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• c++ class member functions selected by traits

  - by Jive Dadson

  I am reluctant to say I can't figure this out, but I can't figure this out. I've googled and searched stackoverflow, and come up empty. The abstract, and possibly overly vague form of the question is, how can I use the traits-pattern to instantiate non-virtual member functions? The question came up while modernizing a set of multivariate function optimizers that I wrote more than 10 years ago. The optimizers all operate by selecting a straight-line path through the parameter space away from the current best point (the "update"), then finding a better point on that line (the "line search"), then testing for the "done" condition, and if not done, iterating. There are different methods for doing the update, the line-search, and conceivably for the done test, and other things. Mix and match. Different update formulae require different state-variable data. For example, the LMQN update requires a vector, and the BFGS update requires a matrix. If evaluating gradients is cheap, the line-search should do so. If not, it should use function evaluations only. Some methods require more accurate line-searches than others. Those are just some examples. The original version instatiates several of the combinations by means of virtual functions. Some traits are selected by setting mode bits. Yuck. It would be trivial to define the traits with #define's and the member functions with #ifdef's and macros. But that's so twenty years ago. It bugs me that I cannot figure out a whiz-bang modern way. If there were only one trait that varied, I could use the curiously recurring template pattern. But I see no way to extend that to arbitrary combinations of traits. I tried doing it using boost::enable_if, etc.. The specialized state info was easy. I managed to get the functions done, but only by resorting to non-friend external functions that have the this-pointer as a parameter. I never even figured out how to make the functions friends, much less member functions. Perhaps tag-dispatch is the key. I haven't gotten very deeply into that. Surely it's possible, right? If so, what is best practice?

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• explicit template instantiations

  - by user323422

  see following code and please clear doubts1. as ABC is template why it not showing error when we put defination of ABC class member function in test.cpp 2.if i put test.cpp code in test.h and remve 2 , then it working fine. // test.h template <typename T> class ABC { public: void foo( T& ); void bar( T& ); }; // test.cpp template <typename T> void ABC<T>::foo( T& ) {} // definition template <typename T> void ABC<T>::bar( T& ) {} // definition template void ABC<char>::foo( char & ); // 1 template class ABC<char>; // 2 // main.cpp #include "test.h" int main() { ABC<char> a; a.foo(); // valid with 1 or 2 a.bar(); // link error if only 1, valid with 2 }

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• How to detect whether there is a specific member variable in class?

  - by Kirill V. Lyadvinsky

  For creating algorithm template function I need to know whether x or X (and y or Y) in class that is template argument. It may by useful when using my function for MFC CPoint class or GDI+ PointF class or some others. All of them use different x in them. My solution could be reduces to the following code: template<int> struct TT {typedef int type;}; template<class P> bool Check_x(P p, typename TT<sizeof(&P::x)>::type b = 0) { return true; } template<class P> bool Check_x(P p, typename TT<sizeof(&P::X)>::type b = 0) { return false; } struct P1 {int x; }; struct P2 {float X; }; // it also could be struct P3 {unknown_type X; }; int main() { P1 p1 = {1}; P2 p2 = {1}; Check_x(p1); // must return true Check_x(p2); // must return false return 0; } But it does not compile in Visual Studio, while compiling in the GNU C++. With Visual Studio I could use the following template: template<class P> bool Check_x(P p, typename TT<&P::x==&P::x>::type b = 0) { return true; } template<class P> bool Check_x(P p, typename TT<&P::X==&P::X>::type b = 0) { return false; } But it does not compile in GNU C++. Is there universal solution? UPD: Structures P1 and P2 here are only for example. There are could be any classes with unknown members.

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• Form resubmission issue-Django html

  - by Hulk

  There is a form ,which is submitted and then the page is redirected to another page.But if the user hits the refresh button again on the new page .the following message is displayed To display this page, Firefox must send information that will repeat any action (such as a search or order confirmation) that was performed earlier. And on press "OK" A duplicate entry is created how do i prevent this or how do i handle this I am using a django,application Thanks..

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• Can I suppress newlines after each template tag with Django's template engine?

  - by ento

  In Rails ERB, you can suppress newlines by adding a trailing hyphen to tags: <ul> <% for @item in @items -%> <li><%= @item %></li> <% end -%> </ul> becomes: <ul> <li>apple</li> <li>banana</li> <li>cacao</li> </ul> Is there a way to do this in Django? (Disclosure: I'm generating a csv file with Django) Edit: Clarified that the newlines I'm hunting down are the ones left behind after the template tags.

