Due to the implementation of Java generics, you can&#39;t have code like this: 

    public class GenSet&lt;E&gt; {
        private E a[];

        public GenSet() {
            a = new E[INITIAL_ARRAY_LENGTH]; // error: generic array creation
        }
    }

How can I implement this while maintaining type safety?

I saw a solution on the Java forums that goes like this:

    import java.lang.reflect.Array;
     
    class Stack&lt;T&gt; {
        public Stack(Class&lt;T&gt; clazz, int capacity) {
            array = (T[])Array.newInstance(clazz, capacity);
        }
     
        private final T[] array;
    }

But I really don&#39;t get what&#39;s going on.

How to create a generic array in Java?

I recently compared the processing speeds of `[]` and `list()` and was surprised to discover that `[]` runs *more than three times faster* than `list()`. I ran the same test with `{}` and `dict()` and the results were practically identical: `[]` and `{}` both took around 0.128sec / million cycles, while `list()` and `dict()` took roughly 0.428sec / million cycles each.

Why is this? Do `[]` and `{}` (and probably `()` and `&#39;&#39;`, too) immediately pass back a copies of some empty stock literal while their explicitly-named counterparts (`list()`, `dict()`, `tuple()`, `str()`) fully go about creating an object, whether or not they actually have elements?

I have no idea how these two methods differ but I&#39;d love to find out.
I couldn&#39;t find an answer in the docs or on SO, and searching for empty brackets turned out to be more problematic than I&#39;d expected.

I got my timing results by calling `timeit.timeit(&quot;[]&quot;)` and `timeit.timeit(&quot;list()&quot;)`, and `timeit.timeit(&quot;{}&quot;)` and `timeit.timeit(&quot;dict()&quot;)`, to compare lists and dictionaries, respectively. I&#39;m running Python 2.7.9.

I recently discovered &quot;https://stackoverflow.com/questions/18123965/why-if-true-is-slower-than-if-1&quot; that compares the performance of `if True` to `if 1` and seems to touch on a similar literal-versus-global scenario; perhaps it&#39;s worth considering as well.



Why is [] faster than list()?

Is there a way to create an instance of a particular class given the class name (dynamic) and pass parameters to its constructor.

Something like:

    Object object = createInstance(&quot;mypackage.MyClass&quot;,&quot;MyAttributeValue&quot;);

Where `&quot;MyAttributeValue&quot;` is an argument to the constructor of `MyClass`.

Creating an instance using the class name and calling constructor

Is there a way to create an instance of a class based on the fact I know the name of the class at runtime. Basically I would have the name of the class in a string.

Create an instance of a class from a string

What is the difference between `Class.forName()` and `Class.forName().newInstance()`?

I do not understand the significant difference (I have read something about them!). Could you please help me?

What is the difference between &quot;Class.forName()&quot; and &quot;Class.forName().newInstance()&quot;?

I have the following class:

    class ReportView: NSView {	
    	var categoriesPerPage = [[Int]]()
    	var numPages: Int = { return categoriesPerPage.count }
    }

Compilation fails with the message:

&gt; Instance member &#39;categoriesPerPage&#39; cannot be used on type
&gt; &#39;ReportView&#39;

 What does this mean? 


Instance member cannot be used on type

I have a file: Base.h

    class Base;
    class DerivedA : public Base;
    class DerivedB : public Base;
    
    /*etc...*/

and another file: BaseFactory.h

    #include &quot;Base.h&quot;
    
    class BaseFactory
    {
    public:
      BaseFactory(const string &amp;sClassName){msClassName = sClassName;};
    
      Base * Create()
      {
        if(msClassName == &quot;DerivedA&quot;)
        {
          return new DerivedA();
        }
        else if(msClassName == &quot;DerivedB&quot;)
        {
          return new DerivedB();
        }
        else if(/*etc...*/)
        {
          /*etc...*/
        }
      };
    private:
      string msClassName;
    };
    
    /*etc.*/

Is there a way to somehow convert this string to an actual type (class), so that BaseFactory wouldn&#39;t have to know all the possible Derived classes, and have if() for each one of them? Can I produce a class from this string? 

I think this can be done in C# through Reflection. Is there something similar in C++?


Is there a way to instantiate objects from a string holding their class name? 

Say I want to `init` a `UIView` subclass with a `String` and an `Int`.

