basic_json::parse¶
// (1)
template<typename InputType>
static basic_json parse(InputType&& i,
const parser_callback_t cb = nullptr,
const bool allow_exceptions = true,
const bool ignore_comments = false);
// (2)
template<typename IteratorType>
static basic_json parse(IteratorType first, IteratorType last,
const parser_callback_t cb = nullptr,
const bool allow_exceptions = true,
const bool ignore_comments = false);
- Deserialize from a compatible input.
-
Deserialize from a pair of character iterators
The value_type of the iterator must be a integral type with size of 1, 2 or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16 and UTF-32.
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
- a compatible iterator type
Parameters¶
i
(in)- Input to parse from.
cb
(in)- a parser callback function of type
parser_callback_t
which is used to control the deserialization by filtering unwanted values (optional) allow_exceptions
(in)- whether to throw exceptions in case of a parse error (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¶
Deserialized JSON value; in case of a parse error and allow_exceptions
set to false
, the return value will be value_t::discarded
. The latter can be checked with is_discarded
.
Exception safety¶
Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
Complexity¶
Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the parser callback function cb
or reading from (1) the input i
or (2) the iterator range [first
, last
] has a super-linear complexity.
Notes¶
(1) A UTF-8 byte order mark is silently ignored.
Examples¶
Parsing from a charater array
The example below demonstrates the parse()
function reading from an array.
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// a JSON text
char 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]
}
}
)";
// parse and serialize JSON
json j_complete = json::parse(text);
std::cout << std::setw(4) << j_complete << "\n\n";
}
Output:
{
"Image": {
"Animated": false,
"Height": 600,
"IDs": [
116,
943,
234,
38793
],
"Thumbnail": {
"Height": 125,
"Url": "http://www.example.com/image/481989943",
"Width": 100
},
"Title": "View from 15th Floor",
"Width": 800
}
}
Parsing from a string
The example below demonstrates the parse()
function with and without callback function.
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
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]
}
}
)";
// parse and serialize JSON
json j_complete = json::parse(text);
std::cout << std::setw(4) << j_complete << "\n\n";
// define parser callback
json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
{
// skip object elements with key "Thumbnail"
if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
{
return false;
}
else
{
return true;
}
};
// parse (with callback) and serialize JSON
json j_filtered = json::parse(text, cb);
std::cout << std::setw(4) << j_filtered << '\n';
}
Output:
{
"Image": {
"Animated": false,
"Height": 600,
"IDs": [
116,
943,
234,
38793
],
"Thumbnail": {
"Height": 125,
"Url": "http://www.example.com/image/481989943",
"Width": 100
},
"Title": "View from 15th Floor",
"Width": 800
}
}
{
"Image": {
"Animated": false,
"Height": 600,
"IDs": [
116,
943,
234,
38793
],
"Title": "View from 15th Floor",
"Width": 800
}
}
Parsing from an input stream
The example below demonstrates the parse()
function with and without callback function.
#include <iostream>
#include <iomanip>
#include <sstream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
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]
}
}
)";
// fill a stream with JSON text
std::stringstream ss;
ss << text;
// parse and serialize JSON
json j_complete = json::parse(ss);
std::cout << std::setw(4) << j_complete << "\n\n";
// define parser callback
json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
{
// skip object elements with key "Thumbnail"
if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
{
return false;
}
else
{
return true;
}
};
// fill a stream with JSON text
ss.clear();
ss << text;
// parse (with callback) and serialize JSON
json j_filtered = json::parse(ss, cb);
std::cout << std::setw(4) << j_filtered << '\n';
}
Output:
{
"Image": {
"Animated": false,
"Height": 600,
"IDs": [
116,
943,
234,
38793
],
"Thumbnail": {
"Height": 125,
"Url": "http://www.example.com/image/481989943",
"Width": 100
},
"Title": "View from 15th Floor",
"Width": 800
}
}
{
"Image": {
"Animated": false,
"Height": 600,
"IDs": [
116,
943,
234,
38793
],
"Title": "View from 15th Floor",
"Width": 800
}
}
Parsing from a contiguous container
The example below demonstrates the parse()
function reading from a contiguous container.
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// a JSON text given as std::vector
std::vector<uint8_t> text = {'[', '1', ',', '2', ',', '3', ']', '\0'};
// parse and serialize JSON
json j_complete = json::parse(text);
std::cout << std::setw(4) << j_complete << "\n\n";
}
Output:
[
1,
2,
3
]
Effect of allow_exceptions
parameter
The example below demonstrates the effect of the allow_exceptions
parameter in the ´parse()` function.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// an invalid JSON text
std::string text = R"(
{
"key": "value without closing quotes
}
)";
// parse with exceptions
try
{
json j = json::parse(text);
}
catch (json::parse_error& e)
{
std::cout << e.what() << std::endl;
}
// parse without exceptions
json j = json::parse(text, nullptr, false);
if (j.is_discarded())
{
std::cout << "the input is invalid JSON" << std::endl;
}
else
{
std::cout << "the input is valid JSON: " << j << std::endl;
}
}
Output:
[json.exception.parse_error.101] parse error at line 4, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \u000A or \n; last read: '"value without closing quotes<U+000A>'
the input is invalid JSON
Version history¶
- Added in version 1.0.0.
- Overload for contiguous containers (1) added in version 2.0.3.
- Ignoring comments via
ignore_comments
added in version 3.9.0.