png85/json_spirit

A C++ JSON Parser/Generator Implemented with Boost Spirit

Introduction

JSON is a text file format similar to XML, but less verbose. It has been called "XML lite". This article describes JSON Spirit, a C++ library that reads and writes JSON files or streams. It is written using the Boost Spirit parser generator. If you are already using Boost, you can use JSON Spirit without any additional dependencies.

Key features:

• supports ASCII or Unicode
• std::vector or std::map implementations for JSON Objects
• object library or header file only use

Using the Code

The JSON Spirit source code is available as a Microsoft Visual Studio 2005 C++ "solution". However, it should compile and work on any platform compatible with Boost. JSON Spirit has been built and tested with Visual C++ 2005, 2008, 2010, and G++ version 4.2.3 on Linux. It has been tested with Visual C++ using Boost versions 1.34.0, 1.37.0, 1.39.0, and 1.42.0. It has also been tested with STLPort.

Platform independent CMake files are included, kindly supplied by Uwe Arzt, with CPack declarations for packaging and a "make install" target supplied by Amzlkej.

The Visual C++ solution consists of five projects:

• The JSON Spirit library and header files
• An application running the library's unit tests
• Three small programs demonstrating how to use JSON Spirit

JSON Spirit Value

A JSON value can hold either a JSON array, JSON object, string, unsigned integer, integer, double, bool, or null. The interface of the JSON Spirit Value class is shown below. The Value class for Unicode is analogous; for details, see the section on Unicode support.

enum Value_type{ obj_type, array_type, str_type,
     bool_type, int_type, real_type, null_type };

class Value
{
    public:

        Value();  // creates null value

        Value( const char*          value );
        Value( const std::string&   value );
        Value( const Object&        value );
        Value( const Array&         value );
        Value( bool                 value );
        Value( int                  value );
        Value( unsigned int         value );
        Value_impl( boost::int64_t  value );
        Value_impl( boost::uint64_t value );
        Value( double               value );

        bool operator==( const Value& lhs ) const;

        Value_type type() const;

        const std::string& get_str()    const;
        const Object&      get_obj()    const;
        const Array&       get_array()  const;
        bool               get_bool()   const;
        int                get_int()    const;
        unsigned int       get_uint()   const;
        boost::int64_t     get_int64()  const;
        boost::uint64_t    get_uint64() const;
        double             get_real()   const;

        Object& get_obj();
        Array&  get_array();

        template< typename > T get_value() const;

        bool is_uint64() const;
        bool is_null() const;

        static const Value null;

    private:

        //...
};

You obtain the Value's type by calling Value::type(). You can then call the appropriate getter function. Generally, you will know a file's format, so you will know what type the JSON values should have. A std::runtime_error exception is thrown if you try to get a value of the wrong type, for example, if you try to extract a string from a value containing an integer.

The template getter function get_value() is an alternative to get_int(), get_real(), etc. Example usage would be:

int    i = value_1.get_value< int >();
double d = value_2.get_value< double >();

A top level Value read from a file or stream normally contains an Array or an Object. An Array is a std::vector of values. An Object is, by default, a std::vector of JSON pairs.

typedef std::vector< Pair > Object;
typedef std::vector< Value > Array;

A Pair is a structure that holds a std::string and a Value.

struct Pair
{
    Pair( const std::string& name, const Value& value );

    bool operator==( const Pair& lhs ) const;

    std::string name_;
    Value value_;
};

JSON Arrays and Objects can themselves contain other Arrays or Objects, forming a tree.

Note, JSON Spirit provides an alternative std::map based Object; see the Map Implementation section below. In this case, a Pair type is not needed as an mObject is a std::map of names to values.

typedef std::map< std::string, mValue > mObject;

Reading JSON

You can read JSON data from a stream or a string:

bool read( const std::string &s, Value& value );
bool read( std::istream &is,     Value& value );

For example:

ifstream is( "json.txt" );
Value value;
read( is, value );

You can also read JSON data by supplying a pair of string iterators.

bool read( std::string::const_iterator &begin,
           std::string::const_iterator  end, Value& value );

After a successful read, the iterator "begin" will point one past the last character of the text for the value just read. This allows the decoding of a string containing multiple top level values. A subsequent call to read will read the next value in the string.

Similarly the stream reading functions now allow a sequence of top-level values to be read one at a time. Before version 3.0, a stream was converted to a string before being parsed. This was fine for files, but not if, for example, you wanted to read multiple JSON values from a socket.

