different between BPL and DLL

之前, 對BPL和DLL的區別, 一直是一知半解的狀態, 大部分的書(我看過的), 就算比較經典的delphi書, 也說得很含糊! 終于找到比較詳細的說明!!!

That is correct. A BPL is a DLL. (But not all DLLs are BPLs.)

> But I still found some different, such as that I can create a
> object from the Host exe and that pass to a BPL and modify it safely, but
> if I do same to a dll, I can not modify any referenced property of the object.

When you use packages, there is only ever one copy of any unit in
memory. One copy of Forms, one copy of SysUtils, one copy of System
(well, most of it), one copy of StdCtrls, etc.

All class-related operations, such as the "is" and "as" operators, rely
on class references. Class references are actually just addresses. They
point to definitions for the layouts of the classes'''' internals. (They
point to what''''s called the virtual-method table, the VMT.) Two classes
are the same if they point to the same VMT -- if the addresses are equal.

When you have a class defined in the EXE''''s copy of StdCtrls and the same
class defined in a DLL''''s copy of StdCtrls, those classes will really
have different addresses. The "is" and "as" operators won''''t work with
cross-module clases. But when you use packages, there is only one copy
of the class, kept in vcl70.bpl, so all modules that reference that
package will share a single class definition.

Another factor is the memory manager in System. All string allocations
ultimately call GetMem and FreeMem. If the EXE allocates a string, it
uses its own GetMem. It then passes the string to a DLL, and the DLL
might try to free it. The DLL will call its own copy of FreeMem, which
won''''t have access to the EXE''''s memory-manager structures, and you''''ll get
errors. With packages, everything will use the same memory manager from
rtl70.bpl. (This can also be solved by using a shared memory manager
between the EXE and the DLL; ShareMem is one example. That won''''t solve
the class-comparison problem, though.)

Above, I said the major difference between BPLs and DLLs is the number
of exported functions. With a DLL, the only things exported are what
appear in the "exports" clause that you write yourself. With a BPL,
everything from all the units'''' "interface" sections gets exported,
including global variables and class definitions. Also exported are the
addresses of the "initialization" and "finalization" sections. And,
internally, that is indeed the major difference. A BPL exports the
functions necessary for the RTL to recognize the file as being a BPL and
not just a generic DLL. If you call LoadPackage, i will call LoadLibrary
to load it like a normal DLL, and then it will call all the package''''s
units'''' initialization sections and do a fe other housekeeping
operations. Calling a package''''s functions generates the same kind of
assembler code as is generated when you call a DLL function.

--
Rob

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