In this article we will discuss about namespace in TCL.
Namespace :
- A namespace allows you to place a bunch of code under the hood of a name.
- Thus any naming conflicts with Classes, Functions and Constants etc are avoided between two namespaces.
- It allows your code to live in that namespace.
- Hence encapsulation of data & functions are done in various programming languages.
- Namespace Hierarchy Separator is Generally “::”
Namespace Hierarchy Visualization :
We will understand the namespace concept using the below info-graphics !
The outer most space , as shown in above picture, is called the Built-In-Namespace. When you start writing any code this is the namespace that you writing the code in. This is true for any programming language , not only TCL. Then one level down , there comes the Global Namespace which is specified by you explicitly. If you go a further level down , you reach up to the Local Namespace. Now, you may go any further level down . Then the levels will be Local Namespace -1 , Local Namespace-2 ......... and so on. You may nominate the nomenclature of these local namespaces as per your own convention.
Namespace Creation in TCL :
To create any namespace of your own , you have to follow the syntax :
namespace eval <Your-Name-space-name>
Here is an example of an namespace with purpose as an up-counter :
namespace eval Counter {
variable num 1
proc increase { } {
variable num
return [incr num]
}
}
As you can see we can define variable and procedures exclusively for that namespace ! Here in the above example, the procedure increase does increases the counter upwards.
Now , at some later part of the code if we decide to include a procedure in the namespace, we do not have to go back and edit it ! Instead we can do it by writing the code snippet below :
namespace eval Counter {
variable clock 5
proc hello { } {
puts "Hello World"
}
}
You can see this block is similar to the previous block , only difference is that this block appends more information to the namespace we have defined earlier.
Important to remember that this block does not re-define or replace the existing namespace declared above !
Once these namespace are defined, we can use the procedures declared inside them as :
Counter::increase and Counter::hello
The entire code is combined in the below snippet :
While executed , it gives the below output !
Various puts statements were inserted to identify the various code-blocks of namespace both in and out !
Namespace :: Hierarchy :: Code Example
We have the hierarchy visualization in previous section . Now we will take a code example to realise the hierarchy.
Here we create the topmost global namespace as : namespace eval top {}
Within which we define our local namespace as : namespace eval within {}
The two names top and within are self-explanatory w.r.to the purpose of this section.
Here goes our entire code snippet for better understanding :
Which gives the output as :
So , the change of the hierarchical data is done by : set ::top::test 25
And the hierarchical access of procedure is done by : ::top::within::hello
In the above code example.
Namespace :: Export/Import :: Code Example
If we want to export any data from withing any namespace , the data transferred to the namespace immediately outside as shown by the first info graphics in this article.
So this export or import operation is very relative and not absolute !
To export everything from current scope to the namespace immediately outside , we can use : namespace export *
To import from an existing namespace we can use : namespace import <name-space-name>::*
To delete everything that we have just imported , we can use : namespace forget space::*
We can combine all these commads in the below example :
Which , upon execution , gives the below output :
The entire article is well narrated in the below video :
Courtesy : Image by www.pngegg.com
