path: root/mimelib/Tutorial

                   T U T O R I A L    F O R   M I M E + + 

1. Introduction

Welcome to MIME++, a C++ class library for creating, parsing, and modifying
messages in MIME format.  MIME++ has been designed specifically with the
following objectives in mind:

  * Create classes that directly correspond to the elements described in
    RFC-822, RFC-2045, and other MIME-related documents.

  * Create a library that is easy to use.  
  
  * Create a library that is extensible.

MIME++ classes directly model the elements of the BNF grammar specified in
RFC-822, RFC-2045, and RFC-2046.  For this reason, I recommend that you
understand these RFCs and keep a copy of them handy as you learn MIME++. 
If you know C++ well, and if you are familiar with the RFCs, you should find
MIME++ easy to learn and use.  If you are new to C++ and object-oriented
programming, you will find in MIME++ some very good object-oriented
techinques, and hopefully you will learn a lot.

Before looking at the MIME++ classes, it is important to understand how
MIME++ represents a message.  There are two representations of a message.
The first is a string representation, in which a message is considered
simply a sequence of characters.  The second is a 'broken-down' -- that is,
parsed -- representation, in which the message is represented as a tree of
components.

The tree will be explained later, but for now, let's consider the
relationship between the string representation and the broken-down
representation.  When you create a new message, the string representation
is initially empty.  After you set the contents of the broken-down
representation, such as the header fields and the message body, you then
assemble the message from its broken-down representation into its string
representation.  The assembling is done through a call to the Assemble()
member function of the DwMessage class.  Conversely, when you receive a
message, it is received in its string representation, and you parse the
message to create its broken-down representation.  The parsing is done
through a call to the Parse() member function of the DwMessage class. 
From the broken-down representation, you can access the header fields, the
body, and so on. If you want to modify a received message, you can change
the contents of the broken-down representation, then assemble the message
to create the modified string representation.  Because of the way MIME++
implements the broken-down representation, only those specific components
that were modified in the broken-down representation will be modified in
the new string representation.

The broken-down representation takes the form of a tree.  The idea for the
tree comes from the idea that a message can be broken down into various
components, and that the components form a hierarchy.  At the highest
level, we have the complete message.  We can break the message down into a
header and a body to arrive at the second-highest level.  We can break the
header down into a collection of header fields.  We can break each header
field down into a field-name and a field-body.  If the header field is a
structured field, we can further break down its field-body into components
specific to that field-body, such as a local-part and domain for a
mailbox.  Now, we can think of each component of the message as a node in
the tree.  The top, or root, node is the message itself.  Below that, the
message node contains child nodes for the header and body; the header node
contains a child node for each header field; and so on.  Each node
contains a substring of the entire message, and a node's string is the
concatenation of all of its child nodes' strings.

In the MIME++ implementation, the abstract base class DwMessageComponent
encapsulates all the common attributes and behavior of the tree's nodes.
The most important member functions of DwMessageComponent are Parse() and
Assemble(), which are declared as pure virtual functions.  Normally, you
would use these member functions only as operations on objects of the
class DwMessage, a subclass of DwMessageComponent.  Parse() builds the
entire tree of components with the DwMessage object at the root.
Assemble() builds the string representation of the DwMessage object by
traversing the tree and concatenating the strings of the leaf nodes.
While every node in the tree is a DwMessageComponent, and therefore has a
Parse() and Assemble() member function, you do not have to call these
member functions for every node in the tree.  The reason is that both of
these functions traverse the subtree rooted at the current node.  Parse()
acts first on the current node, then calls the Parse() member function of
its child nodes.  Assemble() first calls the Assemble() member functions
of a node's child nodes, then concatenates the string representations of
its child nodes.  Therefore, when you call Parse() or Assemble() for an
object of the class DwMessage, Parse() or Assemble() will be called
automatically for every component (that is, child node) in the message.

DwMessageComponent also has one important attribute that you should be aware
of.  That attribute is an is-modified flag (aka dirty flag), which is
cleared whenever Parse() or Assemble() is called, and is set whenever the
broken-down representation is modified.  To understand how this works,
suppose you have just called Parse() on a DwMessage object to create its
broken-down representation.  If you add a new DwField object (representing a
new header field) to the DwHeaders object (representing the header), the
is-modified flag will be set for the DwHeaders object, indicating that the
string representation of the DwHeaders object will have to be re-assembled
from the header fields that it contains.  When a node's is-modified flag is
set, it also notifies its parent node to set its is-modified flag.  Thus,
when the DwHeaders object's is-modified flag is set, the DwMessage object
that is its parent will also have its is-modified flag set.  That way, when
Assemble() is called for the DwMessage object, it will call the Assemble()
member function for the DwHeaders object, as required.  Notice that the value
of having an is-modified flag is that it can purge the tree traversal when
the string representation of a message is being assembled.

