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  • Passing markup into a Rails Partial

    - by 1ndivisible
    Is there any way of doing something equivilant to this: <%= render partial: 'shared/outer' do %> <%= render partial: 'shared/inner' %> <% end %> Resulting in <div class="outer"> <div class="inner"> </div> </div> Obviously there would need to be a way of marking up 'shared/outer.html.erb' to indicate where the passed in partial should be rendered: <div class="outer"> <% render Here %> </div>

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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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  • In the generic programming/TMP world what exactly is a model / a policy and a "concept" ?

    - by Hassan Syed
    I'd like to know the precise yet succinct definitions of these three concepts in one place. The quality of the answer should depend on the following two points. Show a simple code snippet to show how and what the concept/technique is used for. Be simple enough to understand so that a programmer without any exposure to this area can grasp it. Note: There are probably many correct answers since each concept has many different facets. If there are a lot of good answers I will eventually turn the question into CW and aggregate the answers. -- Post Accept Edit -- Boost has a nice article on generic programming concepts

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  • C++ - checking if a class has a certain method at compile time

    - by jetwolf
    Here's a question for the C++ gurus out there. Is there a way to check at compile time where a type has a certain method, and do one thing if it does, and another thing if it doesn't? Basically, I have a template function template <typename T> void function(T t); and I it to behave a certain way if T has a method g(), and another way if it doesn't. Perhaps there is something that can be used together with boost's enable_if? Something like this: template <typename T> enable_if<has_method<T, g, void ()>, void>::type function(T t) { // Superior implementation calling t.g() } template <typename T> disable_if<has_method<T, g, void ()>, void>::type function(T t) { // Inferior implementation in the case where T doesn't have a method g() } "has_method" would be something that preferably checks both that T has a method named 'g', and that the method has the correct signature (in this case, void ()). Any ideas?

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  • Getting a type for a template instantiation?

    - by ebo
    I have the following situation: I have a object of type MyClass, which has a method to cast itself to it's base class. The class includes a typedef for it's base class and a method to do the downcast. template <class T, class B> class BaseClass; template <class T> class NoAccess; template <class T> class MyClass : public BaseClass<T, NoAccess<T> > { private: typedef BaseClass<T, NoAccess<T> > base; public: base &to_base(); }; I need to pass the result of a base call to a functor Operator: template <class Y> class Operator { Operator(Y &x); }; Operator<???> op(myobject.to_base()); Is there a easy way to fill the ??? provided that I do not want to use NoAccess?

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  • Constant template parameter class manages to link externally

    - by the_drow
    I have a class foo with an enum template parameter and for some reason it links to two versions of the ctor in the cpp file. enum Enum { bar, baz }; template <Enum version = bar> class foo { public: foo(); }; // CPP File #include "foo.hpp" foo<bar>::foo() { cout << "bar"; } foo<baz>::foo() { cout << "baz"; } I'm using msvc 2008, is this the standard behavior? Are only type template parameters cannot be linked to cpp files?

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  • Printing factorial at compile time in C++

    - by user519882
    template<unsigned int n> struct Factorial { enum { value = n * Factorial<n-1>::value}; }; template<> struct Factorial<0> { enum {value = 1}; }; int main() { std::cout << Factorial<5>::value; std::cout << Factorial<10>::value; } above program computes factorial value during compile time. I want to print factorial value at compile time rather than at runtime using cout. How can we achive printing the factorial value at compile time? I am using VS2009. Thanks!

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  • I have to generate PL/SQL using Java. Most of the procedures are common. Only a few keeps changing.

    - by blog
    I have to generate PL-SQL code, with some common code(invariable) and a variable code. I don't want to use any external tools. Some ways that I can think: Can I go and maintain the common code in a template and with markers, where my java code will generate code in the markers and generate a new file. Maintain the common code in static constant String and then generate the whole code in StringBuffer and at last write to file. But, I am not at all satisfied with both the ideas. Can you please suggest any better ways of doing this or the use of any design patterns or anything? Thanks in Advance.

