Internet Protocols Module User’s Guide : PART II Packages of the Internet Protocols Module : Chapter 9 Using the MIME Package : Creating a Multipart MIME Part
Creating a Multipart MIME Part
MIME parts often contain more than one document within a single part. Each document contained in a multipart part is a complete MIME part or MIME message. Therefore, the MIME specification refers to messages that contain multiple documents as multipart parts. This section describes the step-by-step process for creating a multipart/mixed part, a multipart type typically used for email attachments. The process for other multipart content types is similar.
To create a multipart/mixed message:
1. Create a MIME part for each part of the message containing the data. The steps for creating a simple MIME part are described in “Creating a Simple MIME Part.”
2. Create headers for a multipart (“Create Headers for a Multipart”).
3. Create an RWMimeMultipart (“Create an RWMimeMultipart”).
4. Add each part to the multipart body (“Add Each Part”).
5. Add the headers to the message (“Add Headers to the Part”).
6. Create a MIME string (“Create a MIME String”).
The section “Program: Creating a Multipart MIME Message” presents a complete example program that creates a multipart MIME message.
Create Parts
Each part of a multipart is a complete, valid MIME part. Construct MIME parts as described in “Creating a Simple MIME Part” above.
Some multipart types require that the parts contain specific headers. For example, a multipart/related message requires each MIME part the message contains to have either a Content-ID or a Content-Location header. See RFC 2387 for details on the multipart/related content type.
Create Headers for a Multipart
Every multipart must contain a Content-Type header. The header must have a boundary string, and the boundary string must match the boundary string in the multipart body of the message. Create a Content-Type header as shown below:
 
RWMimeMultipartType mixed;
RWMimeContentTypeHeader cTHead(mixed);
The default constructor for an RWMimeMultipartType creates a multipart/mixed type with a boundary string created by the getUniqueBoundaryString() function of RWMimeUtils. To create a value with a different subtype, provide the subtype to the constructor. For example, the line below creates a multipart/alternative Content-Type value with an automatically-generated boundary parameter:
 
RWMimeMultipartType alternative("alternative");
MIME requires that a message contain a MIME-Version header. Construct an RWMimeVersionHeader.
 
RWMimeVersionHeader version;
The Content-Transfer-Encoding for a multipart message must be one of the identity encoded types – either 7bit, 8bit, or binary. To determine the Content-Transfer-Encoding:
• If any of the parts to be included in the message has a Content-Transfer-Encoding of binary, use binary.
• Otherwise, if any of the parts to be included in the message has a Content-Transfer-Encoding of 8bit, use 8bit.
• Otherwise, use 7bit.
MIME parts often require protocol-specific headers in addition to MIME headers. For example, email messages require a Date header and a From header. The RWMimeGenericHeader class represents a general-purpose header. If the part requires headers that aren’t part of the MIME format, use instances of RWMimeGenericHeader for those headers. For example, the line below creates a From header for an email message:
 
RWMimeGenericHeader from("From:", "Dev <developer@roguewave.com>");
Create an RWMimeMultipart
Construct the part as an instance of RWMimeMultipart:
 
RWMimeMultipart multipartMessage;
Although a multipart is not required to contain a preamble, most multipart messages that contain human-readable content provide one. Set the preamble using the setPreamble() function:
 
multipartBody.setPreamble("This is a MIME message.\r\n"
"\r\n"
"If you see this message, your user"
"agent is not MIME-compliant\r\n");
Add Each Part
Use the insertPart() function of RWMimeMultipart to add each MIME part to the multipart:
 
RWMimePart part1;
 
// ... fill part1 with content.
 
RWMimePart part2;
 
// ... fill part2 with content.
 
RWMimeMultipart multipartMessage;
 
multipartMessage.insertPart(part1);
multipartMessage.insertPart(part2);
Add the parts in the order in which they should appear in the multipart.
NOTE >> An RWMimeMultipart must not contain itself, either directly or indirectly. A part that contains itself cannot be represented as a string.
Add Headers to the Part
Add the headers to the part using insertHeader() as shown below:
 
multipartMessage.insertHeader(mixed);
multipartMessage.insertHeader(version);
multipartMessage.insertHeader(from);
Remember that a multipart must contain a Content-Type header with a multipart media type, and that every MIME message must contain a MIME-Version header.
Create a MIME String
The MIME part is now complete and ready to use. To send the message over a network, convert the message into a MIME string using the asString() function, as shown:
 
RWMimeMultipart multipartMessage;
 
// ... fill message as described above
 
RWCString msgString;
 
try
{
msgString = multipartMessage.asString();
}
catch (const RWMimeError& msg)
{
// Handle the error
}
The asString() function doesn’t enforce a particular part style, or even guarantee that the part is correct MIME. The function does contain some safeguards that prevent a program from generating an unintelligible message. If the function cannot produce a structurally correct MIME message, the function throws RWMimeError.