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  • Calling different functions depending on the template parameter c++

    - by Noman Javed
    I want to have something like that class A { public: Array& operator()() { . . . } }; class B { public: Element& operator[](int i) { ... } }; template<class T> class execute { public: output_type = operator()(T& t) { if(T == A) Array out = T()(); else { Array res; for(int i=0 ; i < length; ++i) a[i] = t[i]; } } }; There are two issues here 1. Meta-function replacing if-else in the execute operator() 2. Return type of execute operator() Thanks in anticipation Noman

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  • Is there a way to find out whether a class is a direct base of another class?

    - by user176168
    Hi I'm wondering whether there is a way to find out whether a class is a direct base of another class i.e. in boost type trait terms a is_direct_base_of function. As far as I can see boost doesn't see to support this kind of functionality which leads me to think that its impossible with the current C++ standard. The reason I want it is to do some validation checking on two macro's that are used for a reflection system to specify that one class is derived from another e.g. header.h: #define BASE A #define DERIVED B class A {}; class B : public A { #include <rtti.h> }; rtti.h: // I want to check that the two macro's are correct with a compile time assert Rtti<BASE, DERIVED> m_rtti; Although the macro's seem unnecessary in this simple example in my real world scenario rtti.h is a lot more complex. One possible avenue would be to compare the size of the this pointer with the size of a this pointer cast to the base type and some how trying to figure out whether its the size of the base class itself away or something (yeah your right I don't know how that would work either! lol)

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  • Variable function name Javascript.

    - by Vladimir
    I'm sorting array: myArray.sort(comparators.some_comparator); and I have several comparator to choose from: comparators = { asc_firstname_comparator : function(o1, o2){ ... } desc_firstname_comparator : function(o1, o2){ ... } etc... } I want to write function which returns certain comparator depending on input data. It should figure out comparator from string inputs, something like this: function chooseComparator(field, order){ return "comparators."+order+"_"+field+"_comparator"; } So is it possible to pass only function name string to sort() method or I'll need to pass reference to correct comparator somehow?

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  • Cannot Generate ParameterSetMetadata While Programmatically Creating A Parameter Block

    - by Steven Murawski
    I'm trying to programmatically create a parameter block for a function ( along the lines of this blog post ). I'm starting with a CommandMetadata object (from an existing function). I can create the ParameterMetadata object and set things like the ParameterType, the name, as well as some attributes. The problem I'm running into is that when I use the GetParamBlock method of the ProxyCommand class, none of my attributes that I set in the Attributes collection of the ParameterMetadata are generated. The problem this causes is that when the GetParamBlock is called, the new parameter is not annotated with the appropriate Parameter attribute. Example: function test { [CmdletBinding()] param ( [Parameter()] $InitialParameter) Write-Host "I don't matter." } $MetaData = New-Object System.Management.Automation.CommandMetaData (get-command test) $NewParameter = New-Object System.Management.Automation.ParameterMetadata 'NewParameter' $NewParameter.ParameterType = [string[]] $Attribute = New-Object System.Management.Automation.ParameterAttribute $Attribute.Position = 1 $Attribute.Mandatory = $true $Attribute.ValueFromPipeline = $true $NewParameter.Attributes.Add($Attribute) $MetaData.Parameters.Add('NewParameter', $NewParameter) [System.Management.Automation.ProxyCommand]::GetParamBlock($MetaData)

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  • How can I reverse ruby's include function.

    - by Glen
    I'll explain what i'm looking for in code as thats probably the most succinct: module Mixin def method puts "Foo" end end class Whatever include Mixin end w = Whatever.new w.method => "Foo" # some magic here w2 = Whatever.new w.method => NoMethodError I had tried just undefining the Mixin module using remove_const, but this doesn't seem to make any difference to Whatever. I had assumed that #include just added a reference to the module into the class's method resolution chain - but this behaviour doesn't agree with that. Can anyone tell me what include actually does behind the scenes, and how to reverse this?

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  • How do I call +class methods in Objective C without referencing the class?

