What are Early and Late Binding?

Question

In the spirit of asking the questions that need to be asked . . . what is the difference between early and late binding?

Answer 1

Taken directly from http://word.mvps.org/fAQs/InterDev/EarlyvsLateBinding.htm

There are two ways to use Automation (or OLE Automation) to programmatically control another application.

Late binding uses CreateObject to create and instance of the application object, which you can then control. For example, to create a new instance of Excel using late binding:

 Dim oXL As Object
 Set oXL = CreateObject("Excel.Application")

On the other hand, to manipulate an existing instance of Excel (if Excel is already open) you would use GetObject (regardless whether you're using early or late binding):

 Dim oXL As Object
 Set oXL = GetObject(, "Excel.Application")

To use early binding, you first need to set a reference in your project to the application you want to manipulate. In the VB Editor of any Office application, or in VB itself, you do this by selecting Tools + References, and selecting the application you want from the list (e.g. “Microsoft Excel 8.0 Object Library”).

To create a new instance of Excel using early binding:

 Dim oXL As Excel.Application
 Set oXL = New Excel.Application

In either case, incidentally, you can first try to get an existing instance of Excel, and if that returns an error, you can create a new instance in your error handler.

Answer 2

In compiled languages, the difference is stark.

Java:

//early binding:
public create_a_foo(*args) {
 return new Foo(args)
}
my_foo = create_a_foo();

//late binding:
public create_something(Class klass, *args) {
  klass.new_instance(args)
}
my_foo = create_something(Foo);

In the first example, the compiler can do all sorts of neat stuff at compile time. In the second, you just have to hope that whoever uses the method does so responsibly. (Of course, newer JVMs support the Class<? extends Foo> klass structure, which can greatly reduce this risk.)

Another benefit is that IDEs can hotlink to the class definition, since it's declared right there in the method. The call to createsomething(Foo) might be _very far from the method definition, and if you're looking at the method definition, it might be nice to see the implementation.

The major advantage of late binding is that it makes things like inversion-of-control easier, as well as certain other uses of polymorphism and duck-typing (if your language supports such things).

Answer 3

In interpreted languages, the difference is a little more subtle.

Ruby:

# early binding:
def create_a_foo(*args)
  Foo.new(*args)
end
my_foo = create_a_foo

# late binding:
def create_something(klass, *args)
  klass.new(*args)
end
my_foo = create_something(Foo)

Because Ruby is (generally) not compiled, there isn't a compiler to do the nifty up-front stuff. The growth of JRuby means that more Ruby is compiled these days, though, making it act more like Java, above.

The issue with IDEs still stands: a platform like Eclipse can look up class definitions if you hard-code them, but cannot if you leave them up to the caller.

Inversion-of-control is not terribly popular in Ruby, probably because of its extreme runtime flexibility, but Rails makes great use of late binding to reduce the amount of configuration necessary to get your application going.

Answer 4

The short answer is that early (or static) binding refers to compile time binding and late (or dynamic) binding refers to runtime binding (for example when you use reflection).

Answer 5

Late and early binding

Answer 6

http://blog.jeffscudder.com/2008/04/early-vs-late-binding.html