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  • Using member variables inherited from a templated base class (C++)

    - by Aaron Becker
    I'm trying to use member variables of a templated base class in a derived class, as in this example: template <class dtype> struct A { int x; }; template <class dtype> struct B : public A<dtype> { void test() { int id1 = this->x; // always works int id2 = A<dtype>::x; // always works int id3 = B::x; // always works int id4 = x; // fails in gcc & clang, works in icc and xlc } }; gcc and clang are both very picky about using this variable, and require either an explicit scope or the explicit use of "this". With some other compilers (xlc and icc), things work as I would expect. Is this a case of xlc and icc allowing code that's not standard, or a bug in gcc and clang?

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  • Do I need multiple template specializations if I want to specialize for several kinds of strings?

    - by romkyns
    For example: template<typename T> void write(T value) { mystream << value; } template<> void write<const char*>(const char* value) { write_escaped(mystream, value); } template<> void write<char*>(char* value) { write_escaped(mystream, value); } template<> void write<std::string>(std::string value) { write_escaped(mystream.c_str(), value); } This looks like I'm doing it wrong, especially the two variants for const and non-const char*. However I checked that if I only specialize for const char * then passing a char * variable will invoke the non-specialized version, when called like this in VC++10: char something[25]; strcpy(something, "blah"); write(something); What would be the proper way of doing this?

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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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  • Linking a template class using another template class (error LNK2001)

    - by Luís Guilherme
    I implemented the "Strategy" design pattern using an Abstract template class, and two subclasses. Goes like this: template <class T> class Neighbourhood { public: virtual void alter(std::vector<T>& array, int i1, int i2) = 0; }; and template <class T> class Swap : public Neighbourhood<T> { public: virtual void alter(std::vector<T>& array, int i1, int i2); }; There's another subclass, just like this one, and alter is implemented in the cpp file. Ok, fine! Now I declare another method, in another class (including neighbourhood header file, of course), like this: void lSearch(/*parameters*/, Neighbourhood<LotSolutionInformation> nhood); It compiles fine and cleanly. When starting to link, I get the following error: 1>SolverFV.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall lsc::Neighbourhood<class LotSolutionInformation>::alter(class std::vector<class LotSolutionInformation,class std::allocator<class LotSolutionInformation> > &,int,int)" (?alter@?$Neighbourhood@VLotSolutionInformation@@@lsc@@UAEXAAV?$vector@VLotSolutionInformation@@V?$allocator@VLotSolutionInformation@@@std@@@std@@HH@Z)

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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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  • Filtering results and pagination

    - by alj
    I have a template that shows a filter form and below it a list of the result records. I bind the form to the request so that the filter form sets itself to the options the user submitted when the results are returned. I also use pagination. Using the code in the pagination documentation means that when the user clicks for the next page, the form data is lost. What is the best way of dealing with pagination and filtering in this way? Passing the querystring to the paginiation links. Change the pagination links to form buttons and therefore submit the filter form at the same time, but this assumes that the user hasn't messed about with the filter options. As above but with the original data as hidden fields. ALJ

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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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  • Type parameterization in Scala

    - by horatius83
    So I'm learning Scala at the moment, and I'm trying to create an abstract vector class with a vector-space of 3 (x,y,z coordinates). I'm trying to add two of these vectors together with the following code: package math class Vector3[T](ax:T,ay:T,az:T) { def x = ax def y = ay def z = az override def toString = "<"+x+", "+y+", "+z+">" def add(that: Vector3[T]) = new Vector3(x+that.x, y+that.y, z+that.z) } The problem is I keep getting this error: error: type mismatch; found : T required: String def add(that: Vector3[T]) = new Vector3(x+that.x, y+that.y, z+that.z) I've tried commenting out the "toString" method above, but that doesn't seem to have any effect. Can anyone tell me what I'm doing wrong?

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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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  • Lua template processor question

    - by PeterMmm
    I'm going to use that template engine LTP . There is not so much doc available. Now i'm stuck how to pass an environment into the render engine. I have basically this: local ltp = require("ltp.template") ltp.render(io.stdout, 1, "index.dhtm", false, {}, "<?lua", "?>", { total="2400" }) What data structure should be the last parameter (env_code), a string, a table with key=val ?

