Returns a pointer to the first argument in a variable argument list

pointer_variable = va_first()

The va_first function provides an untyped pointer value that points to the first variable argument passed to a function.

Note: Not supported when using gcc.

Function average CDecl(count As Integer, ... ) As Double
    Dim arg As Any Ptr
    Dim sum As Double = 0
    Dim i As Integer
    arg = va_first()

    For i = 1 To count
        sum += va_arg(arg, Double)
        arg = va_next(arg, Double)
    Return sum / count
End Function

Print average(4, 3.4,5.0,3.2,4.1)  '' all passed variable arguments must be of type double
Print average(2, 65.2,454.65481)   '' all passed variable arguments must be of type double

The output would look like:

'' Example of a simple custom printf
Sub myprintf CDecl(ByRef formatstring As String, ...)
    '' Get the pointer to the first var-arg
    Dim As Any Ptr arg = va_first()

    '' For each char in format string...
    Dim As UByte Ptr p = StrPtr(formatstring)
    Dim As Integer todo = Len(formatstring)
    While (todo > 0)
        Dim As Integer char = *p
        p += 1
        todo -= 1

        '' Is it a format char?
        If (char = Asc("%")) Then
            If (todo = 0) Then
                '' % at the end
                Print "%";
                Exit While
            End If

            '' The next char should tell the type
            char = *p
            p += 1
            todo -= 1

            '' Print var-arg, depending on the type
            Select Case char
            '' integer?
            Case Asc("i")
                Print Str(va_arg(arg, Integer));
                '' Note, different from C: va_next() must be
                '' used as va_arg() won't update the pointer.
                arg = va_next(arg, Integer)

            '' long integer? (64-bit)
            Case Asc("l")
                Print Str(va_arg(arg, LongInt));
                arg = va_next(arg, LongInt)

            '' single or double?
            '' Note: because the C ABI, all singles passed on
            '' var-args are converted to doubles.
            Case Asc( "f" ), Asc( "d" )
                Print Str(va_arg(arg, Double));
                arg = va_next(arg, Double)

            '' string?
            Case Asc("s")
                '' Strings are passed byval, so the length is unknown
                Print *va_arg(arg, ZString Ptr);
                arg = va_next(arg, ZString Ptr)

            End Select

        '' Ordinary char, just print as-is
            Print Chr( char );
        End If
End Sub

    Dim As String s = "bar"

    myprintf(!"integer=%i, longint=%l single=%f, double=%d, string=%s, string=%s\n", _
             1, 1ll Shl 32, 2.2, 3.3, "foo", s)


Dialect Differences:
Differences from QB:
See also:
Back to Procedures
Back to Variable Argument list
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