[Public|Private] Function identifier [cdecl|pascal|stdcall] [Overload] [Alias external_identifier] [([parameter_list])] [As return_type] [Static] [Export]

statements
...
{ {Return [return_value]}|{Function = return_value}|{identifier = return_value} }
...

End Function

identifier: the name of the function
external_identifier: externally visible (to the linker) name enclosed in quotes
parameter_list: parameter[, parameter[, ...]]
parameter: [ByRef|ByVal] identifier [As type] [= default_value]
return_type: the type of variable returned by the function
statements: one or more statements that make up the function body
return_value: the value returned from the function

A function defines a block of code which can be executed with a single statement (a function call), and provide a value back to the caller when finished (a return value). There are several reasons to use functions:

• Reduces redundancy in your program
  
• Enables reuse of code in many programs
  
• Improves readability of the program
  
• Improves maintainability of the program
  
• Makes it easy to extend your program
  
  

Access Rights : The Public and Private keywords specify public or private intra module-level access rights, respectively. If neither is given, the function defaults to public access (Public).

Calling Convention : Calling convention, or the order in which arguments are pushed and popped from the stack during function calls, is specified with the cdecl, pascal and stdcall keywords. If none is given, the function uses the standard convention by default (stdcall).

Passing Arguments : Functions may receive one or more variables, or arguments, when called. These arguments are listed as parameters in the parameter_list. The ByRef and ByVal keywords specify whether the argument will be passed by reference or by value, respectively. The argument's type is given by "As type" following the parameter. If a parameter in the declaration is given a default value, the parameter is optional. Array parameters are specified by following an identifier with an empty parenthesis. Note that array parameters are always ByRef and the ByRef keyword is neither required nor allowed for array parameters. When calling a function with an array argument the parenthesis must be supplied there too; see the examples.

Overloaded Functions : An overloaded function may share the same name (identifier) as another with a different signature. The Overload keyword specifies that a function may be overloaded. A function must be defined - or declared - using the Overload keyword prior to any functions that overload them.

Returning values : return_type specifies the data type returned by a function upon exit. If no data type is specified, then the function will return the default data type, which will be Integer unless set to another data type using DefSng, DefDbl, DefStr, etc. Functions can return values using three methods: the Return keyword followed by a value exits the function immediately, and returns that value to the caller. Functions can also return values by assigning the Function keyword or the function's identifier to the desired return value. The latter two methods do not cause the function to exit, however. Return keyword mixed with Function= keyword or function's identifier= or Exit Function keyword in a same function is unsupported when returning objects with constructors. Since functions return values, function calls evaluate to expressions. Thus, function calls can be made wherever an expression is expected, like in Assignments or If statements. Parentheses surrounding the argument list are required on function calls in expressions and even highly recommended if there are no arguments. Functions can also return references by specifying Byref As return_type.

Local Variable Preservation : The Static keyword specifies that a function's locally declared variables are preserved between function calls. Upon entering a function defined with Static, local variables have the same value as when the function was last called.

'' This program demonstrates the declaration of a function 
'' and returning a value using Return command

Declare Function ReturnTen () As Integer

Print ReturnTen () '' ReturnTen returns an integer by default.

Function ReturnTen() As Integer
    Return 10
End Function

'' This program demonstrates the declaration of a function 
'' and returning a value using assignment to function name

Declare Function ReturnTen () As Integer

Print ReturnTen () '' ReturnTen returns an integer by default.

Function ReturnTen() As Integer
    ReturnTen = 10
End Function

'' This program demonstrates function overloading.

'' The overloaded functions must be FIRST.
Declare Function ReturnTen Overload (a As Single) As Integer
Declare Function ReturnTen Overload (a As String) As Integer
Declare Function ReturnTen (a As Integer) As Integer

Print ReturnTen (10.000!) '' ReturnTen will take a single and return an integer
Print ReturnTen (10)      '' ReturnTen will take an integer and return an integer
Print ReturnTen ("10")    '' ReturnTen will take a string and return an integer

Function ReturnTen Overload (a As Single) As Integer
    Return Int(a)
End Function

Function ReturnTen Overload (a As String) As Integer
    Return Val(a)
End Function

Function ReturnTen (a As Integer) As Integer
    Return a
End Function

'' The following example demonstrates optional parameters.

Function TestFunc(P As String = "Default") As String
    Return P
End Function

Print TestFunc("Testing:")
Print TestFunc

'' This example shows how to declare and call 
'' functions taking array arguments.

Function x(b() As Double) As Integer
  x = UBound(b)-LBound(b)+1
End Function

Dim a(1 To 10) As Double
Print x(a())
Dim c(10 To 20) As Double 
Print x(c())

• In the -lang fb dialect, ByVal is the default parameter passing convention for all built-in types except String; String and user-defined Types are passed ByRef by default.
  
• In the -lang qb and -lang fblite dialects, ByRef is the default parameter passing convention.
  
• In the -lang qb dialect, the name of the function must be used in an assignment to specify the return value. Using Function = ..." to specify the return value may not be used.
  
• In the -lang qb and -lang fblite dialects, Return may only be used to return a value when Option Gosub is off. In -lang qb, this must be done explicitly using the Option Nogosub statement.
  
  

• Parameters can be optional in FreeBASIC.
  
• In QBASIC, the return type could only specified with a suffix, not with AS TYPE, and only allowed functions to return a built-in type.
  
• Return value can now be specified by a Return statement.
  
• Function Overloading is supported in FreeBASIC.
  
• The return value of functions can be ignored in the calling code.
  
  

• Sub
  
• Exit
  
• Return
  
• Declare
  
• Public
  
• Private