How would I do this in Swift if I&#39;m just subclassing `UIView`? If I just make a custom `init()` function but the parameters are a String and an Int, it tells me that &quot;super.init() isn&#39;t called before returning from initializer&quot;.

And if I call `super.init()` I&#39;m told I must use a designated initializer. What should I be using there? The frame version? The coder version? Both? Why?

How do I write a custom init for a UIView subclass in Swift?

I was looking as the question : [Instantiate a class from its string name][1] which describes how to instantiate a class when having its name. Is there a way to do it in Java? I will have the package name and class name and I need to be able to create an object having that particular name.


  [1]: https://stackoverflow.com/questions/9854900/instantiate-an-class-from-its-string-name

Is there a way to instantiate a class by name in Java?

I&#39;m a C programmer trying to understand C++.  Many tutorials demonstrate object instantiation using a snippet such as:

    Dog* sparky = new Dog();

which implies that later on you&#39;ll do:

    delete sparky;

which makes sense.  Now, in the case when dynamic memory allocation is unnecessary, is there any reason to use the above instead of

    Dog sparky;

and let the destructor be called once sparky goes out of scope?

Thanks!

C++ Object Instantiation

My colleague has been using &quot;new function()&quot; with a lower case &quot;f&quot; to define new objects in JavaScript.  It seems to work well in all major browsers and it also seems to be fairly effective at hiding private variables.  Here&#39;s an example:

        var someObj = new function () {
            var inner = &#39;some value&#39;;
            this.foo = &#39;blah&#39;;
 
            this.get_inner = function () {
                return inner;
            };

            this.set_inner = function (s) {
                inner = s;
            };
        };

As soon as &quot;this&quot; is used, it becomes a public property of someObj.  So someObj.foo, someObj.get_inner() and someObj.set_inner() are all available publicly.  In addition, set_inner() and get_inner() are privileged methods, so they have access to &quot;inner&quot; through closures.

However, I haven&#39;t seen any reference to this technique anywhere.  Even Douglas Crockford&#39;s JSLint complains about it:

&gt; - weird construction. Delete &#39;new&#39;

We&#39;re using this technique in production and it seems to be working well, but I&#39;m a bit anxious about it because it&#39;s not documented anywhere.  Does anyone know if this is a valid technique?



`new function()` with lower case &quot;f&quot; in JavaScript

I have the need to take a string argument and create an object of the class named in that string in Python.  In Java, I would use `Class.forName().newInstance()`.  Is there an equivalent in Python?


----------
Thanks for the responses.  To answer those who want to know what I&#39;m doing: I want to use a command line argument as the class name, and instantiate it.  I&#39;m actually programming in Jython and instantiating Java classes, hence the Java-ness of the question.  `getattr()` works great.  Thanks much.

Does python have an equivalent to Java Class.forName()?

Take the following class as an example:

    class Sometype
    {
        int someValue;
    
        public Sometype(int someValue)
        {
            this.someValue = someValue;
        }
    }

I then want to create an instance of this type using reflection:

    Type t = typeof(Sometype);
    object o = Activator.CreateInstance(t);

Normally this will work, however because `SomeType` has not defined a parameterless constructor, the call to `Activator.CreateInstance` will throw an exception of type `MissingMethodException` with the message &quot;*No parameterless constructor defined for this object.*&quot; Is there an alternative way to still create an instance of this type? It&#39;d be kinda sucky to add parameterless constructors to all my classes.


Creating instance of type without default constructor in C# using reflection

[Ryan Delucchi][1] asked [here][2] in comment #3 to [Tom Hawtin][3]&#39;s answer:

&gt; why is Class.newInstance() &quot;evil&quot;?

this in response to the code sample:

    // Avoid Class.newInstance, for it is evil.
    Constructor&lt;? extends Runnable&gt; ctor = runClass.getConstructor();
    Runnable doRun = ctor.newInstance();

so, why is it Evil?
  [1]: https://stackoverflow.com/users/9931/ryan-delucchi
  [2]: https://stackoverflow.com/questions/194698/how-to-load-a-jar-file-at-runtime#194712
  [3]: https://stackoverflow.com/users/4725/tom-hawtin-tackline

Why is Class.newInstance() &quot;evil&quot;?

