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:
std::vector
std::map
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:
A JSON value can hold either a JSON array, JSON object, string, 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.
array
object
string
integer
double
bool
null
Value
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_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; 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.
Value::type()
std::runtime_error
The template getter function get_value() is an alternative to get_int(), get_real(), etc. Example usage would be:
get_value()
get_int()
get_real()
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.
stream
Array
Object
typedef std::vector< Pair > Object; typedef std::vector< Value > Array;
A Pair is a structure that holds a std::string and a Value.
Pair
std::string
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.
mObject
typedef std::map< std::string, mValue > mObject;
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.
begin
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.
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:
addr_obj
write
{ "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:
options
enum Output_options { pretty_print = 0x01, raw_utf8 = 0x02, remove_trailing_zeros = 0x04, single_line_arrays = 0x08 };
The pretty_print option causes white-space and line breaks to be added to the JSON text.
pretty_print
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.
raw_utf8
The always_escape_nonascii option escapes all non-ASCII characters, i.e. each such character is outputted as "\uXXXX" where XXXX is its hexadecimal value.
always_escape_nonascii
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".
remove_trailing_zeros
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:
single_line_arrays
{ "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 ] ] }
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_formatted
write( addr_obj, os, pretty_print );
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.
false
json_spirit::Error_position
The Error_position structure holds the line and column number where the first error was found.
Error_position
struct Error_position { ... unsigned int line_; unsigned int column_; std::string reason_; };
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.
std::wstring
wValue
wArray
wObject
wPair
Note that there is no support for reading Unicode files and converting them to wstrings as this is not a task specific to JSON.
wstring
The Value and wValue classes are actually instantiations of the template class Value_impl.
Value_impl
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.
mValue
mArray
wmValue
wmObject
wmArray
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.
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.
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.
Stream_reader
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.
boost.multi_pass
std::istream_iterator
There are two ways to use the library:
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.
json_spirit
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
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.
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]":
[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]":
[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 );
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
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]
c_ecscape_ch_p
const char*
[ 1 2 ]
[ 1, 2 ]
#define BOOST_SPIRIT_THREADSAFE
obj_to_map
map_to_obj
find_value
switch
spirit::multi_pass
CPack
std:runtime_error
<< std::dec
JSON_SPIRIT_VALUE_ENABLED
#define
