Persisting variable values with AVR’s Static() keyword

Another article in this issue of the ASNA newsletter discusses AVR for .NET’s Shared() keyword. In addition to Shared(), there is another little-known, but valuable, variable-related keyword to consider, Static().

Let’s start with a quiz. What is the output of the following code?

The output to the above code is 1 three times (you all should have gotten this one right!). This is because x is declared local to the Test subroutine and each time Test goes out of scope, the value of x is popped off the stack and it effectively disappears.

Let’s change the quiz slightly. What is the output of this code? Be careful with your answer! This one isn’t as easy.

The output of the code above is 1,2,3. The Static(Yes) causes the value of x to persist across calls to the Test subroutine. Don’t get confused here; Static(Yes) doesn’t change the variable’s scope. The x declared in Test() is still only available inside Test(); however its value persists. Static(*Yes) changes the default rules for variable persistence. This is a handy little feature if you need a counter to persist itself across calls; it eliminates the need for what would probably have otherwise been a global variable.

Terminology confusion

Be aware that AVR and VB use Shared to indicate that a variable is owned by a class, as opposed to non-shared members which are owned by class instances. AVR and VB also both use Static to indicate that a variable’s value persists across routine calls.

Java and C# name things a little differently. In these languages, a variable owned by a class is a static variable. Neither Java nor C# implement the concept of a variable that persists its value across routine calls.

Be aware of these terminology differences, especially if you are porting a C# example to AVR.