C++ Boost JSON: Parsing

Parsing is the process where a serialized JSON is validated and decomposed into elements. The library provides some functions:

Name	Description
basic_parser	A SAX push parser implementation which converts a serialized JSON into a series of member function calls to a user provided handler. This allows custom behaviors to be implemented for representing the document in memory.
parse	Parse a string containing a complete serialized JSON, and return a value.

and types to assist with parsing:

Name	Description
parser	A stateful DOM parser object which may be used to efficiently parse a series of JSONs each contained in a single contiguous character buffer, returning each result as a value.
stream_parser	A stateful DOM parser object which may be used to efficiently parse a series of JSONs incrementally, returning each result as a value.
parse_options	A structure used to select which extensions are enabled during parsing.
value_stack	A low level building block used for efficiently building a value. The parsers use this internally, and users may use it to adapt foreign parsers to produce this library's containers.

The parse function offers a simple interface for converting a serialized JSON text to a value in a single function call. This overload uses exceptions to indicate errors:

value jv = parse( "[1,2,3,4,5]" );

With `error_code`*

Alternatively, an error_code can be used:

error_code ec;
value jv = parse( "[1,2,3,4,5]", ec );

if( ec )
    std::cout << "Parsing failed: " << ec.message() << "\n";

Even when using error codes, exceptions thrown from the underlying memory_resource are still possible:

try
{
    error_code ec;
    value jv = parse( "[1,2,3,4,5]", ec );
    if( ec )
        std::cout << "Parsing failed: " << ec.message() << "\n";
}
catch( std::bad_alloc const& e)
{
    std::cout << "Parsing failed: " << e.what() << "\n";
}

Using Memory Resource `memory_resource`

The value returned in the preceding examples use the default memory resource. The following code uses a monotonic_resource, which results in faster parsing. jv is marked const to prevent subsequent modification, because containers using a monotonic resource waste memory when mutated.

{
    monotonic_resource mr;

    value const jv = parse( "[1,2,3,4,5]", &mr );
}

Non-Standard JSON with `parse_options`

Unless otherwise specified, the parser in this library is strict. It recognizes only valid, standard JSON. The parser can be configured to allow certain non-standard extensions by filling in a parse_options structure and passing it by value. By default all extensions are disabled:

parse_options opt;                                  // all extensions default to off
opt.allow_comments = true;                          // permit C and C++ style comments to appear in whitespace
opt.allow_trailing_commas = true;                   // allow an additional trailing comma in object and array element lists
opt.allow_invalid_utf8 = true;                      // skip utf-8 validation of keys and strings

value jv = parse( "[1,2,3,] // comment ", storage_ptr(), opt );

When building with C++20 or later, the use of designated initializers with parse_options is possible:

value jv = parse( "[1,2,3,] // comment ", storage_ptr(),
    {
        .allow_comments = true,             // permit C and C++ style comments to appear in whitespace
        .allow_trailing_commas = true,      // allow a trailing comma in object and array lists
        .allow_invalid_utf8 = true          // skip utf-8 validation of keys and strings
    });

`parser` Object*

Instances of parser and stream_parser offer functionality beyond what is available when using the parse free functions:

More control over memory
Streaming API, parse input JSON incrementally
Improved performance when parsing multiple JSONs
Ignore non-JSON content after the end of a JSON

The parser implementation uses temporary storage space to accumulate values during parsing. When using the parse free functions, this storage is allocated and freed in each call. However, by declaring an instance of parser or stream_parser, this temporary storage can be reused when parsing more than one JSON, reducing the total number of dynamic memory allocations.

To use the parser, declare an instance. Then call write once with the buffer containing representing the input JSON. Finally, call release to take ownership of the resulting value upon success. This example persists the parser instance in a class member to reuse across calls:

class connection
{
    parser p_;                                      // persistent data member

public:
    void do_read( string_view s )                   // called for each complete message from the network
    {
        p_.reset();                                 // start parsing a new JSON using the default resource
        p_.write( s );                              // parse the buffer, using exceptions to indicate error
        do_rpc( p_.release() );                     // process the command
    }

    void do_rpc( value jv );
};

Sometimes a protocol may have a JSON text followed by data that is in a different format or specification. The JSON portion can still be parsed by using the function write_some. Upon success, the return value will indicate the number of characters consumed from the input, which will exclude the non-JSON characters:

stream_parser p;
error_code ec;
string_view s = "[1,2,3] %HOME%";
std::size_t n = p.write_some( s, ec );
assert( ! ec && p.done() && n == 8 );
s = s.substr( n );
value jv = p.release();
assert( s == "%HOME%" );

`stream_parser` Object*

The stream_parser implements a streaming algorithm; it allows incremental processing of large JSON inputs using one or more contiguous character buffers. The entire input JSON does not need to be loaded into memory at once. A network server can use the streaming interface to process incoming JSON in fixed-size amounts, providing these benefits:

CPU consumption per I/O cycle is bounded
Memory consumption per I/O cycle is bounded
Jitter, unfairness, and latency is reduced
Less total memory is required to process the full input

To use the stream_parser, declare an instance. Then call write zero or more times with successive buffers representing the input JSON. When there are no more buffers, call finish. The function done returns true after a successful call to write or finish if parsing is complete.

