This seems to explain it well: Instance initializers are a useful alternative to instance variable initializers whenever: initializer code must catch exceptions, or perform fancy calculations that can’t be expressed with an instance variable initializer. You could, of course, always write such code in constructors. But in a class that had multiple constructors, you would … Read more
I’ve seen that technique before, it’s valid, you are using a function expression as if it were a Constructor Function. But IMHO, you can achieve the same with an auto-invoking function expression, I don’t really see the point of using the new operator in that way: var someObj = (function () { var instance = … Read more
Because [] and {} are literal syntax. Python can create bytecode just to create the list or dictionary objects: >>> import dis >>> dis.dis(compile(‘[]’, ”, ‘eval’)) 1 0 BUILD_LIST 0 3 RETURN_VALUE >>> dis.dis(compile(‘{}’, ”, ‘eval’)) 1 0 BUILD_MAP 0 3 RETURN_VALUE list() and dict() are separate objects. Their names need to be resolved, the … Read more
Yes, something like: Class<?> clazz = Class.forName(className); Constructor<?> ctor = clazz.getConstructor(String.class); Object object = ctor.newInstance(new Object[] { ctorArgument }); That will only work for a single string parameter of course, but you can modify it pretty easily. Note that the class name has to be a fully-qualified one, i.e. including the namespace. For nested classes, … Read more
I have to ask a question in return: is your GenSet “checked” or “unchecked”? What does that mean? Checked: strong typing. GenSet knows explicitly what type of objects it contains (i.e. its constructor was explicitly called with a Class<E> argument, and methods will throw an exception when they are passed arguments that are not of … Read more
Reflection in python is a lot easier and far more flexible than it is in Java. I recommend reading this tutorial There’s no direct function (that I know of) which takes a fully qualified class name and returns the class, however you have all the pieces needed to build that, and you can connect them … Read more
I’ll just copy an example from my “notebook” int foo(void*); template<typename T> struct S { S() { int i = foo(0); } // A standard-compliant compiler is supposed to // resolve the ‘foo(0)’ call here (i.e. early) and // bind it to ‘foo(void*)’ }; void foo(int); int main() { S<int> s; // VS2005 will resolve … Read more
Maybe an example demonstrating how both methods are used will help you to understand things better. So, consider the following class: package test; public class Demo { public Demo() { System.out.println(“Hi!”); } public static void main(String[] args) throws Exception { Class clazz = Class.forName(“test.Demo”); Demo demo = (Demo) clazz.newInstance(); } } As explained in its … Read more
Two ways: Method 1 – only for classes having a no-arg constructor If your class has a no-arg constructor, you can get a Class object using Class.forName() and use the newInstance() method to create an instance (though beware that this method is often considered evil because it can defeat Java’s checked exceptions). For example: Class<?> … Read more
Nope, there is none, unless you do the mapping yourself. C++ has no mechanism to create objects whose types are determined at runtime. You can use a map to do that mapping yourself, though: template<typename T> Base * createInstance() { return new T; } typedef std::map<std::string, Base*(*)()> map_type; map_type map; map[“DerivedA”] = &createInstance<DerivedA>; map[“DerivedB”] = … Read more