One of the first classes you should become familiar with is the DwString
class, which handles character strings in MIME++.  DwString has been
designed to handle very large character strings, so it may be different
from string classes in other libraries.  Most of the standard C library
string functions have DwString counterparts in MIME++.  These functions
all start with "Dw", and include DwStrcpy(), DwStrcmp(), DwStrcasecmp(),
and so on.  In addition, the equality operators and assignment operators
work as expected.  If you have used string classes from other libraries,
you will find DwString fairly intuitive.

The following sections describe how to create, parse, and modify a
message.  You should also look at the example programs included with the
distribution.  These example programs are well-commented and use wrapper
classes.  The wrapper classes BasicMessage, MultipartMessage, and
MessageWithAttachments, are designed with three purposes in mind.  First,
if your requirements are very modest -- say you just want to send a few
files as attachments -- then you may find these classes to be adequate for
your needs, and you will not have to learn the MIME++ library classes.
Second, wrapper classes are the recommended way to use MIME++.  You should
consider starting with these classes and customizing them for your own
application.  Using wrapper classes will simplify the use of the MIME++
library, but will also help to shield your application from future changes
in the MIME++ library.  Third, these classes provide excellent examples for
how to use the MIME++ library classes.

The rest of this tutorial focuses on the library classes themselves.


2. Creating a Message

Creating a message with MIME++ involves instantiating a DwMessage object,
setting values for its parts, and assembling the message into its final
string representation.  The following simple example shows how to
accomplish this.


      void SendMessage(
          const char* aTo,
          const char* aFrom, 
          const char* aSubject,
          const char* aBody)
      {
          // Create an empty message

          DwMessage msg;

          // Set the header fields.  
          // [ Note that a temporary DwString object is created for
          // the argument for FromString() using the
          // DwString::DwString(const char*) constructor. ]

          DwHeaders& headers = msg.Headers();
          headers.MessageId().CreateDefault();
          headers.Date().FromCalendarTime(time(NULL)); //current date, time
          headers.To().FromString(aTo);
          headers.From().FromString(aFrom);
          headers.Subject().FromString(aSubject);

          // Set the message body

          msg.Body().FromString(aBody);

          // Assemble the message from its parts

          msg.Assemble();

          // Finally, send it.  In this example, just print it to the 
          // cout stream.

          cout << msg.AsString();
      }


In this example, we set the fields 'Message-Id', 'Date', 'To', 'From', and
'Subject', which are all documented in RFC-822.  The MIME++ class DwHeaders
directly supports all header fields documented in RFC-822, RFC-2045, and
RFC-1036.  To access the field-body for any one these fields, use the
member function from DwHeaders that has a name corresponding to the
field-name for that field.  The correspondence between a field-name and
the name of the member function in DwHeaders is consistent: hyphens are
dropped and the first character after the hyphen is capitalized.  Thus,
field-name Content-type in RFC-1521 corresponds to the member function
name ContentType.  These field-body access functions create an empty field
in the headers if that field does not already exist.  To check if a
particular field exists already, DwHeaders provides member functions
HasXxxxx(); for example, HasSender(), HasMimeVersion(), or HasXref()
will indicate whether the DwHeaders object has a 'Sender' field, a
'MIME-Version' field, or an 'Xref' field, respectively.

In the example, we used the FromString() member function of
DwMessageComponent to set the string representation of the field-bodies.
This is the simplest way to set the contents of a DwFieldBody object.
Many of the field-bodies also have a broken-down represenation, and it is
possible to set the parts of the broken-down representation.  Consider, for
example, the DwDateTime class, which represents the date-time element of the
BNF grammar specified in RFC-822.  In the example above, we did not set the
string representation -- that would be more difficult and error prone.
Instead we set the contents from the time_t value returned from a call to
the ANSI C function time().  The DwDateTime class also contains member
functions for setting individual attributes.  For example, we could have
used the following code:

