nlohmann::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,
const bool ignore_trailing_commas = 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,
const bool ignore_trailing_commas = false);
- Deserialize from a compatible input.
-
Deserialize from a pair of character iterators
The
value_typeof 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.
Template parameters¶
InputType-
A compatible input, for instance:
- an
std::istreamobject - a
FILEpointer (throws if null) - a C-style array of characters
- a pointer to a null-terminated string of single byte characters (throws if null)
- a
std::string - an object
objfor whichbegin(obj)andend(obj)produces a valid pair of iterators.
- an
IteratorType-
a compatible iterator type, for instance.
- a pair of
std::string::iteratororstd::vector<std::uint8_t>::iterator - a pair of pointers such as
ptrandptr + len
- a pair of
Parameters¶
i(in)- Input to parse from.
cb(in)- a parser callback function of type
parser_callback_twhich 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,
trueby default) ignore_comments(in)- whether comments should be ignored and treated like whitespace (
true) or yield a parse error (false); (optional,falseby default) ignore_trailing_commas(in)- whether trailing commas in arrays or objects should be ignored and treated like whitespace (
true) or yield a parse error (false); (optional,falseby default) first(in)- iterator to the start of a character range
last(in)- iterator to the end of a 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.
Exceptions¶
- Throws
parse_error.101in case of an unexpected token, or empty input like a nullFILE*orchar*pointer. - Throws
parse_error.102ifto_unicodefails or surrogate error. - Throws
parse_error.103ifto_unicodefails.
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¶
A UTF-8 byte order mark is silently ignored.
Examples¶
Parsing from a character 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<std::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
]
Parsing from a non-null-terminated string
The example below demonstrates the parse() function reading from a string that is not null-terminated.
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// a JSON text given as string that is not null-terminated
const char* ptr = "[1,2,3]another value";
// parse and serialize JSON
json j_complete = json::parse(ptr, ptr + 7);
std::cout << std::setw(4) << j_complete << "\n\n";
}
Output:
[
1,
2,
3
]
Parsing from an iterator pair
The example below demonstrates the parse() function reading from an iterator pair.
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// a JSON text given an input with other values
std::vector<std::uint8_t> input = {'[', '1', ',', '2', ',', '3', ']', 'o', 't', 'h', 'e', 'r'};
// parse and serialize JSON
json j_complete = json::parse(input.begin(), input.begin() + 7);
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 (const 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
Effect of ignore_comments parameter
The example below demonstrates the effect of the ignore_comments parameter in the parse() function.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
std::string s = R"(
{
// update in 2006: removed Pluto
"planets": ["Mercury", "Venus", "Earth", "Mars",
"Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
}
)";
try
{
json j = json::parse(s);
}
catch (json::exception& e)
{
std::cout << e.what() << std::endl;
}
json j = json::parse(s,
/* callback */ nullptr,
/* allow exceptions */ true,
/* ignore_comments */ true);
std::cout << j.dump(2) << '\n';
}
Output:
[json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A> {<U+000A> /'; expected string literal
{
"planets": [
"Mercury",
"Venus",
"Earth",
"Mars",
"Jupiter",
"Uranus",
"Neptune"
]
}
Effect of ignore_trailing_commas parameter
The example below demonstrates the effect of the ignore_trailing_commas parameter in the parse() function.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
std::string s = R"(
{
"planets": [
"Mercury",
"Venus",
"Earth",
"Mars",
"Jupiter",
"Uranus",
"Neptune",
]
}
)";
try
{
json j = json::parse(s);
}
catch (json::exception& e)
{
std::cout << e.what() << std::endl;
}
json j = json::parse(s,
/* callback */ nullptr,
/* allow exceptions */ true,
/* ignore_comments */ false,
/* ignore_trailing_commas */ true);
std::cout << j.dump(2) << '\n';
}
Output:
[json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
{
"planets": [
"Mercury",
"Venus",
"Earth",
"Mars",
"Jupiter",
"Uranus",
"Neptune"
]
}
See also¶
- accept - check if the input is valid JSON
- operator>> - deserialize from stream
Version history¶
- Added in version 1.0.0.
- Overload for contiguous containers (1) added in version 2.0.3.
- Ignoring comments via
ignore_commentsadded in version 3.9.0. - Changed runtime assertion in case of
FILE*null pointers to exception in version 3.12.0. - Added
ignore_trailing_commasin version 3.12.1.
Deprecation
Overload (2) replaces calls to 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 parse({ptr, ptr+len}, ...); with parse(ptr, ptr+len, ...);.
You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.