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These helper functions are used to construct a chunk or last chunk directly at call sites.
We demonstrate the use of these objects first by declaring a function which returns the next buffer sequence to use as a chunk body:
This example demonstrates sending a complete chunked message payload manually. No chunk extensions or trailers are emitted:
The following code sends additional chunks, and sets chunk extensions using the helper container. The container automatically quotes values in the serialized output when necessary:
Callers can take over the generation and management of the extensions buffer by passing a non-owning string. Note that this requires the string contents to adhere to the correct syntax for chunk extensions, including the needed double quotes for values which contain spaces:
The next code sample emits a chunked response which promises two trailer fields and delivers them in the last chunk. The implementation allocates memory using the default or a passed-in allocator to hold the state information required to serialize the trailer:
Using a custom allocator to serialize the last chunk:
Alternatively, callers can take over the generation and lifetime management of the serialized trailer fields by passing in a non-owning string:
For the ultimate level of control, a caller can manually compose the chunk itself by first emitting a header with the correct chunk body size, and then by emitting the chunk body in multiple calls to the stream write function. In this case the caller is responsible for also emitting the terminating CRLF (`"\r\n"`):
Parsing Chunks
The __parser__ automatically removes the chunked transfer coding when it is the last encoding in the list. However, it also discards the chunk extensions and does not provide a way to determine the boundaries between chunks. Advanced applications which need to access the chunk extensions or read complete individual chunks may use a callback interface provided by __parser__:
Chunking Parse Callbacks
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Name
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Description
[link beast.ref.boost__beast__http__parser.on_chunk_header `on_chunk_header`]
Set a callback to be invoked on each chunk header.

The callback will be invoked once for every chunk in the message payload, as well as once for the last chunk. The invocation happens after the chunk header is available but before any body octets have been parsed.

The extensions are provided in raw, validated form, use [link beast.ref.boost__beast__http__basic_chunk_extensions.parse `chunk_extensions::parse`] to parse the extensions into a structured container for easier access. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
[link beast.ref.boost__beast__http__parser.on_chunk_body `on_chunk_body`]
Set a callback to be invoked on chunk body data.

The callback will be invoked one or more times to provide buffers corresponding to the chunk body for the current chunk. The callback receives the number of octets remaining in this chunk body including the octets in the buffer provided.

The callback must return the number of octets actually consumed. Any octets not consumed will be presented again in a subsequent invocation of the callback. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
This example will read a message header from the stream, and then manually read each chunk. It recognizes the chunk boundaries and outputs the contents of each chunk as it comes in. Any chunk extensions are printed, each extension on its own line. Finally, any trailers promised in the header are printed.
Given the HTTP response as input on the left, the output of the function shown above is shown on the right:
Chunk Parsing Example Output
Input
Output
HTTP/1.1 200 OK\r\n
Server: test\r\n
Trailer: Expires, Content-MD5\r\n
Transfer-Encoding: chunked\r\n
\r\n
5\r\n
First\r\n
d;quality=1.0\r\n
Hello, world!\r\n
e;file=abc.txt;quality=0.7\r\n
The Next Chunk\r\n
8;last\r\n
Last one\r\n
0\r\n
Expires: never\r\n
Content-MD5: f4a5c16584f03d90\r\n
\r\n
Chunk Body: First
Extension: quality = 1.0
Chunk Body: Hello, world!
Extension: file = abc.txt
Extension: quality = 0.7
Chunk Body: The Next Chunk
Extension: last
Chunk Body: Last one
Expires: never
Content-MD5: f4a5c16584f03d90
Key English Actions
Chunk Parsing Example Output
[link beast.ref.boost__beast__http__parser.on_chunk_body `on_chunk_body`]
Set a callback to be invoked on chunk body data.

The callback will be invoked one or more times to provide buffers corresponding to the chunk body for the current chunk. The callback receives the number of octets remaining in this chunk body including the octets in the buffer provided.

