metaprogramming

问题 I need partial specialization of the struct , but I'd like also use some common functionality. For example, suppose I have the next type: template <typename A, typename B> struct Foo { Foo& func0() { /* common actions with A and B */; return *this; } void func1() { /* common actions with A and B */ } void func2() { /* common actions with A and B */ } } Then I want specialize it for one of the template parameters - for example, I want consider special case when B is int , and I want to

问题 I would like to "spoil" plus method in Groovy in the following way: Integer.metaClass.plus {Integer n -> delegate + n + 1} assert 2+2 == 5 I am getting StackOverflowException (which is not surprising). Is there any way to use "original" plus method inside metaclass' closure? 回答1: The groovy idiomatic way is to save a reference to the old method and invoke it inside the new one. def oldPlus = Integer.metaClass.getMetaMethod("plus", [Integer] as Class[]) Integer.metaClass.plus = { Integer n ->

问题 I have the following snippet: FEED_TYPES = [ ('fan_mail', 'Fan Mail'), ('review', 'Review'), ('tip', 'Tip'), ('fan_user', 'Fan User'), ('fan_song', 'Fan Song'), ('fan_album', 'Fan Album'), ('played_song', 'Played Song'), ('played_album', 'Played Album'), ('played_radio', 'Played Radio'), ('new_event', 'New Event'), ] class Feed: @classmethod def do_create(cls, **kwargs): print kwargs @classmethod def create(cls, type, **kwargs): kwargs['feed_type'] = type cls.do_create(**kwargs) for type

问题 I want to generate unique numbers for each class in my header, primes in my case primes but let's say this should only be consecutive numbers i.e. 1,2,3,4,etc. Of course I can hardcode these: struct A { enum { ID = 1; }; }; struct B { enum { ID = 2; }; }; struct C { enum { ID = 3; }; }; struct D { enum { ID = 4; }; }; This is very error-prone since in reality the classes are not that small and if I add a new class in the middle I have to change all the following numbers if I don't want to

问题 In the code below, method roar is not defined in class Lion , but still can be called using method_missing . class Lion def method_missing(name, *args) puts "Lion will #{name}: #{args[0]}" end end lion = Lion.new lion.roar("ROAR!!!") # => Lion will roar: ROAR!!! In which situations and how should I use this method_missing ? And is it safe to use? 回答1: It's entirely safe to use provided you use it in expected ways and don't get carried away. Not everything you can do is worth doing, after all.

问题 At compile time in C++11 in a template function that takes 2 template parameters, both of which must be unsigned integer types, I'd like to have a local variable have the type of whichever of the two template parameters has more bits. In C++03 I might write something like: template<bool, class T, class U> struct pick_first; template<class T, class U> struct pick_first<true, T, U> { typedef T type; }; template<class T, class U> struct pick_first<false, T, U> { typedef U type; }; template<class

问题 At compile time in C++11 in a template function that takes 2 template parameters, both of which must be unsigned integer types, I'd like to have a local variable have the type of whichever of the two template parameters has more bits. In C++03 I might write something like: template<bool, class T, class U> struct pick_first; template<class T, class U> struct pick_first<true, T, U> { typedef T type; }; template<class T, class U> struct pick_first<false, T, U> { typedef U type; }; template<class

问题 Is it possible to add some magic construct around a Scala expression so that it prints the type during compilation? E.g. have some class, magic function, meta programming type, which does: val i = 1 Some(11).map(Trace(_ + 1)) // compile // prints: Int 回答1: Not exactly, but how 'bout this $ cat Test.scala def Trace[T] = identity[T] _ val i = 1 Some(11) map {x => Trace(x + 1)} $ scala -Xprint:typer Test.scala 2>&1 | egrep --o 'Trace\[.*\]' Trace[T >: Nothing <: Any] Trace[Int] The first Trace

问题 I have a boiler platey class that delegates some actions to a reference class. It looks like this: class MyClass(): def __init__(self, someClass): self.refClass = someClass def action1(self): self.refClass.action1() def action2(self): self.refClass.action2() def action3(self): self.refClass.action3() This is the refClass: class RefClass(): def __init__(self): self.myClass = MyClass(self) def action1(self): #Stuff to execute action1 def action2(self): #Stuff to execute action2 def action3(self

问题 I'm able to get access to a Ruby method's arguments using the TracePoint API: def foo(foo_arg) end trace = TracePoint.trace(:call, :c_call) do |tp| tp.disable case tp.method_id when :foo, :sub method = eval("method(:#{tp.method_id})", tp.binding) method.parameters.each do |p| puts "#{p.last}: #{tp.binding.local_variable_get(p.last)}" end end tp.enable end trace.enable foo(10) # => foo_arg: 10 However when I try this with a c method call, I get an error. "foo".sub(/(f)/) { $1.upcase } script