Parameter-less plug-in functions
Occasionally, it is useful to invoke a function or method within a plug-in which requires, and returns, no parameters. This is simply achieved by having the function/method ignore the function/method parameters and defining a function which takes no parameters and returns void in the function table. Examples are given below.
In C++ the method is defined as:
void AP_PLUGIN_CALL Analytic::SilentInitialisation (
const AP_Context& ctx,
const AP_TypeList& args,
AP_Type& rval,
AP_TypeDiscriminator rtype) {
// Custom Code Here
// Ignoring the args, rval and rtype parameters
}
And, in C as:
static void AP_PLUGIN_CALL SilentInitialisation (
const AP_PluginContext* ctx,
const AP_PluginTypeList* args,
AP_PluginType* rval,
AP_TypeDiscriminator rtype) {
// Custom Code Here
// Ignoring the args, rval and rtype parameters
}
Then the function table would appear thus, in C++:
Static AP_Function Functions[1] = {
{
"SilentInit", &Analytics:: SilentInitialisation, 0, NULL,
"void" },
};
And as below in C:
Static AP_Function Functions[1] = {
{ "SilentInit", &SilentInitialisation, 0, NULL, "void" },
};
In EPL, the plug-in function/method is then invoked as:
import "analytics" as a;
action onload {
a.SilentInit();
// Custom Code Here
}
Each call to a plug-in function returns a single value. Occasionally, it is necessary for an operation to return multiple values; there are various techniques that can be used to achieve this:
Provide multiple functions which are called in turn, each of which returns one of the values.
Return a
chunk expressing the composite value, and provide functions that interrogate the
chunk to extract each individual value.
Return a
string that can be parsed as an event that expresses the composite value.
Enqueue an event that expresses the composite value.
Pass the function a
string and modify the elements.
See
The chunk type for details of how chunks are used and
Asynchronous plug-ins for how to enqueue an event from a plug-in.