The callback must return the number of octets actually consumed. Any octets not consumed will be presented again in a subsequent invocation of the callback. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
This example will read a message header from the stream, and then manually read each chunk. It recognizes the chunk boundaries and outputs the contents of each chunk as it comes in. Any chunk extensions are printed, each extension on its own line. Finally, any trailers promised in the header are printed.
Given the HTTP response as input on the left, the output of the function shown above is shown on the right:
A buffer sequence representing a last chunk. The last chunk indicates the end of the chunked message payload, and may contain optional trailer fields.
[link beast.ref.boost__beast__http__make_chunk `make_chunk`]

[link beast.ref.boost__beast__http__make_chunk_last `make_chunk_last`]
These helper functions are used to construct a chunk or last chunk directly at call sites.
We demonstrate the use of these objects first by declaring a function which returns the next buffer sequence to use as a chunk body:
This example demonstrates sending a complete chunked message payload manually. No chunk extensions or trailers are emitted:
The following code sends additional chunks, and sets chunk extensions using the helper container. The container automatically quotes values in the serialized output when necessary:
Callers can take over the generation and management of the extensions buffer by passing a non-owning string. Note that this requires the string contents to adhere to the correct syntax for chunk extensions, including the needed double quotes for values which contain spaces:
The next code sample emits a chunked response which promises two trailer fields and delivers them in the last chunk. The implementation allocates memory using the default or a passed-in allocator to hold the state information required to serialize the trailer:
Using a custom allocator to serialize the last chunk:
Alternatively, callers can take over the generation and lifetime management of the serialized trailer fields by passing in a non-owning string:
For the ultimate level of control, a caller can manually compose the chunk itself by first emitting a header with the correct chunk body size, and then by emitting the chunk body in multiple calls to the stream write function. In this case the caller is responsible for also emitting the terminating CRLF (`"\r\n"`):
Parsing Chunks
The __parser__ automatically removes the chunked transfer coding when it is the last encoding in the list. However, it also discards the chunk extensions and does not provide a way to determine the boundaries between chunks. Advanced applications which need to access the chunk extensions or read complete individual chunks may use a callback interface provided by __parser__:
Chunking Parse Callbacks
[link beast.ref.boost__beast__http__parser.on_chunk_header `on_chunk_header`]
Set a callback to be invoked on each chunk header.

The callback will be invoked once for every chunk in the message payload, as well as once for the last chunk. The invocation happens after the chunk header is available but before any body octets have been parsed.

The extensions are provided in raw, validated form, use [link beast.ref.boost__beast__http__basic_chunk_extensions.parse `chunk_extensions::parse`] to parse the extensions into a structured container for easier access. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
158
Description
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Description
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Name

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Set a callback to be invoked on each chunk header.

The callback will be invoked once for every chunk in the message payload, as well as once for the last chunk. The invocation happens after the chunk header is available but before any body octets have been parsed.

The extensions are provided in raw, validated form, use [link beast.ref.boost__beast__http__basic_chunk_extensions.parse `chunk_extensions::parse`] to parse the extensions into a structured container for easier access. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
Set a callback to be invoked on each chunk header.

The callback will be invoked once for every chunk in the message payload, as well as once for the last chunk. The invocation happens after the chunk header is available but before any body octets have been parsed.

The extensions are provided in raw, validated form, use [link beast.ref.boost__beast__http__basic_chunk_extensions.parse `chunk_extensions::parse`] to parse the extensions into a structured container for easier access. The implementation type-erases the callback without requiring a dynamic allocation. For this reason, the callback object is passed by a non-constant reference.

The function object will be called with this equivalent signature:
06/05/2026
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Component
Boost Beast Translation (zh_Hans)/Doc / Qbk / 04 Http / 08 Chunked Encoding (qbk)
Source string description
type: table cell
Flags
read-only
Unit identifier
57408
Source string location
string ID 158
String age
06/05/2026
Last updated
06/05/2026
Source string age
06/05/2026
Translation file QuickBook file
doc/qbk/04_http/08_chunked_encoding.qbk, string 35