Writing JSON

To output JSON, you first create a Value object containing your data, then write the created Value to a stream or string. For example, the following code will create a small JSON file containing an object with three members:

Object addr_obj;

addr_obj.push_back( Pair( "house_number", 42 ) );
addr_obj.push_back( Pair( "road",         "East Street" ) );
addr_obj.push_back( Pair( "town",         "Newtown" ) );

ofstream os( "address.txt" );

write( addr_obj, os, pretty_print );

os.close();

The object addr_obj is automatically converted into a Value as it is passed to the write function. The file address.txt will contain:

{
    "house_number" : 42,
    "road" : "East Street",
    "town" : "Newtown"
}

There are two versions of each function, one to write to strings, the other to write to files:

std::string write( const Value&  value, unsigned int options = 0 );
void write( const Value& value, std::ostream&  os, unsigned int options = 0 );

The options parameter controls how the data is written

enum Output_options
{ 
    pretty_print = 0x01, 
    raw_utf8 = 0x02, 
    remove_trailing_zeros = 0x04,
    single_line_arrays = 0x08,
    always_escape_nonascii = 0x10,
    no_quote_strings = 0x20
};

The pretty_print option causes white-space and line breaks to be added to the JSON text.

The raw_utf8 option is an extension to the JSON standard. It disables the escaping of non-printable characters, allowing UTF-8 sequences held in 8 bit char strings to pass through unaltered.

The remove_trailing_zeros option removes trailing zeros when outputting double precision floating point numbers; e.g., without the option, 1.2 will be written as "1.200000000000000"; with the option, it will be written as "1.2".

The single_line_arrays option has the same effect as the pretty printing option except that arrays printed on single lines unless they contain composite elements, i.e. objects or arrays. This produced output like:

{
    "1" : [ 0, 0, 0, 0, 0 ],
    "2" : [
        [ 0, 0, 0, 0, 0 ],
        [ 0, 1, 2, 3, 4 ],
        [ 0, 2, 4, 6, 8 ],
        [ 0, 3, 6, 9, 12 ],
        [ 0, 4, 8, 12, 16 ]
    ]
}

The always_escape_nonascii option escapes all unicode wide characters, i.e. outputed as "\uXXXX" even if they are printable under the current locale, ascii printable chars are not escaped.

You can apply more than one option by using the OR operator, i.e.:

write( addr_obj, os, pretty_print | raw_utf8 );

The default is to write the JSON text without any white-space and to escape non-printable characters. There is also a function called write_formatted. This is equivalent to:

write( addr_obj, os, pretty_print );

Error Detection

From version 3.00, JSON Spirit provides functions that report the position of format errors in the text being parsed. These functions are identical to the normal read functions except that instead of returning false if an error is found, they throw a json_spirit::Error_position exception. Note, these functions run about three times slower than the normal read functions.

The Error_position structure holds the line and column number where the first error was found.

struct Error_position
{
    ...
    unsigned int line_;
    unsigned int column_;
    std::string reason_;
};

Unicode Support

Unicode support is provided by std::wstring versions of the JSON Spirit Value, Array, Object, and Pair types. These are called wValue, wArray, wObject, and wPair. There are also std::wstring versions of each reader and writer function.

To enable Unicode support set the JSON_SPIRIT_WVALUE_ENABLED and/or JSON_SPIRIT_WMVALUE_ENABLED cmake options.

Note that there is no support for reading Unicode files and converting them to wstrings as this is not a task specific to JSON.

The Value and wValue classes are actually instantiations of the template class Value_impl.

Std::map Implementation

Before version 4.00, the JSON Spirit Object type was a std::vector of name/value Pairs. You now have the option of using mObject which is a name/value std::map. For the std::map version, use mValue instead of Value, mObject instead of Object, and mArray instead of Array. For the Unicode map version, use wmValue, wmObject, and wmArray.

To enable map support set the JSON_SPIRIT_MVALUE_ENABLED and optionally set JSON_SPIRIT_WMVALUE_ENABLED cmake options.