      DwDateTime& date = msg.Headers().Date();
      time_t t = time(NULL);
      struct tm stm = *localtime(&t);
      date.SetYear(stm.tm_year);
      date.SetMonth(stm.tm_mon);
      date.SetDay(stm.tm_mday);
      date.SetHour(stm.tm_hour);
      date.SetMinute(stm.tm_min);


3. Parsing a Message

Parsing a received message with MIME++ involves instantiating a DwMessage
object, setting its string representation to contain the message, and then
calling the Parse() member function of the DwMessage object.  The
following simple example shows how to accomplish this.

      void ParseMessage(DwString& aMessageStr)
      {
          // Create a message object
          // We can set the message's string representation directly from the
          // constructor, as in the uncommented version.  Or, we can use the
          // default constructor and set its string representation using
          // the member function DwMessage::FromString(), as in the
          // commented version.

          DwMessage msg(aMessageStr);
    
          // Alternate technique:
          // DwMessage msg;                 // Default constructor
          // msg.FromString(aMessageStr);   // Set its string representation

          // Execute the parse method, which will create the broken-down
          // representation (the tree representation, if you recall)

          msg.Parse();

          // Print some of the header fields, just to show how it's done
    
          // Date field.  First check if the field exists, since
		  // DwHeaders::Date() will create it if is not found.

          if (msg.Headers().HasDate()) {
              cout << "Date of message is " 
                   << msg.Headers().Date().AsString()
                   << '\n';
          }

          // From field.  Here we access the broken-down field body, too,
		  // to get the full name (which may be empty), the local part,
		  // and the domain of the first mailbox.  (The 'From' field can
		  // have a list of mailboxes).

          if (msg.Headers().HasFrom()) {
              DwMailboxList& from = msg.Headers().From();
              cout << "Message is from ";

              // Get first mailbox, then iterate through the list

              int isFirst = 1;
              DwMailbox* mb = from.FirstMailbox();
              while (mb) {
                  if (isFirst) {
                      isFirst = 0;
                  }
                  else {
                      cout << ", ";
                  }
                  DwString& fullName = mb->FullName();
                  if (fullName != "") {
                      cout << fullName << '\n';
                  }
                  else {
                      // Apparently, there is no full name, so use the email
                      // address
                      cout << mb->LocalPart() << '@' << mb->Domain() << '\n';
                  }
                  mb = mb->Next();
              }

          }
    
          // Finally, print the message body, just to show how the body is
          // retrieved.

          cout << msg.Body().AsString() << '\n';
      }

Once you have parsed the message, you can access any of its parts.  The
field-bodies of well-known header fields can be accessed by calling member
functions of DwHeaders.  Some examples follow.

    DwMediaType& contType = msg.Headers().ContentType();
    DwMechanism& cte = msg.Headers().ContentTransferEncoding();
    DwDateTime& date = msg.Headers().Date();

The various subclasses of DwFieldBody, including DwMediaType, DwMechanism,
and DwDateTime above, have member functions that allow you to access the parts
of the field-body.  For example, DwMediaType has member functions to allow
you to access its type, subtype, and parameters.  If the message is a
multipart message, you may access the body parts by calling member
functions of the class DwBody.  See the example code in multipar.cpp for
an example of how to do this.


4. Modifying a Message

Modifying a message combines the procedures of parsing a message and
creating a message.  First, parse the message, as explained above.  Then
set the values of the components -- field-bodies, new fields, new body
parts, or what have you -- that you wish to modify.  Finally, call the
Assemble() member function of the DwMessage object to reassemble the
message.  You can then access the modified message by calling
DwMessage::AsString().  These final steps are the same as those involved
in creating a new message.


5. Customizing MIME++ Classes

MIME++ has been designed to be easily customizable.  Typically, you
customize C++ library classes through inheritance.  MIME++ allows you to
create subclasses of most of its library classes in order to change their
behavior.  MIME++ also includes certain 'hooks', which make it far easier
to customize certain parts of the library.

The most common customization is that of changing the way header fields
are dealt with.  This could include adding the ability to handle certain
non-standard header fields, or to change the way the field-bodies of
certain standard header fields are interpreted or parsed.  As an example of
the former customization, you may want to add the 'X-status' field or
'X-sender' field to your messages.  As an example of the latter, you may
want to change DwMediaType so that it will handle other MIME subtypes.

