Some general context for my problem first.
I need to bind some arguments of a prepared statement using GNATCOLL
SQLite bindings. These bindings expect C character pointer as an input (beside other things). This requirements creates two problems on Ada's end:
Problem 1
The variable pointed to by the "bound" pointer must not perish until the prepared statement is finalized (otherwise it will store a pointer to garbage). But, for queries that operate on the same type of record it would be desirable to extract the part of binding of arguments (which are obtained from the record fields) into a separate procedure. If such procedure returns before the statement is finalized the variables (on the stack of such procedure) will be deleted, and pointers now point to garbage.
Problem 2
I only know of three instances in Ada of creating pointers: new
-ing, taking a pointer to a function / procedure and taking a pointer to a variable. Since the bindings want a pointer, I don't know how to extract such a pointer from a record unless I "manually unpack" it into a bunch of local variables. Needless to say this is ugly, repetitive and very prone to copy-and-paste error. This also leads to the lifetime issues (since variables "unpacked" in such a way will be deleted before the actual value they are used to capture still exists.)
Example
type Tag is record
Field : String := "value";
end record;
type Tag_Access is access all Tag;
procedure Bind_Tag (T : Tag_Access; Stmt : Gnade.Statement) is
-- This variable will vanish before the statement is executed
Field : aliased constant String := T.Field;
begin
Gnade.Bind_Text (Stmt, Field'Address, Field'Length);
end Bind_Tag;
procedure Insert_Tag (T : Tag) is
-- Necessary connection initialization and building of prepared statement
Tc : Tag := T; -- Creating a useless variable only to please the compiler
Ta : Tag_Access := Tc'Access;
begin
Bind_Tag (Ta, Stmt);
-- Here, bindings are dead, so we are executing garbage
Gnade.Step (Db, Stmt);
end Insert_Tag;
If I may enter a plea
I suspect this may be helped by using objects (i.e. new
-ing something). I haven't researched this approach because my first experience with Ada (I'm still learning) was very negative when contracting objects. Deallocation combined with absence of convenient object lifetime management (eg. equivalent of C++ RAII) makes using objects a very daunting task. I would like to stay away from this functionality as much as possible.
Edit
I found a way out of this particular conundrum: turns out SQlite can be instructed to make copies when binding strings. This isn't ideal, but at least I can get the strings into the database.
This doesn't mean that the question is solved though. I'd still like to know a more general way of dealing with record fields. Something that in eg. C would be accomplished by taking a pointer to the struct and then adding the size of the fields preceding the field of interest and adding that the the pointer.