metaprogramming

问题 Is it possible to transform the types of a parameter pack and pass it on? E.g. given the following: template<class... Args> struct X {}; template<class T> struct make_pointer { typedef T* type; }; template<class T> struct make_pointer<T*> { typedef T* type; }; Can we define a template magic or something similar so that the following assertion holds: typedef magic<X, make_pointer, int, char>::type A; typedef X<int*, char*> B; static_assert(is_same<A, B>::value, ":("); 回答1: Yes we can do that

问题 I am looking for best, easiest way to do something like: $var1="value"; bunch of code..... **print allVariablesAndTheirValuesCurrentlyDefined;** 回答1: Package variables? Lexical variables? Package variables can be looked up via the symbol table. Try Devel::Symdump: #!/path/to/perl use Devel::Symdump; package example; $var = "value"; @var = ("value1", "value2"); %var = ("key1" => "value1", "key2" => "value2"); my $obj = Devel::Symdump->new('example'); print $obj->as_string(); Lexical variables

问题 What exactly is the difference? It seems like the terms can be used somewhat interchangeably, but reading the wikipedia entry for Objective-c, I came across: In addition to C’s style of procedural programming, C++ directly supports certain forms of object-oriented programming, generic programming, and metaprogramming. in reference to C++. So apparently they're different? 回答1: Programming : Writing a program that creates, transforms, filters, aggregates and otherwise manipulates data.

问题 What exactly is the difference? It seems like the terms can be used somewhat interchangeably, but reading the wikipedia entry for Objective-c, I came across: In addition to C’s style of procedural programming, C++ directly supports certain forms of object-oriented programming, generic programming, and metaprogramming. in reference to C++. So apparently they're different? 回答1: Programming : Writing a program that creates, transforms, filters, aggregates and otherwise manipulates data.

问题 How would I get something similar to the following?: (evaluate-text "(+ 1 2)") ; resolves to 3 回答1: (load-string "(+ 1 2)") 回答2: user> (eval (read-string "(+ 1 2)")) 3 You probably shouldn't ever need to do this. Macros and fns make this kind of thing unnecessary 99% of the time. This is quite brittle, and can be unsafe if these strings are coming from user input, and so on. 回答3: How similar does it have to be? Clojure's eval works on lists, so: (eval (list + 1 2)) #=> 3 来源： https:/

问题 I just found out how to check if operator<< is provided for a type. template<class T> T& lvalue_of_type(); template<class T> T rvalue_of_type(); template<class T> struct is_printable { template<class U> static char test(char(*)[sizeof( lvalue_of_type<std::ostream>() << rvalue_of_type<U>() )]); template<class U> static long test(...); enum { value = 1 == sizeof test<T>(0) }; typedef boost::integral_constant<bool, value> type; }; Is this trick well-known, or have I just won the metaprogramming

问题 You can use templates to find the length of an array. template<typename T, size_t N> size_t arraylen( T(&)[N] ) { return N; } I'd like to take this idea one step further. struct Foo { template< typename T, size_t N > Foo( /* ??? */ ) : ptr(?), size(?) { } char* ptr; size_t size; }; int main() { Foo foo("test"); const char bar[] = "test2"; Foo foo2(bar); const char* baz = bar; Foo foo3(baz); // compiler error. } However, for the life of me I can't get the syntax to compile. I think part of

问题 I am currrently working on a project that has to implement dynamic workflow. Dynamic : I store workflow's states in database table called wf_steps and the workflow gem has to create states for a particular workflow from the database For that I am trying to use the workflow gem. You can see how it initializes states and corresponding events in the gem's github-page. My code: class SpsWorkflow < ActiveRecord::Base belongs_to :user has_many :wf_steps include Workflow workflow do # binding.pry #

问题 I have a custom Array type class in Typescript. I didn't extend it because of the difficulties that comes with that, so I just use a private array which I have to directly reference each time I want to use an array method. export class ObjectCollection<T extends ICollection> { public _Collection: T; public get Collection(): T { return this._Collection; } public getInstance: (id: string) => IListItem; constructor(collection: T) { this._Collection = collection; // used for getting instance from

问题 So I have AR model like the following, and I want to dynamically generate a few instance methods like #fallbackable_header_script, #fallbackable_header_content... etc, just like the #fallbackable_background I've already written. What's the best way to do this? class Course < ActiveRecord::Base FALLBACKABLE_ATTRIBUTES = :header_script, :header_content, :footer_content OTHER_FALLBACKABLE_ATTRIBUTES = :css_config def fallbackable_background read_attribute(:background) ? background : self.user