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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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  • std::basic_string full specialization (g++ conflict)

    - by SoapBox
    I am trying to define a full specialization of std::basic_string< char, char_traits<char>, allocator<char> > which is typedef'd (in g++) by the <string> header. The problem is, if I include <string> first, g++ sees the typedef as an instantiation of basic_string and gives me errors. If I do my specialization first then I have no issues. I should be able to define my specialization after <string> is included. What do I have to do to be able to do that? My Code: #include <bits/localefwd.h> //#include <string> // <- uncommenting this line causes compilation to fail namespace std { template<> class basic_string< char, char_traits<char>, allocator<char> > { public: int blah() { return 42; } size_t size() { return 0; } const char *c_str() { return ""; } void reserve(int) {} void clear() {} }; } #include <string> #include <iostream> int main() { std::cout << std::string().blah() << std::endl; } The above code works fine. But, if I uncomment the first #include <string> line, I get the following compiler errors: blah.cpp:7: error: specialization of ‘std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ after instantiation blah.cpp:7: error: redefinition of ‘class std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ /usr/include/c++/4.4/bits/stringfwd.h:52: error: previous definition of ‘class std::basic_string<char, std::char_traits<char>, std::allocator<char> >’ blah.cpp: In function ‘int main()’: blah.cpp:22: error: ‘class std::string’ has no member named ‘blah’ Line 52 of /usr/include/c++/4.4/bits/stringfwd.h: template<typename _CharT, typename _Traits = char_traits<_CharT>, typename _Alloc = allocator<_CharT> > class basic_string; As far as I know this is just a forward delcaration of the template, NOT an instantiation as g++ claims. Line 56 of /usr/include/c++/4.4/bits/stringfwd.h: typedef basic_string<char> string; As far as I know this is just a typedef, NOT an instantiation either. So why are these lines conflicting with my code? What can I do to fix this other than ensuring that my code is always included before <string>?

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  • Better way to write an object generator for an RAII template class?

    - by Dan
    I would like to write an object generator for a templated RAII class -- basically a function template to construct an object using type deduction of parameters so the types don't have to be specified explicitly. The problem I foresee is that the helper function that takes care of type deduction for me is going to return the object by value, which will result in a premature call to the RAII destructor when the copy is made. Perhaps C++0x move semantics could help but that's not an option for me. Anyone seen this problem before and have a good solution? This is what I have: template<typename T, typename U, typename V> class FooAdder { private: typedef OtherThing<T, U, V> Thing; Thing &thing_; int a_; // many other members public: FooAdder(Thing &thing, int a); ~FooAdder(); void foo(T t, U u); void bar(V v); }; The gist is that OtherThing has a horrible interface, and FooAdder is supposed to make it easier to use. The intended use is roughly like this: FooAdder(myThing, 2) .foo(3, 4) .foo(5, 6) .bar(7) .foo(8, 9); The FooAdder constructor initializes some internal data structures. The foo and bar methods populate those data structures. The ~FooAdder dtor wraps things up and calls a method on thing_, taking care of all the nastiness. That would work fine if FooAdder wasn't a template. But since it is, I would need to put the types in, more like this: FooAdder<Abc, Def, Ghi>(myThing, 2) ... That's annoying, because the types can be inferred based on myThing. So I would prefer to create a templated object generator, similar to std::make_pair, that will do the type deduction for me. Something like this: template<typename T, typename U, typename V> FooAdder<T, U, V> AddFoo(Thing &thing, int a) { return FooAdder<T, U, V>(thing, a); } That seems problematic: because it returns by value, the stack temporary object will be destructed, which will cause the RAII dtor to run prematurely. One thought I had was to give FooAdder a copy ctor with move semantics, kinda like std::auto_ptr. But I would like to do this without dynamic memory allocation, so I thought the copy ctor could set a flag within FooAdder indicating the dtor shouldn't do the wrap-up. Like this: FooAdder(FooAdder &rhs) // Note: rhs is not const : thing_(rhs.thing_) , a_(rhs.a_) , // etc... lots of other members, annoying. , moved(false) { rhs.moved = true; } ~FooAdder() { if (!moved) { // do whatever it would have done } } Seems clunky. Anyone got a better way?

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  • Should this work?

    - by Noah Roberts
    I am trying to specialize a metafunction upon a type that has a function pointer as one of its parameters. The code compiles just fine but it will simply not match the type. #include <iostream> #include <boost/mpl/bool.hpp> #include <boost/mpl/identity.hpp> template < typename CONT, typename NAME, typename TYPE, TYPE (CONT::*getter)() const, void (CONT::*setter)(TYPE const&) > struct metafield_fun {}; struct test_field {}; struct test { int testing() const { return 5; } void testing(int const&) {} }; template < typename T > struct field_writable : boost::mpl::identity<T> {}; template < typename CONT, typename NAME, typename TYPE, TYPE (CONT::*getter)() const > struct field_writable< metafield_fun<CONT,NAME,TYPE,getter,0> > : boost::mpl::false_ {}; typedef metafield_fun<test, test_field, int, &test::testing, 0> unwritable; int main() { std::cout << typeid(field_writable<unwritable>::type).name() << std::endl; std::cin.get(); } Output is always the type passed in, never bool_.

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  • In Jeditable, how do I make it so that when I click the div to edit, the text box content has initial value that is processed?