Under what conditions am I supposed to make the `:base()` and `:this()`  constructor calls following my constructor&#39;s parentheses (or even in other places in the code). When are these calls good practices and when are they mandatory? 

base() and this() constructors best practices

In other words, why would you need an instance initializer? What difference or advantage do you have in writing a instance initializer over a constructor?

How is an instance initializer different from a constructor?

**How we can instantiate class from it&#39;s name string in Ruby-on-Rails?**

For example we have it&#39;s name in database in format like &quot;ClassName&quot; or &quot;my_super_class_name&quot;.

How we can create object from it?

**Solution:**

Was looking for it myself, but not found, so here it is.
[Ruby-on-Rails API Method][1]    
 

    name = &quot;ClassName&quot;
    instance = name.constantize.new  

It can be even not formatted, we can user string method *.classify* 

    name = &quot;my_super_class&quot;
    instance = name.classify.constantize.new
    
Of course maybe this is not very &#39;Rails way&#39;, but it solves it&#39;s purpose.


  [1]: http://api.rubyonrails.org/classes/String.html#method-i-constantize

How to instantiate class from name string in Rails?

Is it possible to declare a variable in c++ without instantiating it? I want to do something like this:

    Animal a;
    if( happyDay() ) 
        a( &quot;puppies&quot; ); //constructor call
    else
        a( &quot;toads&quot; );

Basially, I just want to declare a outside of the conditional so it gets the right scope.

Is there any way to do this without using pointers and allocating `a` on the heap? Maybe something clever with references?


Declaring an object before initializing it in c++

Is there a way to circumvent the constructor `__init__` of a class in python?

Example:

    class A(object):	
	    def __init__(self):
    		print &quot;FAILURE&quot;
	
    	def Print(self):
    		print &quot;YEHAA&quot;

Now I would like to create an instance of `A`. It could look like this, however this syntax is not correct.

    a = A
    a.Print()

EDIT:

An even more complex example:

Suppose I have an object `C`, which purpose it is to store one single parameter and do some computations with it. The parameter, however, is not passed as such but it is embedded in a huge parameter file. It could look something like this:

    class C(object):
        def __init__(self, ParameterFile):
            self._Parameter = self._ExtractParamterFile(ParameterFile)
        def _ExtractParamterFile(self, ParameterFile):
            #does some complex magic to extract the right parameter
            return the_extracted_parameter

Now I would like to dump and load an instance of that object `C`. However, when I load this object, I only have the single variable `self._Parameter` and I cannot call the constructor, because it is expecting the parameter file. 

        @staticmethod
        def Load(file):
            f = open(file, &quot;rb&quot;)
            oldObject = pickle.load(f)
            f.close()

            #somehow create newObject without calling __init__
            newObject._Parameter = oldObject._Parameter
            return newObject

In other words, it is not possible to create an instance without passing the parameter file. In my &quot;real&quot; case, however, it is not a parameter file but some huge junk of data I certainly not want to carry around in memory or even store it to disc.

And since I want to return an instance of `C` from the method `Load` I do somehow have to call the constructor.

OLD EDIT:

A more complex example, which explains __why__ I am asking the question:

    class B(object):	
        def __init__(self, name, data):
            self._Name = name
            #do something with data, but do NOT save data in a variable

        @staticmethod
        def Load(self, file, newName):
            f = open(file, &quot;rb&quot;)
            s = pickle.load(f)
            f.close()

            newS = B(???)
            newS._Name = newName
            return newS

As you can see, since `data` is not stored in a class variable I cannot pass it to `__init__`. Of course I could simply store it, but what if the data is a huge object, which I do not want to carry around in memory all the time or even save it to disc?

Is there a way to instantiate a class without calling __init__?

So let&#39;s say I wanna make a dictionary. We&#39;ll call it `d`. But there are multiple ways to initialize a dictionary in Python! For example, I could do this:

    d = {&#39;hash&#39;: &#39;bang&#39;, &#39;slash&#39;: &#39;dot&#39;}

Or I could do this:

    d = dict(hash=&#39;bang&#39;, slash=&#39;dot&#39;)

Or this, curiously:

    d = dict({&#39;hash&#39;: &#39;bang&#39;, &#39;slash&#39;: &#39;dot&#39;})

Or this:
    
    d = dict([[&#39;hash&#39;, &#39;bang&#39;], [&#39;slash&#39;, &#39;dot&#39;]])

And a whole other multitude of ways with the `dict()` function. So obviously one of the things `dict()` provides is flexibility in syntax and initialization. But that&#39;s not what I&#39;m asking about.