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• Get the signed/unsigned variant of an integer template parameter without explicit traits

  - by Blair Holloway

  I am looking to define a template class whose template parameter will always be an integer type. The class will contain two members, one of type T, and the other as the unsigned variant of type T -- i.e. if T == int, then T_Unsigned == unsigned int. My first instinct was to do this: template <typename T> class Range { typedef unsigned T T_Unsigned; // does not compile public: Range(T min, T_Unsigned range); private: T m_min; T_Unsigned m_range; }; But it doesn't work. I then thought about using partial template specialization, like so: template <typename T> struct UnsignedType {}; // deliberately empty template <> struct UnsignedType<int> { typedef unsigned int Type; }; template <typename T> class Range { typedef UnsignedType<T>::Type T_Unsigned; /* ... */ }; This works, so long as you partially specialize UnsignedType for every integer type. It's a little bit of additional copy-paste work (slash judicious use of macros), but serviceable. However, I'm now curious - is there another way of determining the signed-ness of an integer type, and/or using the unsigned variant of a type, without having to manually define a Traits class per-type? Or is this the only way to do it?

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• Partial template specialization: matching on properties of specialized template parameter

  - by Kenzo

  template <typename X, typename Y> class A {}; enum Property {P1,P2}; template <Property P> class B {}; class C {}; Is there any way to define a partial specialization of A such that A<C, B<P1> > would be A's normal template, but A<C, B<P2> > would be the specialization? Replacing the Y template parameter by a template template parameter would be nice, but is there a way to partially specialize it based on P then? template <typename X, template <Property P> typename Y> class A {}; // template <typename X> class A<X,template<> Y<P2> > {}; <-- not valid Is there a way by adding traits to a specialization template<> B<P2> and then using SFINAE in A?

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• How do I apply a "template" or "skeleton" of code in C# here?

  - by Scott Stafford

  In my business layer, I need many, many methods that follow the pattern: public BusinessClass PropertyName { get { if (this.m_LocallyCachedValue == null) { if (this.Record == null) { this.m_LocallyCachedValue = new BusinessClass( this.Database, this.PropertyId); } else { this.m_LocallyCachedValue = new BusinessClass( this.Database, this.Record.ForeignKeyName); } } return this.m_LocallyCachedValue; } } I am still learning C#, and I'm trying to figure out the best way to write this pattern once and add methods to each business layer class that follow this pattern with the proper types and variable names substituted. BusinessClass is a typename that must be substituted, and PropertyName, PropertyId, ForeignKeyName, and m_LocallyCachedValue are all variables that should be substituted for. Are attributes usable here? Do I need reflection? How do I write the skeleton I provided in one place and then just write a line or two containing the substitution parameters and get the pattern to propagate itself? EDIT: Modified my misleading title -- I am hoping to find a solution that doesn't involve code generation or copy/paste techniques, and rather to be able to write the skeleton of the code once in a base class in some form and have it be "instantiated" into lots of subclasses as the accessor for various properties. EDIT: Here is my solution, as suggested but left unimplemented by the chosen answerer. // I'll write many of these... public BusinessClass PropertyName { get { return GetSingleRelation(ref this.m_LocallyCachedValue, this.PropertyId, "ForeignKeyName"); } } // That all call this. public TBusinessClass GetSingleRelation<TBusinessClass>( ref TBusinessClass cachedField, int fieldId, string contextFieldName) { if (cachedField == null) { if (this.Record == null) { ConstructorInfo ci = typeof(TBusinessClass).GetConstructor( new Type[] { this.Database.GetType(), typeof(int) }); cachedField = (TBusinessClass)ci.Invoke( new object[] { this.Database, fieldId }); } else { var obj = this.Record.GetType().GetProperty(objName).GetValue( this.Record, null); ConstructorInfo ci = typeof(TBusinessClass).GetConstructor( new Type[] { this.Database.GetType(), obj.GetType()}); cachedField = (TBusinessClass)ci.Invoke( new object[] { this.Database, obj }); } } return cachedField; }

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• How to check if a variable exists in a FreeMarker template?