In the following example a JSON [object] is parsed from standard input a line at a time. Error codes are used instead. The function finish is used to indicate the end of the input:

value read_json( std::istream& is, error_code& ec )
{
    stream_parser p;
    std::string line;
    while( std::getline( is, line ) )
    {
        p.write( line, ec );
        if( ec )
            return nullptr;
    }
    p.finish( ec );
    if( ec )
        return nullptr;
    return p.release();
}

The stream_parser instance may be constructed with parse options which allow some non-standard JSON extensions to be recognized:

parse_options opt;                                  // All extensions default to off
opt.allow_comments = true;                          // Permit C and C++ style comments to appear in whitespace
opt.allow_trailing_commas = true;                   // Allow an additional trailing comma in object and array element lists
opt.allow_invalid_utf8 = true;                      // Skip utf-8 validation of keys and strings
stream_parser p( storage_ptr(), opt );              // The stream_parser will use the options

Controlling Memory

After default construction, or after reset is called with no arguments, the value produced after a successful parse operation uses the default memory resource. To use a different memory resource, call reset with the resource to use. Here we use a monotonic_resource, which is optimized for parsing but not subsequent modification:

{
    monotonic_resource mr;

    stream_parser p;
    p.reset( &mr );                                 // Use mr for the resulting value
    p.write( "[1,2,3,4,5]" );                       // Parse the input JSON
    value const jv = p.release();                   // Retrieve the result
    assert( *jv.storage() == mr );                  // Same memory resource
}

To achieve performance and memory efficiency, the parser uses a temporary storage area to hold intermediate results. This storage is reused when parsing more than one JSON, reducing the total number of calls to allocate memory and thus improving performance. Upon construction, the memory resource used to perform allocations for this temporary storage area may be specified. Otherwise, the default memory resource is used. In addition to a memory resource, the parser can make use of a caller-owned buffer for temporary storage. This can help avoid dynamic allocations for small inputs. The following example uses a four kilobyte temporary buffer for the parser, and falls back to the default memory resource if needed:

unsigned char temp[ 4096 ];                                 // Declare our buffer
stream_parser p(
    storage_ptr(),                                          // Default memory resource
    parse_options{},                                        // Default parse options (strict parsing)
    temp);                                                  // Use our buffer for temporary storage

Avoiding Dynamic Allocations*

Through careful specification of buffers and memory resources, it is possible to eliminate all dynamic allocation completely when parsing JSON, for the case where the entire JSON is available in a single character buffer, as shown here:

/*  Parse JSON and invoke the handler

    This function parses the JSON specified in `s`
    and invokes the handler, whose signature must
    be equivalent to:

        void( value const& jv );

    The operation is guaranteed not to perform any
    dynamic memory allocations. However, some
    implementation-defined upper limits on the size
    of the input JSON and the size of the resulting
    value are imposed.

    Upon error, an exception is thrown.
*/
template < class Handler >
void do_rpc( string_view s, Handler&& handler )
{
    unsigned char temp[ 4096 ];                 // The parser will use this storage for its temporary needs
    parser p(                                   // Construct a strict parser using the temp buffer and no dynamic memory
        get_null_resource(),                    // The null resource never dynamically allocates memory
        parse_options(),                        // Default constructed parse options allow only standard JSON
        temp );

    unsigned char buf[ 16384 ];                 // Now we need a buffer to hold the actual JSON values
    static_resource mr2( buf );                 // The static resource is monotonic, using only a caller-provided buffer
    p.reset( &mr2 );                            // Use the static resource for producing the value
    p.write( s );                               // Parse the entire string we received from the network client

    // Retrieve the value and invoke the handler with it.
    // The value will use `buf` for storage. The handler
    // must not take ownership, since monotonic resources
    // are inefficient with mutation.
    handler( p.release() );
}

C++ Boost JSON: Parsing

With error_code*

Using Memory Resource memory_resource

Non-Standard JSON with parse_options

parser Object*