Foo<char> x; Foo<Bar> y; std::cout << "x " << x.doX() << std::endl; return EXIT_SUCCESS; } Thank you everyone for your suggestions. Two valid solutions have been presented. I can either specialize the doX method, or specialize m_getY() method. At the end of the day I prefer to keep my specializations private rather than public so I'm accepting Krill's answer.

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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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  • Simplest way to mix sequences of types with iostreams?

    - by Kylotan
    I have a function void write<typename T>(const T&) which is implemented in terms of writing the T object to an ostream, and a matching function T read<typename T>() that reads a T from an istream. I am basically using iostreams as a plain text serialisation format, which obviously works fine for most built-in types, although I'm not sure how to effectively handle std::strings just yet. I'd like to be able to write out a sequence of objects too, eg void write<typename T>(const std::vector<T>&) or an iterator based equivalent (although in practice, it would always be used with a vector). However, while writing an overload that iterates over the elements and writes them out is easy enough to do, this doesn't add enough information to allow the matching read operation to know how each element is delimited, which is essentially the same problem that I have with a single std::string. Is there a single approach that can work for all basic types and std::string? Or perhaps I can get away with 2 overloads, one for numerical types, and one for strings? (Either using different delimiters or the string using a delimiter escaping mechanism, perhaps.)

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  • template function roundTo int, float -> truncation

    - by Oops
    Hi, according to this question: http://stackoverflow.com/questions/2833730/calling-template-function-without-type-inference the round function I will use in the future now looks like: template < typename TOut, typename TIn > TOut roundTo( TIn value ) { return static_cast<TOut>( value + 0.5 ); } double d = 1.54; int i = rountTo<int>(d); However it makes sense only if it will be used to round to integral datatypes like char, short, int, long, long long int, and it's unsigned counterparts. If it ever will be used with a TOut As float or long double it will deliver s***. double d = 1.54; float f = roundTo<float>(d); // aarrrgh now float is 2.04; I was thinking of a specified overload of the function but ... that's not possible... How would you solve this problem? many thanks in advance Oops

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  • How can I reuse a base class function in a derived class

    - by Armen Ablak
    Let's say we have these four classes: BinaryTree, SplayTree (which is a sub-class of BinaryTree), BinaryNode and SplayNode (which is a sub-class of BinaryNode). In class BinaryTree I have 2 Find functions, like this bool Find(const T &) const; virtual Node<T> * Find(const T &, Node<T> *) const; and in SplayTree I would like to reuse the second one, because it works in the same way (for example) as in SplayTree, the only thing different is the return type, which is SplayNode. I thought it might be enough if I use this line in SplayTree.cpp using BinaryTree::Find; but it isn't. So, how can I do this?

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  • fancybox image sometimes renders outside box

    - by Colleen
    I have the following django template: <script type="text/javascript" src="{{ STATIC_URL }}js/ jquery.fancybox-1.3.4.pack.js"></script> <link rel="stylesheet" href="{{ STATIC_URL }}css/ jquery.fancybox-1.3.4.css" type="text/css" media="screen" /> {% include "submission-form.html" with section="photos" %} <div class="commentables"> {% load thumbnail %} {% for story in objects %} <div class="image {% if forloop.counter|add:"-1"| divisibleby:picsinrow %}left{% else %}{% if forloop.counter| divisibleby:picsinrow %}right{% else %}middle{% endif %}{% endif %}"> {% if story.image %} {% thumbnail story.image size crop="center" as full_im %} <a rel="gallery" href="{% url post slug=story.slug %}"> <img class="preview" {% if story.title %} alt="{{ story.title }}" {% endif %} src="{{ full_im.url }}" full- image="{% if story.image_url %}{{ story.image_url }}{% else %} {{ story.image.url }}{% endif %}"> </a> {% endthumbnail %} {% else %} {% if story.image_url %} {% thumbnail story.image_url size crop="center" as full_im %} <a rel="gallery" href="{% url post slug=story.slug %}"> <img class="preview" {% if story.title %} alt="{{ story.title }}" {% endif %} src="{{ full_im.url }}" full- image="{{ story.image_url }}"> </a> {% endthumbnail %} {% endif %} {% endif %} </div> {% endfor %} {% if rowid != "last" %} <br style="clear: both" /> {% endif %} {% if not no_more_button %} <p style="text-align: right;" class="more-results"><a href="{% url images school_slug tag_slug %}">more...</a></p> {% endif %} </div> <script> $(document).ready(function(){ function changeattr(e){ var f = $(e.clone()); $(f.children()[0]).attr('src', $(f.children() [0]).attr("full-image")); $(f.children()[0]).attr('height', '500px'); return f[0].outerHTML; } $('.image a').each(function(idx, elem) { var e = $(elem); e.fancybox({ title: $(e.children()[0]).attr('alt'), content: changeattr(e) }); }); }); </script> and I'm occasionally getting weird display errors where the box will either not render anything at all (so it will show up as just a thin white bar, basically) or it will render only about 30 px wide, and position itself halfway down the page. In both cases, if I inspect element, I can see the "shadow" of the full picture, at the right size, with the right url. Image source doesnt' seem to make a difference, I'm getting no errors, and this is happening in both chrome and firefox. Does anyone have any ideas?

