nlohmann::basic_json::sax_parse¶
// (1)
template <typename InputType, typename SAX>
static bool sax_parse(InputType&& i,
SAX* sax,
input_format_t format = input_format_t::json,
const bool strict = true,
const bool ignore_comments = false);
// (2)
template<class IteratorType, class SAX>
static bool sax_parse(IteratorType first, IteratorType last,
SAX* sax,
input_format_t format = input_format_t::json,
const bool strict = true,
const bool ignore_comments = false);
Read from input and generate SAX events
- Read from a compatible input.
-
Read from a pair of character iterators
The value_type of the iterator must be an integral type with size of 1, 2 or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16 and UTF-32.
The SAX event lister must follow the interface of json_sax
.
Template parameters¶
InputType
-
A compatible input, for instance:
- an
std::istream
object - a
FILE
pointer - a C-style array of characters
- a pointer to a null-terminated string of single byte characters
- an object
obj
for whichbegin(obj)
andend(obj)
produces a valid pair of iterators.
- an
IteratorType
- Description
SAX
- Description
Parameters¶
i
(in)- Input to parse from.
sax
(in)- SAX event listener
format
(in)- the format to parse (JSON, CBOR, MessagePack, or UBJSON) (optional,
input_format_t::json
by default), seeinput_format_t
for more information strict
(in)- whether the input has to be consumed completely (optional,
true
by default) ignore_comments
(in)- whether comments should be ignored and treated like whitespace (
true
) or yield a parse error (false
); (optional,false
by default) first
(in)- iterator to start of character range
last
(in)- iterator to end of character range
Return value¶
return value of the last processed SAX event
Exception safety¶
Complexity¶
Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the SAX consumer sax
has a super-linear complexity.
Notes¶
A UTF-8 byte order mark is silently ignored.
Examples¶
Example
The example below demonstrates the sax_parse()
function reading from string and processing the events with a user-defined SAX event consumer.
#include <iostream>
#include <iomanip>
#include <sstream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
// a simple event consumer that collects string representations of the passed
// values; not inheriting from json::json_sax_t is not required, but can
// help not to forget a required function
class sax_event_consumer : public json::json_sax_t
{
public:
std::vector<std::string> events;
bool null() override
{
events.push_back("value: null");
return true;
}
bool boolean(bool val) override
{
events.push_back("value: " + std::string(val ? "true" : "false"));
return true;
}
bool number_integer(number_integer_t val) override
{
events.push_back("value: " + std::to_string(val));
return true;
}
bool number_unsigned(number_unsigned_t val) override
{
events.push_back("value: " + std::to_string(val));
return true;
}
bool number_float(number_float_t val, const string_t& s) override
{
events.push_back("value: " + s);
return true;
}
bool string(string_t& val) override
{
events.push_back("value: " + val);
return true;
}
bool start_object(std::size_t elements) override
{
events.push_back("start: object");
return true;
}
bool end_object() override
{
events.push_back("end: object");
return true;
}
bool start_array(std::size_t elements) override
{
events.push_back("start: array");
return true;
}
bool end_array() override
{
events.push_back("end: array");
return true;
}
bool key(string_t& val) override
{
events.push_back("key: " + val);
return true;
}
bool binary(json::binary_t& val) override
{
events.push_back("binary");
return true;
}
bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override
{
events.push_back("error: " + std::string(ex.what()));
return false;
}
};
int main()
{
// a JSON text
auto text = R"(
{
"Image": {
"Width": 800,
"Height": 600,
"Title": "View from 15th Floor",
"Thumbnail": {
"Url": "http://www.example.com/image/481989943",
"Height": 125,
"Width": 100
},
"Animated" : false,
"IDs": [116, 943, 234, 38793],
"Distance": 12.723374634
}
}
)";
// create a SAX event consumer object
sax_event_consumer sec;
// parse and serialize JSON
bool result = json::sax_parse(text, &sec);
// output the recorded events
for (auto& event : sec.events)
{
std::cout << "(" << event << ") ";
}
// output the result of sax_parse
std::cout << "\nresult: " << std::boolalpha << result << std::endl;
}
Output:
(start: object) (key: Image) (start: object) (key: Width) (value: 800) (key: Height) (value: 600) (key: Title) (value: View from 15th Floor) (key: Thumbnail) (start: object) (key: Url) (value: http://www.example.com/image/481989943) (key: Height) (value: 125) (key: Width) (value: 100) (end: object) (key: Animated) (value: false) (key: IDs) (start: array) (value: 116) (value: 943) (value: 234) (value: 38793) (end: array) (key: Distance) (value: 12.723374634) (end: object) (end: object)
result: true
Version history¶
- Added in version 3.2.0.
- Ignoring comments via
ignore_comments
added in version 3.9.0.
Deprecation
Overload (2) replaces calls to sax_parse
with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like sax_parse({ptr, ptr+len});
with sax_parse(ptr, ptr+len);
.