The purpose of this class is much like Emacs' (interactive) form: it
allows a value expression function to declare exactly how many
arguments, and of what type, it intends to receive. It then offers
type-safe access to theese arguments in a consistent manner.
An example value expression function definition in C++:
value_t fn_foo(call_scope_t& scope) {
// We expect a string, an integer, and an optional date
interactive_t args(scope, "sl&d");
std::cout << "String = " << args.get<string>(0)
<< "Integer = " << args.get<long>(1) << std::endl;
if (args.has(2)) // was a date provided?
std::cout << "Date = " << args.get<date_t>(2) << std::endl;
return NULL_VALUE;
}
There is also an in_context_t<T> template, which finds the context type
T in the current scope hierarchy. The in_context_t then also acts as a
smart pointer to reference this context object, in addition to serving
the same duty as interactive_t. This combination of intent is solely
for the sake of brevity.
value_t fn_bar(call_scope_t& scope) {
in_context_t<account_t> env(scope, "sl&d");
std::cout << "Account name = " << env->fullname()
<< "String arg = " << env.get<string>(0)
<< std::endl;
return NULL_VALUE;
}
As you can see here, 'env' acts as a smart pointer to the required
context, and an object to extract the typed arguments.
The previous method bent over backwards to try and avoid multiple passes
through the account tree, but the result was a horribly complicated mess
that never ceased to dredge up obscure bugs. The new scheme is a very,
very simple two-pass algorithm, with multiple subpasses during the
second pass for refining the output based on the report options.