    - by TimM
    I have a series of "policy" objects which I thought would be convenient to implement as class methods on a set of policy classes. I have specified a protocol for this, and created classes to conform to (just one shown below) @protocol Counter +(NSInteger) countFor: (Model *)model; @end @interface CurrentListCounter : NSObject <Counter> +(NSInteger) countFor: (Model *)model; @end I then have an array of the classes that conform to this protocol (like CurrentListCounter does) +(NSArray *) availableCounters { return [[[NSArray alloc] initWithObjects: [CurrentListCounter class],[AllListsCounter class], nil] autorelease]; } Notice how I am using the classes like objects (and this might be my problem - in Smalltalk classes are objects like everything else - I'm not sure if they are in Objective-C?) My exact problem is when I want to call the method when I take one of the policy objects out of the array: id<Counter> counter = [[MyModel availableCounters] objectAtIndex: self.index]; return [counter countFor: self]; I get a warning on the return statement - it says -countFor: not found in protocol (so its assuming its an instance method where I want to call a class method). However as the objects in my array are instances of class, they are now like instance methods (or conceptually they should be). Is there a magic way to call class methods? Or is this just a bad idea and I should just create instances of my policy objects (and not use class methods)?

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  • Checking if a function has C-linkage at compile-time

    - by scjohnno
    Is there any way to check if a given function is declared with C-linkage (that is, with extern "C") at compile-time? I am developing a plugin system. Each plugin can supply factory functions to the plugin-loading code. However, this has to be done via name (and subsequent use of GetProcAddress or dlsym). This requires that the functions be declared with C-linkage so as to prevent name-mangling. It would be nice to be able to throw a compiler error if the referred-to function is declared with C++-linkage (as opposed to finding out at runtime when a function with that name does not exist). Here's a simplified example of what I mean: extern "C" void my_func() { } void my_other_func() { } // Replace this struct with one that actually works template<typename T> struct is_c_linkage { static const bool value = true; }; template<typename T> void assertCLinkage(T *func) { static_assert(is_c_linkage<T>::value, "Supplied function does not have C-linkage"); } int main() { assertCLinkage(my_func); // Should compile assertCLinkage(my_other_func); // Should NOT compile } Thanks.

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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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  • Programatically importing a file and creating an instance of a class in Python

    - by benofsky
    I have a (python) list of strings which refer to python source files and subsequently classes within those files which I want to import and then create an instance of the classes within the files (everything follows a strict naming convention, making this theoretically possible), in Ruby I would do something like: require "lib/sources/#{source}.rb" s = source.constantize.new How would I do something similar in Python? Thanks, Ben

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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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  • Creating serializeable unique compile-time identifiers for arbitrary UDT's.

    - by Endiannes
    I would like a generic way to create unique compile-time identifiers for any C++ user defined types. for example: unique_id<my_type>::value == 0 // true unique_id<other_type>::value == 1 // true I've managed to implement something like this using preprocessor meta programming, the problem is, serialization is not consistent. For instance if the class template unique_id is instantiated with other_type first, then any serialization in previous revisions of my program will be invalidated. I've searched for solutions to this problem, and found several ways to implement this with non-consistent serialization if the unique values are compile-time constants. If RTTI or similar methods, like boost::sp_typeinfo are used, then the unique values are obviously not compile-time constants and extra overhead is present. An ad-hoc solution to this problem would be, instantiating all of the unique_id's in a separate header in the correct order, but this causes additional maintenance and boilerplate code, which is not different than using an enum unique_id{my_type, other_type};. A good solution to this problem would be using user-defined literals, unfortunately, as far as I know, no compiler supports them at this moment. The syntax would be 'my_type'_id; 'other_type'_id; with udl's. I'm hoping somebody knows a trick that allows implementing serialize-able unique identifiers in C++ with the current standard (C++03/C++0x), I would be happy if it works with the latest stable MSVC and GNU-G++ compilers, although I expect if there is a solution, it's not portable.

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  • How can I write a function template for all types with a particular type trait?