  - by Don

  Hi, I have a Freemarker template which contains a bunch of placeholders for which values are supplied when the template is processed. I want to conditionally include part of the template if the userName variable is supplied, something like: [#if_exists userName] Hi ${userName}, How are you? [/#if_exists] However, the FreeMarker manual seems to indicate that if_exists is deprecated, but I can't find another way to achieve this. Of course, I could simple providing an additional boolean variable isUserName and use that like this: [#if isUserName] Hi ${userName}, How are you? [/#if] But if there's a way of checking whether userName exists then I can avoid adding this extra variable. Cheers, Don

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• Sticky footer with CSS Layout Template Module?

  - by boris callens

  I'm currently looking into the JQuery library for CSS Layout Template module Is it possible to define the height of a placeholder with *? Meaning it will push content down if it has to, or fill up any remaining space if the complete content is less then the view port. This would be really nice to make the whole sticky footer thing much easier and robust.

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• Template error with django-photologue: 'Permission Denied'

  - by Ellie P.

  I'm in the process of re-setting up my Django development environment after reinstalling my OS. We use django-photologue with our project, which I installed using easy_install. However, I'm getting a template error on every template that has a photologue photo: TemplateSyntaxError at /newsroom/news/ Caught an exception while rendering: (13, 'Permission denied') One example of the line where the error is located: <img src="{{ photo.get_list_url }}"> I'm totally stumped--I've used photologue for this project on many different platforms and have never had this problem. Google yields nothing. I'm currently using Jolicloud (based on Ubuntu Jaunty). I'm working with a pre-existing database, so all of the photo sizes and everything have already been set up.

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• Twig Template For loop Results

  - by user981480

  I am trying to print out a the contents of an array but am not getting the expected results. any help is much appreciated: PHP code: $list[1]['first_name'] = 'Joe'; $list[1]['last_name'] = 'Smith'; $list[2]['first_name'] = 'John'; $list[2]['last_name'] = 'brand'; $data = array( 'customer' => $list ); echo $template->render($data); Template Markup: <html> <head><title>My first Twig template!</title></head> <body> {% for person in customer %} {{ person.first_name }} {% endfor %} </body> RESULTS: Array ( [first_name] = Joe [last_name] = Smith ) first_name Joe Array ( [first_name] = John [last_name] = brand ) first_name John

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• Template operator linker error

  - by Dani

  I have a linker error I've reduced to a simple example. The build output is: debug/main.o: In function main': C:\Users\Dani\Documents\Projects\Test1/main.cpp:5: undefined reference tolog& log::operator<< (char const (&) [6])' collect2: ld returned 1 exit status It looks like the linker ignores the definition in log.cpp. I also cant put the definition in log.h because I include the file alot of times and it complains about redefinitions. main.cpp: #include "log.h" int main() { log() << "hello"; return 0; } log.h: #ifndef LOG_H #define LOG_H class log { public: log(); template<typename T> log &operator <<(T &t); }; #endif // LOG_H log.cpp: #include "log.h" #include <iostream> log::log() { } template<typename T> log &log::operator <<(T &t) { std::cout << t << std::endl; return *this; }

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• Choosing the right template engine

  - by asrijaal

  Hi there, since today - I always used simple JSP Tags and JSTL which works but now I'm looking for a little bit more features and maybe more usability. With googling I've found Freemaker, Velocity and Tiles which seem to be very famous. Does someone of you has some further experiences with some of these and can give me a short brief about it. Or are there other frameworks which are worth to look into?

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• Display value of a django form field in a template?

  - by Eran Kampf

  I have a form with an email property. When using {{ form.email }} in case of some validation error, django still renders the previous value in the input tag's value attribute: <input type="text" id="id_email" maxlength="75" class="required" value="[email protected]" name="email"> I want to render the input tag myself (to add some javascript code and an error class in case of an error). For example this is my template instead of {{ form.email }}: <input type="text" autocomplete="on" id="id_email" name="email" class="email {% if form.email.errors %}error{% endif %}"> However this does not display the errorneous value ("[email protected]" in this example) to the user. How do I get the field's value in the template?

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• if we put template class member function defination in .cpp(not in headerfile) with explicit instant

  - by user323422

  For example //temp.h class ABC { pubilc: void temp(T &a); }; //temp.cpp( ) void ABc<T>::temp(T &a){} //defination ( it should be in header file?????????) template class ABC<int>; //main.cpp int main() { int i=10; ABC<int> a; a.temp(i); return 0; }

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• Template metaprogram converting type to unique number

  - by daramarak

  I just started playing with metaprogramming and I am working on different tasks just to explore the domain. One of these was to generate a unique integer and map it to type, like below: int myInt = TypeInt<AClass>::value; I want to know if this is at all possible, and in that case how. Because although I have learned much about exploring this subject I still have failed to come up with an answer. (P.S. A yes/no answer is much more gratifying than a c++ solution that doesn't use metaprogramming, as this is the domain that I am exploring)

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• Can I suppress newlines with Django's template engine?

