CVA_LIST


Variadic argument list object type

Syntax:
dim variable as cva_list

Description:
cva_list is a built in data type for working with the variable length argument list in a variadic procedure.

Use cva_start to initialize the cva_list variable (constructor).
Use cva_copy to copy a cva_list variable (copy constructor).
Use cva_arg to obtain the value of the next argument.
Use cva_end when finished with a cva_list variable (destructor).

The exact type and size of cva_list varies depending on the target platform. This builtin type provides a layer of abstraction over the underlying mechanism for obtaining the values passed to a variadic procedure.

fbc will select a default cva_list type depending on -target, -arch, and -gen command line options, as described in the following code.
#if (__FB_BACKEND__ = "gcc")
    #if defined( __FB_64BIT__ )
        #if defined( __FB_ARM__ )
            Type __va_list Alias "__va_list"
                As Any Ptr __stack
                As Any Ptr __gr_top
                As Any Ptr __vr_top
                As Long __gr_offs
                As Long __vr_offs
            End Type
            Type cva_list As __va_list Alias "__builtin_va_list"
        #elseif defined( __FB_WIN32__ )
            Type cva_list As Any Alias "__builtin_va_list" Ptr
        #else
            Type __va_list_tag Alias "__va_list_tag"
                As ulong gp_offset
                As ulong fp_offset
                As Any Ptr overflow_arg_area
                As Any Ptr reg_save_area
            End Type  
            Type cva_list As __va_list_tag Alias "__builtin_va_list[]"
        #endif 
    #else
        Type cva_list As Any Alias "__builtin_va_list" Ptr
    #endif
#else
    Type cva_list As Any Alias "char" Ptr
#endif


Examples:
Function average CDecl(count As Integer, ... ) As Double

    Dim sum As Double = 0
    Dim i As Integer

    Dim args As cva_list '' argument list object
    cva_start( args, count ) '' constructor
    For i = 1 To count
        sum += cva_arg(args, Double)
    Next
    cva_end( args ) '' destructor
   
    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:
 3.925
 259.927405


'' pass the args list to a function taking an cva_list type argument
#include "crt/stdio.bi"

Sub myprintf CDecl( fmt As ZString Ptr, ... )
    Dim args As cva_list
    cva_start( args, fmt )
    vprintf( fmt, args )
    cva_end( args )
End Sub

Dim As String s = "bar"

myprintf( !"integer=%i, longint=%lli float=%f\n", _
    1, 1ll Shl 32, 3.3 )

myprintf( !"string=%s, string=%s\n", "foo", s )


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