    - by TC
    Consider the following example: struct Scanner { template <typename T> T get(); }; template <> string Scanner::get() { return string("string"); } template <> int Scanner::get() { return 10; } int main() { Scanner scanner; string s = scanner.get<string>(); int i = scanner.get<int>(); } The Scanner class is used to extract tokens from some source. The above code works fine, but fails when I try to get other integral types like a char or an unsigned int. The code to read these types is exactly the same as the code to read an int. I could just duplicate the code for all other integral types I'd like to read, but I'd rather define one function template for all integral types. I've tried the following: struct Scanner { template <typename T> typename enable_if<boost::is_integral<T>, T>::type get(); }; Which works like a charm, but I am unsure how to get Scanner::get<string>() to function again. So, how can I write code so that I can do scanner.get<string>() and scanner.get<any integral type>() and have a single definition to read all integral types? Update: bonus question: What if I want to accept more than one range of classes based on some traits? For example: how should I approach this problem if I want to have three get functions that accept (i) integral types (ii) floating point types (iii) strings, respectively.

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  • Good real-world uses of metaclasses (e.g. in Python)

    - by Carles Barrobés
    I'm learning about metaclasses in Python. I think it is a very powerful technique, and I'm looking for good uses for them. I'd like some feedback of good useful real-world examples of using metaclasses. I'm not looking for example code on how to write a metaclass (there are plenty examples of useless metaclasses out there), but real examples where you have applied the technique and it was really the appropriate solution. The rule is: no theoretical possibilities, but metaclasses at work in a real application. I'll start with the one example I know: Django models, for declarative programming, where the base class Model uses a metaclass to fill the model objects of useful ORM functionality from the attribute definitions. Looking forward to your contributions.

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  • Annotation based data structure visualization - are there similar tools out there?

    - by Helper Method
    For a project at university I plan to build an annotation based tool to visualize/play around with data structures. Here's my idea: Students which want to try out their self-written data structures need to: mark the type of their data structures using some sort of marker annotation e.g. @List public class MyList { ... } so that I know how to represent the data structure need to provide an iterator so that I can retrieve the elements in the right order need to annotate methods for insertion and removal, e.g. @add public boolean insert(E e) { ... } so that I can "bind" that method to some button. Do similar applications exist? I googled a little bit around but didn't find anything like that.

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  • ruby confusing -- local variable or instance_method ?

    - by boblu
    I have the following program. module C def self.included(base) base.extend(ClassMethods) end module ClassMethods def test_for class_eval <<-DEFINECLASSMETHODS def self.my_method(param_a) puts "SELF is: #{self.inspect}" puts param_a puts "#{param_a}" end DEFINECLASSMETHODS end end end class A include C end class B < A test_for end when I run B.new.my_method("aaa"), I got this error NameError: undefined local variable or method `param_a' for B:Class I am quite confused. I define param_a as a local variable in class method my_method, puts param_a runs good, and will output the "aaa". however, puts "#{param_a}" output that error. why? Can anyone explain this?

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  • Enumerate over an enum in C++

    - by jameszhao00
    In C++, Is it possible to enumerate over an enum (either runtime or compile time (preferred)) and call functions/generate code for each iteration? Sample use case: enum abc { start a, b, c, end } for each (__enum__member__ in abc) { function_call(__enum__member__); } Plausible duplicates: C++: Iterate through an enum Enum in C++ like Enum in Ada?

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  • Python metaclass to run a class method automatically on derived class

    - by Barry Steyn
    I want to automatically run a class method defined in a base class on any derived class during the creation of the class. For instance: class Base(object): @classmethod def runme(): print "I am being run" def __metclass__(cls,parents,attributes): clsObj = type(cls,parents,attributes) clsObj.runme() return clsObj class Derived(Base): pass: What happens here is that when Base is created, ''runme()'' will fire. But nothing happens when Derived is created. The question is: How can I make ''runme()'' also fire when creating Derived. This is what I have thought so far: If I explicitly set Derived's metclass to Base's, it will work. But I don't want that to happen. I basically want Derived to use the Base's metaclass without me having to explicitly set it so.

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