FUNCTION

Defines a procedure returning a value

Syntax:[Public|Private] Function identifier [CDecl|Pascal|StdCall] [Overload] [Alias external_identifier] [([parameter_list])] [ ByRef ] 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 (default_value can be a literal, or a constant, or a shared variable)

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 (but Function keyword or function's identifier does not allow to evaluate the current assigned 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. Functions can also return references by specifying ByRef As return_type.
Warning: Whatever the output branch used, the return value must be always defined, otherwise an unexpected behavior may occur.

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.

Global Variable Access: To access duplicated symbols defined as global outside the function body, add one or preferably two dot(s) as prefix: .SomeSymbol or preferably ..SomeSymbol (or only ..SomeSymbol if inside a With..End With block).

When calling a function, parentheses surrounding the argument list (if any) are required only for function calls in expressions. If there is no argument to pass, the parentheses become optional, but it is a common convention to place empty parentheses '()' after the function name, to signify a function call.

Warning for 64-bit compiler only: See the Identifier Rules page for the choice of user procedure identifier names (and specially the 'Platform Differences' paragraph).

Examples:

'' 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 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.

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 Overloading is supported in FreeBASIC.
The return value of functions can be ignored in the calling code.

See also:

Back to Procedures

FUNCTION - FreeBASIC Wiki Manual | FBWiki

FUNCTION