Say I were to make `d` just an empty dictionary. What goes on behind the scenes of the Python interpreter when I do `d = {}` versus `d = dict()`? Is it simply two ways to do the same thing? Does using `{}` have the &lt;i&gt;additional&lt;/i&gt; call of `dict()`? Does one have (even negligible) more overhead than the other? While the question is really completely unimportant, it&#39;s a curiosity I would love to have answered.

What&#39;s the difference between dict() and {}?

In Javascript it would be:

&lt;!-- language: lang-js --&gt;

    var newObject = { &#39;propertyName&#39; : &#39;propertyValue&#39; };
    newObject.propertyName;  // returns &quot;propertyValue&quot;

But the same syntax in Python would create a dictionary, and that&#39;s not what I want

    new_object = {&#39;propertyName&#39;: &#39;propertyValue&#39;}
    new_object.propertyName  # raises an AttributeError

How to create inline objects with properties?

I&#39;ve always assumed that - in the absence of constructor parameters - the parentheses (curly brackets) follow the class name when creating a class instance, were optional, and that you could include or exclude them at your own personal whim.

That these two statements were equal:

    $foo = new bar;
    $foo = new bar();

Am I right? Or is there some significance to the brackets that I am unaware of?
 
I know this sounds like a RTM question, but I&#39;ve been searching for a while (including the entire PHP OOP section) and I can&#39;t seem to find a straight answer.

PHP class instantiation. To use or not to use the parentheses?

I just noticed that using `{}` instead of `()` gives the same results when constructing an object.

    class Customer
    {
        public string name;
        public string ID {get; set;}
    }

    static void Main()
    {  
        Customer c1= new Customer{}; //Is this a constructor? 
        Customer c2= new Customer();
        
        //what is the concept behind the ability to assign values for properties 
        //and fields inside the {} and is not allowable to do it inside () 
        //without defining a constructor:
        
        Customer c3= new Customer{name= &quot;John&quot;, ID=&quot;ABC&quot;};
    }

Does `{}` act like `()` when creating a new object in C#?

Does { } act like ( ) when creating a new object in C#?

See title. I have a template. I want to force a particular instance of a template to instantiate. How do I do this?

More specifically, can you force an abstract template class to instantiate?

******

I might elaborate as I have the same question.  In my case I am building a library, some of the template implementations are large and include lots of stuff, but are only generated for a couple of types.  I want to compile them in the library and export all the methods, but not include the header with the code everywhere.

ie:

    template&lt;class T&gt;
    OS_EXPORT_DECL class MyTmpl
    {
        T *item1;
    public:
        inline T *simpleGetT() { return(item1); } /* small inline code in here */ } 
        T *doSomeReallyBigMergeStuff(T *b); // note only declaration here
    };

    // *** implementation source file only seen inside library

    template&lt;class T&gt;
    MyTmpl&lt;T&gt;::doSomeReallyBigMergeStuff(T *b)
    {
        ... a really big method, but don&#39;t want to duplicate it, 
            so it is a template ...
    }

I could of course reference all the methods inside the library which would force them to compile and export but the desire isn&#39;t to add un-needed code to the library like the argument formatting for the items and the code to call them etc.

?????  specifically I am building the library for several versions of MSC and GCC and intel compilers.
  

How do I force a particular instance of a C++ template to instantiate?

I have a class and an interface, and when I try to instantiate the interface, I get an error:

&gt; Cannot create an instance of the abstract class or interface

My code is below:


    namespace MyNamespace
    {
        public interface IUser
        {
            int Property1 { get; set; }
            string Property2 { get; set; }
            string Property3 { get; set; }
            void GetUser();
        }
    
        public class User : IUser
        {
            public int Property1 { get; set; }
            public string Property2 { get; set; }
            public string Property3 { get; set; }
            
            public void GetUser()
            {
               //some logic here...... 
            }
    
        }
    }

When I try to instantiate `IUser user = new IUser();` I get an error:

&gt; Cannot create an instance of the abstract class or interface


What am I doing wrong here?








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