  - by ento

  In Rails ERB, you can suppress newlines by adding a trailing hyphen to tags: <ul> <% for @item in @items -%> <li><%= @item %></li> <% end -%> </ul> becomes: <ul> <li>apple</li> <li>banana</li> <li>cacao</li> </ul> Is there a way to do this in Django? (Disclosure: I'm generating a csv file with Django)

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• Policy-based template design: How to access certain policies of the class?

  - by dehmann

  I have a class that uses several policies that are templated. It is called Dish in the following example. I store many of these Dishes in a vector (using a pointer to simple base class), but then I'd like to extract and use them. But I don't know their exact types. Here is the code; it's a bit long, but really simple: #include <iostream> #include <vector> struct DishBase { int id; DishBase(int i) : id(i) {} }; std::ostream& operator<<(std::ostream& out, const DishBase& d) { out << d.id; return out; } // Policy-based class: template<class Appetizer, class Main, class Dessert> class Dish : public DishBase { Appetizer appetizer_; Main main_; Dessert dessert_; public: Dish(int id) : DishBase(id) {} const Appetizer& get_appetizer() { return appetizer_; } const Main& get_main() { return main_; } const Dessert& get_dessert() { return dessert_; } }; struct Storage { typedef DishBase* value_type; typedef std::vector<value_type> Container; typedef Container::const_iterator const_iterator; Container container; Storage() { container.push_back(new Dish<int,double,float>(0)); container.push_back(new Dish<double,int,double>(1)); container.push_back(new Dish<int,int,int>(2)); } ~Storage() { // delete objects } const_iterator begin() { return container.begin(); } const_iterator end() { return container.end(); } }; int main() { Storage s; for(Storage::const_iterator it = s.begin(); it != s.end(); ++it){ std::cout << **it << std::endl; std::cout << "Dessert: " << *it->get_dessert() << std::endl; // ?? } return 0; } The tricky part is here, in the main() function: std::cout << "Dessert: " << *it->get_dessert() << std::endl; // ?? How can I access the dessert? I don't even know the Dessert type (it is templated), let alone the complete type of the object that I'm getting from the storage. This is just a toy example, but I think my code reduces to this. I'd just like to pass those Dish classes around, and different parts of the code will access different parts of it (in the example: its appetizer, main dish, or dessert).

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• Need help with auto-scaffolding template in ASP.NET MVC

  - by DanM

  I'm trying to write an auto-scaffolder for Index views. I'd like to be able to pass in a collection of models or view-models (e.g., IQueryable<MyViewModel>) and get back an HTML table that uses the DisplayName attribute for the headings (th elements) and Html.Display(propertyName) for the cells (td elements). Each row should correspond to one item in the collection. Here's what I have so far: <%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl" %> <% var items = (IQueryable<TestProj.ViewModels.TestViewModel>)Model; // Should be generic! var properties = items.First().GetMetadata().Properties .Where(pm => pm.ShowForDisplay && !ViewData.TemplateInfo.Visited(pm)); %> <table> <tr> <% foreach(var property in properties) { %> <th> <%= property.DisplayName %> </th> <% } %> </tr> <% foreach(var item in items) { %> <tr> <% foreach(var property in properties) { %> <td> <%= Html.Display(property.DisplayName) %> // This doesn't work! </td> <% } %> </tr> <% } %> </table> Two problems with this: I'd like it to be generic. So, I'd like to replace var items = (IQueryable<TestProj.ViewModels.TestViewModel>)Model; with var items = (IQueryable<T>)Model; or something to that effect. The <td> elements are not working because the Html in <%= Html.Display(property.DisplayName) %> contains the model for the view, which is a collection of items, not the item itself. Somehow, I need to obtain an HtmlHelper object whose Model property is the current item, but I'm not sure how to do that. How do I solve these two problems?

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• Help with django filters

  - by Cato Johnston

  In a django template, I want to get an value from the first object in a list's field. I have the following code {{ list.object|first.field }} But it results in an error. Is there a way to achieve this without writing a loop or doing it in the view?

