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# 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);
```
1. Deserialize from a compatible input.
1. Deserialize 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.
## Template parameters
`InputType` : A compatible input, for instance:
```
- an `std::istream` object
- a `FILE` pointer (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 `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
```
`IteratorType` : a compatible iterator type, for instance.
```
- a pair of `std::string::iterator` or `std::vector<std::uint8_t>::iterator`
- a pair of pointers such as `ptr` and `ptr + len`
```
## Parameters
`i` (in) : Input to parse from.
`cb` (in) : a parser callback function of type [`parser_callback_t`](https://json.nlohmann.me/api/basic_json/parser_callback_t/index.md) 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)
`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, `false` by 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`](https://json.nlohmann.me/api/basic_json/is_discarded/index.md).
## Exception safety
Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
## Exceptions
- Throws [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) in case of an unexpected token, or empty input like a null `FILE*` or `char*` pointer.
## 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.
Invalid Unicode escapes and unpaired surrogates in the input are reported as [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) with a detailed message.
## 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](https://json.nlohmann.me/api/basic_json/accept/index.md) - check if the input is valid JSON
- [sax_parse](https://json.nlohmann.me/api/basic_json/sax_parse/index.md) - parse input using the SAX interface
- [operator>>](https://json.nlohmann.me/api/operator_gtgt/index.md) - 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_comments` added in version 3.9.0.
- Changed [runtime assertion](https://json.nlohmann.me/features/assertions/index.md) in case of `FILE*` null pointers to exception in version 3.12.0.
- Added `ignore_trailing_commas` in version 3.12.x.
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.
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