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  • Re render template when submit a form

    - by Agusti-N
    Hi, i've this problem: I've a view that render a template and then retrive to the user the rendered template. Yesterday i want to add a form. The problem is when i submit this form to the same url that the view render the template, i have to re render the template with previusly data and then show the errors (if they are) in the form with the new renderd data, so i have to re render all the time the previusly information. I've view a lot of examples, but ALL examples have the form in an externar html(separated) How can avoid this ?

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  • If you use MVC in your web app then you dont need to use Smarty(TemplateEngine) Right?

    - by Imran
    I'm just trying to understand the Templating(system). If you use MVC in your web application then you don't need to use something like Smarty(template engine) as you are already separating application code from presentation code anyway by using MVC right? please correct me? So am i correct in thinking it's MVC OR Templating or do you use both in your apps?If any one could explain this in detail it would be great. Thank you in advance;-)

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  • clang does not compile but g++ does

    - by user1095108
    Can someone help me with this code: #include <type_traits> #include <vector> struct nonsense { }; template <struct nonsense const* ptr, typename R> typename std::enable_if<!std::is_void<R>::value, int>::type fo(void* const) { return 0; } template <struct nonsense const* ptr, typename R> typename std::enable_if<std::is_void<R>::value, int>::type fo(void* const) { return 1; } typedef int (*func_type)(void*); template <std::size_t O> void run_me() { static struct nonsense data; typedef std::pair<char const* const, func_type> pair_type; std::vector<pair_type> v; v.push_back(pair_type{ "a", fo<&data, int> }); v.push_back(pair_type{ "b", fo<&data, void> }); } int main(int, char*[]) { run_me<2>(); return 0; } clang-3.3 does not compile this code, but g++-4.8.1 does, which of the two compiler is right? Is something wrong with the code, as I suspect? The error reads: a.cpp:32:15: error: no matching constructor for initialization of 'pair_type' (aka 'pair<const char *const, func_type>') v.push_back(pair_type{ "a", fo<&data, int> }); ^ ~~~~~~~~~~~~~~~~~~~~~~~ a.cpp:33:15: error: no matching constructor for initialization of 'pair_type' (aka 'pair<const char *const, func_type>') v.push_back(pair_type{ "b", fo<&data, void> }); ^ ~~~~~~~~~~~~~~~~~~~~~~~~

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  • Reducing template bloat with inheritance

    - by benoitj
    Does anyone have experience reducing template code bloat using inheritance? i hesitate rewriting our containers this way: class vectorBase { public: int size(); void clear(); int m_size; void *m_rawData; //.... }; template< typename T > class vector : public vectorBase { void push_back( const T& ); //... }; I should keep maximum performance while reducing compile time I'm also wondering why stl implementations do not uses this approach Thanks for your feedbacks

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  • Forcing a templated object to construct from a pointer

    - by SalamiArmi
    I have a fictional class: template<typename T> class demonstration { public: demonstration(){} ... T *m_data; } At some point in the program's execution, I want to set m_data to a big block of allocated memory and construct an object T there. At the moment, I've been using this code: void construct() { *m_data = T(); } Which I've now realised is probably not the best idea... wont work under certain cirumstances, if T has a private assignment operator for example. Is there a normal/better way to do what I'm attempting here?

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  • How to generate a line break in Django template

    - by Iamamac
    I want to give default value to a textarea. The code is something like this: <textarea>{{userSetting.list | join:"NEWLINE"}}</textarea> where userSetting.list is a string list, each item of whom is expected to show in one line. textarea takes the content between the tags as the default value, preserving its line breaks and not interpreting any HTML tags (which means <br>,\n won't work). I have found a solution: {{userSetting.list | join:" " | wordwrap:0}} (there is no whitespace in the list). But obviously it is NOT a good one. Any help would be appreciated.

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  • Template inheritance: X is not a template

    - by user2923917
    I am trying to build a inheritance-structure which looks like: Base - template Grandpa - template Father class Base {}; template <int x> class Grandpa: public Base {}; template <int x> class Father: public Grandpa<x> {}; However, the compiler complains when compiling Father, that Grandpa is not a template. I guess it is just some synthatic issue, however everything I've tried so far led to even more compiler complaints ;) Any idea whats wrong?

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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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  • 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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  • 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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