The following table shows the time in seconds it takes on my PC to extract the data from a single object of varying sizes. The methods used are as per the demo programs. The vector version is faster until the number of object members reach around 10, but then gets exponentially slower.

size	vector	map
2	2.03253e-007	3.24672e-007
5	7.63788e-007	9.59516e-007
10	2.26948e-006	2.00929e-006
15	4.68965e-006	3.28509e-006
20	8.19667e-006	4.75871e-006
30	1.72695e-005	7.64189e-006
50	4.63171e-005	1.39316e-005
75	0.000102418	2.27205e-005
100	0.000179923	3.17732e-005
200	0.000709335	7.3061e-005
500	0.00440368	0.00022076
1000	0.0175385	0.000495554
10000	1.76533	0.00658268
100000	178.718	0.127325
1000000	17800.718	1.99467

Note: with a vector, object members will be written out in the same order they were read in. A map will sort members alphabetically. A vector object also allows members to have duplicate names. This might be useful in some circumstances but would be non-standard.

Stream_reader Class

There is a Stream_reader class to work around a bug that prevents multiple top level values that are not separated by white space from being read from a stream. The standard stream reading functions would fail in this case.

istringstream is( "[1][1,2][1,2,3]" );  // no white space separating arrays

Stream_reader< istringstream, Value > reader( is );

Value value;

const bool ok = reader.read_next( value ); // read first array
reader.read_next( value );                 // read second array
reader.read_next( value );                 // read third array

I originally thought that this was caused by a bug in the boost.multi_pass iterator. However, I now realise that it basically results from the intrinsic behaviour of the std::istream_iterator constructor in immediately reading a character from its stream.

Compiling and linking

There are two ways to use the library:

• Build and link in the JSON Spirit an object library.
• Just include the relevant header files, i.e., "header only" use.

Linking to the object library has the advantage that the heavily templated code only has to be compiled once. Compiling the code can take some time. If you just include the header file implementation, the code will be compiled each time it is included. The use of precompiled headers should help, but I have not investigated this.

The include files needed to read JSON text using the object library are json_spirit_reader.h and json_spirit_value.h. To generate JSON text, you need json_spirit_writer.h and json_spirit_value.h. Alternately, you can include json_spirit.h, which includes all three of the above.

The include files needed when using the header only implementation are json_spirit_reader_template.h and json_spirit_writer_template.h.

All JSON Spirit declarations are in the namespace json_spirit

Reducing Build Times

As already described, you can choose between four different types of JSON value classes: Value, mValue, wValue and wmValue. You would normally chose a single type and stick with it, however the compiler will process the templates to generate sets of classes for all four types. This results in longer than necessary build times and larger intermediate files. You can disable value types you don't want by commenting out the relevant lines in json_spirit_value.h below:

#define JSON_SPIRIT_VALUE_ENABLED
#define JSON_SPIRIT_WVALUE_ENABLED
#define JSON_SPIRIT_MVALUE_ENABLED
#define JSON_SPIRIT_WMVALUE_ENABLED

Using JSON Spirit with Multiple Threads

If you intend to use JSON Spirit in more than one thread, you will need to uncomment the following line near the top of json_spirit_reader.cpp.

//#define BOOST_SPIRIT_THREADSAFE

In this case, Boost Spirit will require you to link against Boost Threads.

Advanced

Container Constructor

An advanced feature introduced in version 4.05 is a constructor in the Value classes for containers. A container is passed in as an iterator range. This allows you to create an Array from a container such as an std::vector. For example, the following code creates and outputs the array [1,2,3]:

std::vector< int > int_vect;

int_vect.push_back( 1 );
int_vect.push_back( 2 );
int_vect.push_back( 3 );

const Value val( int_vect.begin(), int_vect.end() );

cout << write( val );

Any compatible container type can be used. The following code creates and outputs the array [1.0,2.0,3.0]:

std::list< float > float_list;

double_vect.push_back( 1.0 );
double_vect.push_back( 2.0 );
double_vect.push_back( 3.0 );

const Value val( double_vect.begin(), double_vect.end() );

cout << write( val, remove_trailing_zeros );

Variant Constructor

Another advanced feature introduced in version 4.05 is a constructor in the Value classes for boost::variant objects. Any compatible variant type can be used. For example:

boost::variant< int, string, std::vector< bool > > var;

var = 1;
Value val( var );
cout << write( val ) << endl;

var = string( "hello" );
val = var;
cout << write( val ) << endl;

std::vector< bool > bool_vect;
bool_vect.push_back( true );
bool_vect.push_back( false );
bool_vect.push_back( true );
var = bool_vect;
val = var;
cout << write( val ) << endl;

Output:

1
"hello"
[true,false,true]