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• C++ Using a class template argument as a template argument for another type

  - by toefel

  Hey Everyone, I'm having this problem while writing my own HashTable. It all works, but when I try to templatize the thing, it gave me errors. I recreated the problem as follows: THIS CODE WORKS: typedef double Item; class A { public: A() { v.push_back(pair<string, Item>("hey", 5.0)); } void iterate() { for(Iterator iter = v.begin(); iter != v.end(); ++iter) cout << iter->first << ", " << iter->second << endl; } private: vector<pair<string, double> > v; typedef vector< pair<string, double> >::iterator Iterator; }; THIS CODE DOES NOT: template<typename ValueType> class B { public: B(){} void iterate() { for(Iterator iter = v.begin(); iter != v.end(); ++iter) cout << iter->first << ", " << iter->second << endl; } private: vector<pair<string, ValueType> > v; typedef vector< pair<string, ValueType> >::iterator Iterator; }; the error messages: g++ -O0 -g3 -Wall -c -fmessage-length=0 -omain.o ..\main.cpp ..\main.cpp:50: error: type std::vector<std::pair<std::string, ValueType>, std::allocator<std::pair<std::string, ValueType> > >' is not derived from typeB' ..\main.cpp:50: error: ISO C++ forbids declaration of `iterator' with no type ..\main.cpp:50: error: expected `;' before "Iterator" ..\main.cpp: In member function `void B::iterate()': ..\main.cpp:44: error: `Iterator' was not declared in this scope ..\main.cpp:44: error: expected `;' before "iter" ..\main.cpp:44: error: `iter' was not declared in this scope Does anybody know why this is happening? Thanks!

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• Templating engine's for Asp.net

  - by FosterZ

  Are there any third-party Templating Engines for Asp.net like we have smarty,savant for php ?

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• perl dancer: passing database info to template

  - by Bubnoff

  Following Dancer tutorial here: http://search.cpan.org/dist/Dancer/lib/Dancer/Tutorial.pod I'm using my own sqlite3 database with this schema CREATE TABLE if not exists location (location_code TEXT PRIMARY KEY, name TEXT, stations INTEGER); CREATE TABLE if not exists session (id INTEGER PRIMARY KEY, date TEXT, sessions INTEGER, location_code TEXT, FOREIGN KEY(location_code) REFERENCES location(location_code)); My dancer code ( helloWorld.pm ) for the database: package helloWorld; use Dancer; use DBI; use File::Spec; use File::Slurp; use Template; our $VERSION = '0.1'; set 'template' => 'template_toolkit'; set 'logger' => 'console'; my $base_dir = qq(/home/automation/scripts/Area51/perl/dancer); # database crap sub connect_db { my $db = qw(/home/automation/scripts/Area51/perl/dancer/sessions.sqlite); my $dbh = DBI->connect("dbi:SQLite:dbname=$db", "", "", { RaiseError => 1, AutoCommit => 1 }); return $dbh; } sub init_db { my $db = connect_db(); my $file = qq($base_dir/schema.sql); my $schema = read_file($file); $db->do($schema) or die $db->errstr; } get '/' => sub { my $branch_code = qq(BPT); my $dbh = connect_db(); my $sql = q(SELECT * FROM session); my $sth = $dbh->prepare($sql) or die $dbh->errstr; $sth->execute or die $dbh->errstr; my $key_field = q(id); template 'show_entries.tt', { 'branch' => $branch_code, 'data' => $sth->fetchall_hashref($key_field), }; }; init_db(); true; Tried the example template on the site, doesn't work. <% FOREACH id IN data.keys.nsort %> <li>Date is: <% data.$id.sessions %> </li> <% END %> Produces page but with no data. How do I troubleshoot this as no clues come up in the console/cli? Thanks Bubnoff

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• Django how to handle # in variable name.

  - by Jack

  I've got a dictionary in python which is assigned as a template variable. One of the keys is named "#text" but when i try to access it using {{ artist.image.3."#text"}} I get an error which is File "/home/jack/Desktop/test/appengine/lib/django/django/template/__init__.py", line 558, in __init__ raise TemplateSyntaxError, "Could not parse the remainder: %s" % token[upto:] TemplateSyntaxError: Could not parse the remainder: "#text" So how can I use this in the template? I've tried putting quotes around it but to no avail. I'd like to not modify the dictionary if possible, but if its easy enough to do then I guess its okay. Thanks

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