Declare

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Declare

Declares a module-level or member procedure

Syntax

Declare Sub name [ param_list ]
Declare Function name [ param_list ] [ ByRef ] As return_type
Declare Operator op_symbol param_list [ [ ByRef ] As return_type ]
Type T
Declare Constructor [ param_list ]
Declare Destructor
Declare Sub name [ param_list ]
Declare Function name [ param_list ] [ ByRef ] As return_type

Parameters

• param_list
• return_type
• name
• op_symbol
• T

Description

The Declare statement declares a Sub, Function, Operator, Constructor, or Destructor.
The procedure can be referred to in code without seeing its definition, although it must be defined somewhere. Essentially, the Declare statement introduces a procedure, and states that its definition is elsewhere. For example, a function can be declared at the top of a source module, called, then defined at the bottom of the source file, as shown below the example.

A procedure's declaration is almost identical to the first line of its definition, except the declaration is preceded by the Declare keyword and has no body. Also, attributes such as Export are left off the declaration.

FreeBASIC, as QB, does not require the declaration of the functions unless they are defined in a different source file or in the same file past the point where they are called. This is no longer true for procedures declared inside a Type body, which must always be declared first in the Type's body before use. If you do not declare Type procedures you will receive an error.

As every file using a function must have its declaration, declarations are usually kept in one or more include files to allow usage of the function by any module that needs it using the #include statement.

Remarks

Dialect Differences

• In the -lang fb dialect, ByVal is the default parameter passing convention. In the -lang qb and -lang deprecated dialects, ByRef is the default parameter passing convention.
  Type-level Sub/Function/Operator/Constructor/Destructor's are only allowed in -lang fb

Differences from QB

• In FreeBASIC, the parameter names are optional.

See also

• Sub
• Function
• Operator
• Property
• Constructor
• Destructor
• Constructor (Module)
• Destructor (Module)
• Type
• Dim
• Alias (Name)
• Alias (Modifier)

Examples

Module-level Function:

'' declare the function sum which takes two integers and returns an integer

Declare Function sum( As Integer, As Integer ) As Integer

   Print "the sum of 420 and 69 is: " & sum( 420, 69 )    '' call the function sum

'' define the function sum which takes two integers and returns an integer

Function sum( a As Integer, b As Integer ) As Integer

   Return a + b

End Function

Type-level Sub:

Type my_type

    my_data As Integer

    Declare Sub increment_data( )

End Type

Sub my_type.increment_data( )

    my_data += 1

End Sub

Dim As my_type an_instance

an_instance.my_data = 68

an_instance.increment_data( )

Print an_instance.my_data

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Category: Procedures
Kind: function

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Reproduced from the FreeBASIC Wiki Manual under the GNU Free Documentation License 1.3 — with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. Copyright © the FreeBASIC project and wiki contributors. See License.