Exit Zen
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string ID 97
type: paragraph
English
Use of the flexible __Body__ concept customization point enables authors to preserve the self-contained nature of the __message__ object while allowing domain specific behaviors. Common operations for HTTP servers include sending responses which deliver file contents, and allowing for file uploads. In this example we build the [link beast.ref.boost__beast__http__basic_file_body `basic_file_body`] type which supports both reading and writing to a file on the file system. The interface is a class templated on the type of file used to access the file system, which must meet the requirements of __File__.
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string ID 108
type: paragraph
English
First we declare the type with its nested types:
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string ID 112
type: paragraph
English
We will start with the definition of the `value_type`. Our strategy will be to store the file object directly in the message container through the `value_type` field. To use this body it will be necessary to call `msg.body.file().open()` first with the required information such as the path and open mode. This ensures that the file exists throughout the operation and prevent the race condition where the file is removed from the file system in between calls.
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string ID 122
type: paragraph
English
Our implementation of __BodyWriter__ will contain a small buffer from which the file contents are read. The buffer is provided to the implementation on each call until everything has been read in.
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string ID 128
type: paragraph
English
And here are the definitions for the functions we have declared:
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string ID 132
type: paragraph
English
Files can be read now, and the next step is to allow writing to files by implementing the __BodyReader__. The style is similar to the writer, except that buffers are incoming instead of outgoing. Here's the declaration:
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string ID 139
type: paragraph
English
Finally, here is the implementation of the reader member functions:
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string ID 143
type: paragraph
English
We have created a full featured body type capable of reading and writing files on the filesystem, integrating seamlessly with the HTTP algorithms and message container. The body type works with any file implementation meeting the requirements of __File__ so it may be transparently used with solutions optimized for particular platforms. Example HTTP servers which use file bodies are available in the example directory.
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