Function
Defines a procedure returning a value
Syntax
`
[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
`
Parameters
`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]`
`identifier`: the name of the variable referenced in the function. If the argument is an array then the identifier must be followed by an empty parenthesis.
`type`: the type of variable
`default_value`: the value of the argument if none is specified in the call
`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
Description
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.
Example
`
'' 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())
`
Dialect Differences

- In the -lang fb dialect, `ByVal` is the default parameter passing convention for all built-in types except `String`; String and user-defined `Type`s 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.

Differences from QB

- 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 `Overload`ing is supported in FreeBASIC.
- The return value of functions can be ignored in the calling code.

See also

- `Sub`
- `Exit`
- `Return`
- `Declare`
- `Public`
- `Private`