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nlohmann
@@ -0,0 +1,624 @@
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# nlohmann::basic_json::parse
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```
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// (1)
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template<typename InputType>
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static basic_json parse(InputType&& i,
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const parser_callback_t cb = nullptr,
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const bool allow_exceptions = true,
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const bool ignore_comments = false,
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const bool ignore_trailing_commas = false);
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// (2)
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template<typename IteratorType>
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static basic_json parse(IteratorType first, IteratorType last,
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const parser_callback_t cb = nullptr,
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const bool allow_exceptions = true,
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const bool ignore_comments = false,
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const bool ignore_trailing_commas = false);
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```
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1. Deserialize from a compatible input.
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1. Deserialize from a pair of character iterators
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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.
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## Template parameters
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`InputType` : A compatible input, for instance:
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```
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- an `std::istream` object
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- a `FILE` pointer (throws if null)
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- a C-style array of characters
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- a pointer to a null-terminated string of single byte characters (throws if null)
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- a `std::string`
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- an object `obj` for which `begin(obj)` and `end(obj)` produces a valid pair of iterators.
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```
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`IteratorType` : a compatible iterator type, for instance.
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```
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- a pair of `std::string::iterator` or `std::vector<std::uint8_t>::iterator`
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- a pair of pointers such as `ptr` and `ptr + len`
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```
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## Parameters
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`i` (in) : Input to parse from.
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`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)
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`allow_exceptions` (in) : whether to throw exceptions in case of a parse error (optional, `true` by default)
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`ignore_comments` (in) : whether comments should be ignored and treated like whitespace (`true`) or yield a parse error (`false`); (optional, `false` by default)
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`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)
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`first` (in) : iterator to the start of a character range
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`last` (in) : iterator to the end of a character range
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## Return value
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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).
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## Exception safety
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Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
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## Exceptions
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- 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.
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## Complexity
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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.
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## Notes
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A UTF-8 byte order mark is silently ignored.
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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.
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## Examples
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Parsing from a character array
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The example below demonstrates the `parse()` function reading from an array.
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```
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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||||
// a JSON text
|
||||
char text[] = R"(
|
||||
{
|
||||
"Image": {
|
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"Width": 800,
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"Height": 600,
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"Title": "View from 15th Floor",
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"Thumbnail": {
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"Url": "http://www.example.com/image/481989943",
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"Height": 125,
|
||||
"Width": 100
|
||||
},
|
||||
"Animated" : false,
|
||||
"IDs": [116, 943, 234, 38793]
|
||||
}
|
||||
}
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)";
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||||
|
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// parse and serialize JSON
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json j_complete = json::parse(text);
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std::cout << std::setw(4) << j_complete << "\n\n";
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}
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```
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Output:
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||||
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||||
```
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{
|
||||
"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
|
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}
|
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}
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```
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Parsing from a string
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The example below demonstrates the `parse()` function with and without callback function.
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```
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||||
#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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|
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int main()
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{
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// a JSON text
|
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auto text = R"(
|
||||
{
|
||||
"Image": {
|
||||
"Width": 800,
|
||||
"Height": 600,
|
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"Title": "View from 15th Floor",
|
||||
"Thumbnail": {
|
||||
"Url": "http://www.example.com/image/481989943",
|
||||
"Height": 125,
|
||||
"Width": 100
|
||||
},
|
||||
"Animated" : false,
|
||||
"IDs": [116, 943, 234, 38793]
|
||||
}
|
||||
}
|
||||
)";
|
||||
|
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// parse and serialize JSON
|
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json j_complete = json::parse(text);
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std::cout << std::setw(4) << j_complete << "\n\n";
|
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// define parser callback
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json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
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{
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// skip object elements with key "Thumbnail"
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if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
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||||
{
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return false;
|
||||
}
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||||
else
|
||||
{
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return true;
|
||||
}
|
||||
};
|
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// parse (with callback) and serialize JSON
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json j_filtered = json::parse(text, cb);
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std::cout << std::setw(4) << j_filtered << '\n';
|
||||
}
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||||
```
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||||
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||||
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
|
||||
}
|
||||
}
|
||||
```
|
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|
||||
Parsing from an input stream
|
||||
|
||||
The example below demonstrates the `parse()` function with and without callback function.
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||||
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||||
```
|
||||
#include <iostream>
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#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]
|
||||
}
|
||||
}
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)";
|
||||
|
||||
// fill a stream with JSON text
|
||||
std::stringstream ss;
|
||||
ss << text;
|
||||
|
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// parse and serialize JSON
|
||||
json j_complete = json::parse(ss);
|
||||
std::cout << std::setw(4) << j_complete << "\n\n";
|
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|
||||
// define parser callback
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json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)
|
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{
|
||||
// skip object elements with key "Thumbnail"
|
||||
if (event == json::parse_event_t::key and parsed == json("Thumbnail"))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
else
|
||||
{
|
||||
return true;
|
||||
}
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};
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||||
|
||||
// fill a stream with JSON text
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ss.clear();
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||||
ss << text;
|
||||
|
||||
// parse (with callback) and serialize JSON
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json j_filtered = json::parse(ss, cb);
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std::cout << std::setw(4) << j_filtered << '\n';
|
||||
}
|
||||
```
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||||
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||||
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
|
||||
}
|
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}
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```
|
||||
|
||||
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.
|
||||
Reference in New Issue
Block a user