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v3:subroutines [2022/01/21 22:13] – [Overloading] shawnv3:subroutines [2024/07/30 22:43] (current) – [Overloading] neils
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 While you can use the [[GOSUB]] command in pair with [[RETURN]] to call parts of code as subroutines, the more sophisticated way of implementing subroutines is using the ''SUB ... END SUB'' block. While you can use the [[GOSUB]] command in pair with [[RETURN]] to call parts of code as subroutines, the more sophisticated way of implementing subroutines is using the ''SUB ... END SUB'' block.
  
-===== Defining subroutines =====+===== Defining Subroutines =====
  
 Subroutines are named routines that accept zero or more arguments. The simplest syntax to define a subroutine is the following: Subroutines are named routines that accept zero or more arguments. The simplest syntax to define a subroutine is the following:
  
-  SUB <rountine_name> (arg1 AS <type>, arg2 AS <type>, ...)+  SUB <rountine_name> ([arg1 AS <type>, arg2 AS <type>, ...])
     <statements>     <statements>
   END SUB   END SUB
  
-It is worth noting that a subroutine does not require arguments. In this case you still must add the empty parentheses after the routine name, though, like so:+It is worth noting that the argument list is optional. If you omit the arguments, you still must add the empty parentheses after the routine name, like so:
  
   SUB <routine_name> ()   SUB <routine_name> ()
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   END SUB   END SUB
  
-===== Calling subroutines =====+===== Calling Subroutines =====
  
 You can use the [[CALL]] keyword to call a subroutine. It behaves similarly to [[GOSUB]] with an important difference: ''CALL'' can pass arguments to the subroutine. Consider the following example: You can use the [[CALL]] keyword to call a subroutine. It behaves similarly to [[GOSUB]] with an important difference: ''CALL'' can pass arguments to the subroutine. Consider the following example:
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   END SUB   END SUB
      
-  CALL greet("Emily") : REM will display: Hello, Emily +  CALL greet("Emily") ' will display: Hello, Emily 
-  CALL greet("Mark") : REM will display: Hello, Mark+  CALL greet("Mark") ' will display: Hello, Mark
  
 The ''CALL'' command will evaluate the argument list in the parentheses, pass all arguments to the subroutine and then instruct the computer to continue the program at the top of the subroutine. The ''CALL'' command will evaluate the argument list in the parentheses, pass all arguments to the subroutine and then instruct the computer to continue the program at the top of the subroutine.
  
-===== Exiting subroutines =====+===== Exiting Subroutines =====
  
 The subroutine will be exited at the ''END SUB'' statement. If you want to exit a subroutine earlier, use the ''EXIT SUB'' command: The subroutine will be exited at the ''END SUB'' statement. If you want to exit a subroutine earlier, use the ''EXIT SUB'' command:
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   END SUB   END SUB
   CALL test(-1)   CALL test(-1)
-===== Local and global variables =====+===== Local and Global Variables =====
  
 Variables defined inside a subroutine are local variables, i. e. they are only accessible within that subroutine. Global variables (the ones defined outside subroutines) are visible from within all subroutines. Variables defined inside a subroutine are local variables, i. e. they are only accessible within that subroutine. Global variables (the ones defined outside subroutines) are visible from within all subroutines.
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 ==== Shadowing ==== ==== Shadowing ====
  
-A local variable may have the same name as a global variable. In such cases the local variable will be used inside the subroutine. Consider the following example:+A local variable may have the same name as a global variable. In such cases the local variable will be used inside the subroutine. This is know as a "shadow variable." Consider the following example:
  
   a = 42   a = 42
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 </adm> </adm>
  
-==== Static variables inside dynamic subroutines ====+==== Static Variables Inside Dynamic Subroutines ====
  
 You can mix static and dynamic behaviour using the ''STATIC'' keyword instead of ''DIM'' to mark local variables static when a subroutine is otherwise dynamic. You can mix static and dynamic behaviour using the ''STATIC'' keyword instead of ''DIM'' to mark local variables static when a subroutine is otherwise dynamic.
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 ===== Overloading ===== ===== Overloading =====
  
-When passing arguments to a subroutine, the compiler will match the number of arguments in the ''CALL'' statement to the number of arguments in the subroutine declaration. If the number or arguments do not match, a compile-time error is emitted.+Subroutine overloading, commonly known as method overloading in object-oriented programming languages, refers to the ability to create multiple subroutines with the same name but different parameters. This feature allows a programmer to define different ways to call a subroutine based on the types and number of arguments passed.
  