    - by TIMEX
    When the user clicks on the div, jeditable will make a text box. However, I want the initial text to be done with function stripTags(), instead of what's on the page. The reason is that I'm using some URL techniques to turn plain text links into URLs. When the user clicks on the div, jeditable is turning them into <a href=>..</a> Is there a "beforeSubmit" option in jeditable? http://www.appelsiini.net/projects/jeditable

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  • C++ : Swapping template class elements of different types?

    - by metamemetics
    template< class T1, class T2 > class Pair { T1 first; T2 second; }; I'm being asked to write a swap() method so that the first element becomes the second and the second the first. I have: Pair<T2,T1> swap() { return Pair<T2,T1>(second, first); } But this returns a new object rather than swapping, where I think it needs to be a void method that changes its own data members. Is this possible to do since T1 and T2 are potentially different class types? In other words I can't simply set temp=first, first=second, second=temp because it would try to convert them to different types. I'm not sure why you would potentially want to have a template object that changes order of its types as it seems that would cause confusion but that appears to be what I'm being asked to do.

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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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  • C++ creating generic template function specialisations

    - by Fire Lancer
    I know how to specialise a template function, however what I want to do here is specialise a function for all types which have a given method, eg: template<typename T> void foo(){...} template<typename T, if_exists(T::bar)>void foo(){...}//always use this one if the method T::bar exists T::bar in my classes is static and has different return types. I tried doing this by having an empty base class ("class HasBar{};") for my classes to derive from and using boost::enable_if with boost::is_base_of on my "specialised" version. However the problem then is that for classes that do have bar, the compiler cant resolve which one to use :(. template<typename T> typename boost::enable_if<boost::is_base_of(HasBar, T>, void>::type f() {...} I know that I could use boost::disable_if on the "normal" version, however I do not control the normal version (its provided by a third party library and its expected for specialisations to be made, I just don't really want to make explicit specialisations for my 20 or so classes), nor do I have that much control over the code using these functions, just the classes implementing T::bar and the function that uses it. Is there some way to tell the compiler to "always use this version if possible no matter what" without altering the other versions?

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  • C++ template type deduction problem

    - by hamishmcn
    motivation: I would like to create a utility class so that instead of having to write: if( someVal == val1 || someVal == val2 || someVal == val3 ) I could instead write: if( is(someVal).in(val1, val2, val3) ) which is much closer to the mathematical 'a is an element of (b,c,d)' and also would save on a lot of typing when the variable name 'someVal' is long. Here is the code I have so far (for 2 and 3 values): template<class T> class is { private: T t_; public: is(T t) : t_(t) { } bool in(const T& v1, const T& v2) { return t_ == v1 || t_ == v2; } bool in(const T& v1, const T& v2, const T& v3) { return t_ == v1 || t_ == v2 || t_ == v3; } }; However it fails to compile if I write: is(1).in(3,4,5); instead I have to write is<int>(1).in(3,4,5); Which isn't too bad, but it would be better if somehow the compiler could figure out that the type is int with out me having to explicitly specify it. Is there anyway to do this or I am stuck with specifying it explicitly?

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  • Java template classes using generator or similar?

    - by Hugh Perkins
    Is there some library or generator that I can use to generate multiple templated java classes from a single template? Obviously Java does have a generics implementation itself, but since it uses type-erasure, there are lots of situations where it is less than adequate. For example, if I want to make a self-growing array like this: class EasyArray { T[] backingarray; } (where T is a primitive type), then this isn't possible. This is true for anything which needs an array, for example high-performance templated matrix and vector classes. It should probably be possible to write a code generator which takes a templated class and generates multiple instantiations, for different types, eg for 'double' and 'float' and 'int' and 'String'. Is there something that already exists that does this? Edit: note that using an array of Object is not what I'm looking for, since it's no longer an array of primitives. An array of primitives is very fast, and uses only as much space a sizeof(primitive) * length-of-array. An array of object is an array of pointers/references, that points to Double objects, or similar, which could be scattered all over the place in memory, require garbage collection, allocation, and imply a double-indirection for access. Edit2: good god, voted down for asking for something that probably doesn't currently exist, but is technically possible and feasible? Does that mean that people looking for ways to improve things have already left the java community? Edit3: Here is code to show the difference in performance between primitive and boxed arrays: int N = 10*1000*1000; double[]primArray = new double[N]; for( int i = 0; i < N; i++ ) { primArray[i] = 123.0; } Object[] objArray = new Double[N]; for( int i = 0; i < N; i++ ) { objArray[i] = 123.0; } tic(); primArray = new double[N]; for( int i = 0; i < N; i++ ) { primArray[i] = 123.0; } toc(); tic(); objArray = new Double[N]; for( int i = 0; i < N; i++ ) { objArray[i] = 123.0; } toc(); Results: double[] array: 148 ms Double[] array: 4614 ms Not even close!