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  • velocity: join optional fields with a separator/prefix

    - by SlowStrider
    What would be the most concise/readable way in a velocity template to join multiple fields with a separator while leaving out empty or null Strings without adding excess separators? As an example we have a tooltip or appointments that goes like: Appointment ($number) [with $employee] [-] [$remarks] [-] [$roomToVisit] Where I used brackets to indicate optional data. When filled in it would normally show as Appointment (3) with John - ballroom - serve Java coffee When $remarks is empty but $roomToVisit is not, this becomes: Appointment (3) with John - ballroom When $remarks is "serve Java coffee" but $roomToVisit is empty we get: Appointment (3) with John - serve Java coffee When both are empty: Appointment (3) with John Bonus: also make the field prefix optional. When only $employee is empty we should get: Appointment (2) serve Java coffee - ballroom Ideally I would like the velocity template to look very similar to the first code box. If this is not possible, how would you achieve this with a minimum of distracting code tags? Similar ideas (first is much more verbose): Join with intelligent separators velocity: do something except in last loop iteration

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  • Strange Template error : error C2783: could not deduce template argument

    - by osum
    Hi, I have created a simple function with 2 diffrernt template arguments t1, t2 and return type t3. So far no compilation error. But when Itry to call the function from main, I encounter error C2783. I needed to know If the following code is legally ok? If not how is it fixed? please help! template <typename t1, typename t2, typename t3> t3 adder1 (t1 a , t2 b) { return int(a + b); }; int main() { int sum = adder1(1,6.0); // error C2783 could not deduce template argument for t3 return 0; }

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  • c++ global operator not playing well with template class

    - by John
    ok, i found some similar posts on stackoverflow, but I couldn't find any that pertained to my exact situation and I was confused with some of the answers given. Ok, so here is my problem: I have a template matrix class as follows: template <typename T, size_t ROWS, size_t COLS> class Matrix { public: template<typename, size_t, size_t> friend class Matrix; Matrix( T init = T() ) : _matrix(ROWS, vector<T>(COLS, init)) { /*for( int i = 0; i < ROWS; i++ ) { _matrix[i] = new vector<T>( COLS, init ); }*/ } Matrix<T, ROWS, COLS> & operator+=( const T & value ) { for( vector<T>::size_type i = 0; i < this->_matrix.size(); i++ ) { for( vector<T>::size_type j = 0; j < this->_matrix[i].size(); j++ ) { this->_matrix[i][j] += value; } } return *this; } private: vector< vector<T> > _matrix; }; and I have the following global function template: template<typename T, size_t ROWS, size_t COLS> Matrix<T, ROWS, COLS> operator+( const Matrix<T, ROWS, COLS> & lhs, const Matrix<T, ROWS, COLS> & rhs ) { Matrix<T, ROWS, COLS> returnValue = lhs; return returnValue += lhs; } To me, this seems to be right. However, when I try to compile the code, I get the following error (thrown from the operator+ function): binary '+=' : no operator found which takes a right-hand operand of type 'const matrix::Matrix<T,ROWS,COLS>' (or there is no acceptable conversion) I can't figure out what to make of this. Any help if greatly appreciated!

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  • Visual Studio - How to use an existing vsproj's project settings as a template for new project?

    - by Jakobud
    There is some software I want to write a plugin for. The software includes some sample plugins. I want to create a new fresh project but I want to use one of the sample plugin vsproj's project settings as a template. It doesn't seem very clear on how to do this. If I do "New Project From Existing Code" that only imports the cpp, h, etc files into the new project. Right now the only way I can see to copy a sample projects settings is to open two instances of VS2005 next to each other and simply mimic the settings... Surely there is a built in method of doing this?

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  • Can I write a test that succeeds if and only if a statement does not compile?

    - by Billy ONeal
    I'd like to prevent clients of my class from doing something stupid. To that end, I have used the type system, and made my class only accept specific types as input. Consider the following example (Not real code, I've left off things like virtual destructors for the sake of example): class MyDataChunk { //Look Ma! Implementation! }; class Sink; class Source { virtual void Run() = 0; Sink *next_; void SetNext(Sink *next) { next_ = next; } }; class Sink { virtual void GiveMeAChunk(const MyDataChunk& data) { //Impl }; }; class In { virtual void Run { //Impl } }; class Out { }; //Note how filter and sorter have the same declaration. Concrete classes //will inherit from them. The seperate names are there to ensure only //that some idiot doesn't go in and put in a filter where someone expects //a sorter, etc. class Filter : public Source, public Sink { //Drop objects from the chain-of-command pattern that don't match a particular //criterion. }; class Sorter : public Source, public Sink { //Sorts inputs to outputs. There are different sorters because someone might //want to sort by filename, size, date, etc... }; class MyClass { In i; Out o; Filter f; Sorter s; public: //Functions to set i, o, f, and s void Execute() { i.SetNext(f); f.SetNext(s); s.SetNext(o); i.Run(); } }; What I don't want is for somebody to come back later and go, "Hey, look! Sorter and Filter have the same signature. I can make a common one that does both!", thus breaking the semantic difference MyClass requires. Is this a common kind of requirement, and if so, how might I implement a test for it?

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  • Can we have an anonymous struct as template argument?

    - by nonoitall
    The title is pretty self-explanatory, but here's a simplified example: #include <cstdio> template <typename T> struct MyTemplate { T member; void printMemberSize() { printf("%i\n", sizeof(T)); } }; int main() { MyTemplate<struct { int a; int b; }> t; // <-- compiler doesn't like this t.printMemberSize(); return 0; } The compiler complains when I try to use an anonymous struct as a template argument. What's the best way to achieve something like this without having to have a separate, named struct definition?

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  • Why is this std::bind not converted to std::function?

    - by dauphic
    Why is the nested std::bind in the below code not implicitly converted to an std::function<void()> by any of the major compilers (VS2010/2012, gcc, clang)? Is this standard behavior, or a bug? #include <functional> void bar(int, std::function<void()>) { } void foo() { } int main() { std::function<void(int, std::function<void()>)> func; func = std::bind(bar, 5, std::bind(foo)); std::cin.get(); return 0; }

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