-If the number of arguments match, the compiler will compare each passed argument's type to the variable type the subroutine accepts. If the passed type can be converted to the accepted type, it will be silently converted. If however the types are not convertible (for example the subroutine accepts a numeric argument and the calling statement attempts to pass a string), compilation will fail with an error.+Overloaded subroutines have the same name but differ in the type, number, or both type and number of parameters.
  
-But what if the programmer desires a subroutine that behaves differently depending on the number or type of arguments that are passed? This is possible using overloading. A subroutine may have as many variations as desired. If there is more than one variation, the compiler will locate the best match among the candidates for the call. Consider the following example:+==== Compile-Time Polymorphism ====
  
-  SUB test (a AS INT) STATIC +The appropriate subroutine to call is determined at compile-time based on the arguments provided in the call. 
-    PRINT "a is an integer: "; a+ 
 +Consider the following example: 
 + 
 +  SUB PrintMessage(msg AS STRING * 16) 
 +      PRINT msg
   END SUB   END SUB
      
-  SUB test (a AS STRING * 16) OVERLOAD STATIC +  SUB PrintMessage(msg AS STRING * 16, num AS INT) OVERLOAD 
-    PRINT "a is a string: "; a+      PRINT msg; " "; num
   END SUB   END SUB
      
-  CALL test(5) +  CALL PrintMessage("Hello, XC=BASIC!") 
-  CALL test("hello")+  CALL PrintMessage("The number is", 42)
  
 <adm warning> <adm warning>
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 It is possible to overload the built-in XC=BASIC functions in your code, too. It is possible to overload the built-in XC=BASIC functions in your code, too.
 </adm> </adm>
-===== Forward declaration =====+===== Forward Declaration =====
  
 A subroutine can not be called before it was defined. This often makes it hard to organize your code in a clean and readable way. You may want to put subroutines at the end of your code and that's a perfectly valid requirement. A subroutine can not be called before it was defined. This often makes it hard to organize your code in a clean and readable way. You may want to put subroutines at the end of your code and that's a perfectly valid requirement.
  
-This is where forward declaration comes in handy. Forward declaration means that you declare a subroutine's all important properties (or the //header// of the subroutine) beforehands, and leave the actual implementation for later. The following example tries to illustrate this.+This is where forward declaration comes in handy. Forward declaration means that you declare a subroutine's all important properties (or the //header// of the subroutine) beforehand, and leave the actual code implementation for later. Consider the following example:
  
   REM -- the top of the program   REM -- the top of the program
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 <adm warning> <adm warning>
-The implementation must use the same number and type of arguments as the declaration. Overloading is possible though: you can declare overloaded variations of the subroutine and imlement each variation later.+The implementation of the subroutine later in the code must use the same number and type of arguments as the declaration. Overloading is still possible, though: you may declare overloaded variations of the subroutine and implement each variation later on in the program.
 </adm> </adm>
-===== Subroutine visibility =====+===== Subroutine Visibility =====
  
-Subroutines, as well as variables can also be defined with different visibility levels. XC=BASIC offers two options:+Subroutines, as well as variables, may be defined with different visibility levels. XC=BASIC offers two options:
  
-  * //Global// visibility: the subroutine is callable from within the entire code module where it was defined.+  * //Global// visibility: the subroutine is callable from within the entire code module it was defined (but not outside the code module).
   * //Shared// visibility: the subroutine is callable from within all code modules.   * //Shared// visibility: the subroutine is callable from within all code modules.
  
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   END SUB   END SUB
  
-This way you make sure it is callable from within other code modules. Read more about [[code_modules|Code Modules here]].+This will ensure the subroutine is callable from within other code modules. Read more about [[code_modules|Code Modules here]].
  
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