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  • Is there a Visual Studio (or freeware) equivalent for Expression Blend's "Edit Template" feature?

    - by DanM
    In Expression Blend, you can view and edit the control template of objects in the "Objects and Timeline" panel. I'm wondering if there's an equivalent feature in Visual Studio or if there's something free (or very inexpensive) I can download that will allow me to do this. Here's a screen cap from Expression Blend that shows what I'm talking about: Doing this for DataGrid results in the following: <Style x:Key="DataGridStyle1" TargetType="{x:Type Custom:DataGrid}"> ... <Setter Property="Template"> <Setter.Value> <ControlTemplate TargetType="{x:Type Custom:DataGrid}"> ... </ControlTemplate> </Setter.Value> </Setter> <Style.Triggers> <Trigger Property="IsGrouping" Value="True"> <Setter Property="ScrollViewer.CanContentScroll" Value="False"/> </Trigger> </Style.Triggers> </Style> (The ... is of course replaced with setters and the contents of the control template.) This is a very useful starting point if you want to create a custom style and template for a control. It seems like you can do pretty much anything you can do in Blend in Studio, but this one is eluding me. Any ideas? Edit I'm also curious if this feature will be in Visual Studio 2010. Anyone know?

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  • Strange overloading rules in C++

    - by bucels
    I'm trying to compile this code with GCC 4.5.0: #include <algorithm> #include <vector> template <typename T> void sort(T, T) {} int main() { std::vector<int> v; sort(v.begin(), v.end()); } But it doesn't seem to work: $ g++ -c nm.cpp nm.cpp: In function ‘int main()’: nm.cpp:9:28: error: call of overloaded ‘sort(std::vector<int>::iterator, std::vector<int>::iterator)’ is ambiguous nm.cpp:4:28: note: candidates are: void sort(T, T) [with T = __gnu_cxx::__normal_iterator<int*, std::vector<int> >] /usr/lib/gcc/i686-pc-linux-gnu/4.5.0/../../../../include/c++/4.5.0/bits/stl_algo.h:5199:69: note: void std::sort(_RAIter, _RAIter) [with _RAIter = __gnu_cxx::__normal_iterator<int*, std::vector<int> >] Comeau compiles this code without errors. (4.3.10.1 Beta2, strict C++03, no C++0x) Is this valid C++?

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  • Mapping integers to types using C++ template fails in a specific case

    - by Shailesh Kumar
    I am attempting to compile the following template based code in VC++ 2005. #include <iostream> using namespace std; /* * T is a template which maps an integer to a specific type. * The mapping happens through partial template specialization. * In the following T<1> is mapped to char, T<2> is mapped to long * and T<3> is mapped to float using partial template specializations */ template <int x> struct T { public: }; template<> struct T<1> { public: typedef char xType; }; template<> struct T<2> { public: typedef long xType; }; template<> struct T<3> { public: typedef float xType; }; // We can easily access the specific xType for a specific T<N> typedef T<3>::xType x3Type; /*! * In the following we are attempting to use T<N> inside another * template class T2<R> */ template<int r> struct T2 { //We can map T<r> to some other type T3 typedef T<r> T3; // The following line fails typedef T3::xType xType; }; int main() { T<1>::xType a1; cout << typeid(a1).name() << endl; T<2>::xType a2; cout << typeid(a2).name() << endl; T<3>::xType a3; cout << typeid(a3).name() << endl; return 0; } There is a particular line in the code which doesn't compile: typedef T3::xType xType; If I remove this line, compilation goes fine and the result is: char long float If I retain this line, compilation errors are observed. main.cpp(53) : warning C4346: 'T<x>::xType' : dependent name is not a type prefix with 'typename' to indicate a type main.cpp(54) : see reference to class template instantiation 'T2<r>' being compiled main.cpp(53) : error C2146: syntax error : missing ';' before identifier 'xType' main.cpp(53) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int I am not able to figure out how to make sure that T::xType can be treated as a type inside the T2 template. Any help is highly appreciated.

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  • Specializing a template member function of a template class?

    - by uj2
    I have a template class that has a template member function that needs to be specialized, as in: template <typename T> class X { public: template <typename U> void Y() {} template <> void Y<int>() {} }; Altough VC handles this correctly, apperantly this isn't standard and GCC complains: explicit specialization in non-namespace scope 'class X<T>' I tried: template <typename T> class X { public: template <typename U> void Y() {} }; template <typename T> // Also tried `template<>` here void X<T>::Y<int>() {} But this causes both VC and GCC to complain. What's the right way to do this?

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