Let's begin with the latter situation -- that of subclassing DwMediaType.
Obviously, you will have to become familiar with DwMediaType and its
superclasses before you change its behavior.  Then, at a minimum, you will
want to provide your own implementation of the virtual member functions
Parse() and Assemble().  Once you feel comfortable with the behavior of
the behavior of your new class -- call it MyMediaType -- you will have to
take the right steps to ensure that the MIME++ library internal routines
will create objects of type MyMediaType, and not DwMediaType.  There are
three such steps.

First, define a function NewMyMediaType(), matching the prototype

      DwMediaType* NewMyMediaType(
	      const DwString& aStr, 
		  DwMessage* aParent)

that creates a new instance of MyMediaType and returns it.  Set the static
data member DwMediaType::sNewMediaType to point to this function.
DwMediaType::sNewMediaType is normally NULL, meaning that no user-defined
function is available.  When you set this static data member, however,
MIME++'s internal routines will call your own function, and will therefore
be able to create instances of your subclass.

Second, make sure you have reimplemented the virtual function
DwMediaType::Clone() to return a clone of your own subclassed object.
Clone() serves as a 'virtual constructor'.  (See the discussion of virtual
constructors in Stroustrup's _The C++ Programming Language_, 2nd Ed).

Third, you should define a function CreateFieldBody(), matching the
prototype

      DwFieldBody* CreateFieldBody(
          const DwString& aFieldName,
          const DwString& aFieldBody, 
          DwMessageComponent* aParent)

that returns an object of a subclass of DwFieldBody.  (DwFieldBody is a
superclass of MyMediaType).  CreateFieldBody() is similar to the
NewMyMediaType() function already described, except that its first
argument supplies the field-name for the particular field currently being
handled by MIME++.  CreateFieldBody() should examine the field-name,
create an object of the appropriate subclass of DwFieldBody, and return a
pointer to the object.  In this particular case, you need to make sure
that when the field-name is 'Content-Type' you return an object of the
class MyMediaType.  Set the hook for CreateFieldBody() setting the static
data member DwField::sCreateFieldBody to point to your CreateFieldBody()
function.  DwField::sCreateFieldBody is normally NULL when no user
function is provided.

These three steps are sufficient to ensure that your subclass of
DwMediaType is integrated with the other MIME++ classes.

The other customization task mentioned above is that of adding support for
a non-standard header field.  There is a simple way to do this, and a way
that involves creating a subclass of DwHeaders.  You can access any header
field by calling DwHeaders's member functions.  In fact, you can iterate
over all the header fields if you would like.  Therefore, the really
simple way is just to not change anything and just use existing member
functions.  The relevant functions include DwHeaders::HasField(), which will
return a boolean value indicating if the header has the specified field,
and DwHeaders::FieldBody(), which will return the DwFieldBody object
associated with a specified field.  [ Note that DwHeaders::FieldBody() will
create a field if it is not found. ] The default DwFieldBody subclass,
which applies to all header fields not recognized by MIME++, is DwText,
which is suitable for the unstructured field-bodies described in RFC-822
such as 'Subject', 'Comments', and so on.  If a DwText object is suitable
for your non-standard header field, then you don't have to do anything at all.
Suppose, however, that you want an object of your own subclass of
DwFieldBody, say StatusFieldBody, to be attached to the 'X-status' field.
In this case, you will need to set the hook DwField::sCreateFieldBody as
discussed above.  Your CreateFieldBody() function should return an
instance of StatusFieldBody whenever the field-name is 'X-status'.

Finally, while you can access any header field using DwHeaders's member
functions, you may want to create your own subclass of DwHeaders for some
reason or other -- maybe to add a convenience function to access the
'X-status' header field.  To ensure that your new class is integrated with
the library routines, you basically follow steps 1 and 2 above for
subclassing DwFieldBody.  First, define a function NewMyHeaders() and set the
static data member DwHeaders::sNewHeaders to point to your function. Second,
make sure you have reimplemented the virtual function DwHeaders::Clone() to
return an instance of your subclass.  Step 3 for subclassing DwFieldBody
does not apply when subclassing DwHeaders.


6. Getting Help

I will try to help anyone who needs help specific to MIME++.  I won't try
to answer general questions about C++ that could be answered by any C++
expert.  Bug reports will receive the highest priority.  Other questions
about how to do something I will try to answer in time, but I ask for your
patience.  If you have any comments -- perhaps maybe you know of a better
way to do something -- please send them.  My preferred email is
[email protected], but [email protected] is also acceptable.

Good luck!