An overview of what you need to know to use simdjson, with examples.
DOM vs On-Demand
The simdjson library offers two distinct approaches on how to access a JSON document. We support a conventional Document-Object-Model (DOM) front-end. In such a scenario, the JSON document is entirely parsed, validated and materialized in memory as the first step. The programmer may then access the parsed data using this in-memory model.
The Basics: Loading and Parsing JSON Documents using the DOM front-end
The simdjson library offers a simple DOM tree API, which you can access by creating a dom::parser and calling the load() method:
{c++}
dom::parser parser;
dom::element doc = parser.load(filename); // load and parse a file
Or by creating a padded string (for efficiency reasons, simdjson requires a string with SIMDJSON_PADDING bytes at the end) and calling parse():
{c++}
dom::parser parser;
dom::element doc = parser.parse("[1,2,3]"_padded); // parse a string, the _padded suffix creates a simdjson::padded_string instance
You can copy your data directly on a simdjson::padded_string as follows:
{c++}
const char * data = "my data"; // 7 bytes
simdjson::padded_string my_padded_data(data, 7); // copies to a padded buffer
Or as follows...
{c++}
std::string data = "my data";
simdjson::padded_string my_padded_data(data); // copies to a padded buffer
You can then parse the JSON document from the simdjson::padded_string instance:
A persistent document parser.
simdjson_result< element > parse(const uint8_t *buf, size_t len, bool realloc_if_needed=true) &noexcept
Parse a JSON document and return a temporary reference to it.
Whenever you pass an std::string reference to parser::parse, the parser will access the bytes beyond the end of the string but before the end of the allocated memory (std::string::capacity()). If you are using a sanitizer that checks for reading uninitialized bytes or std::string's container-overflow checks, you may encounter sanitizer warnings. You can safely ignore these warnings. Or you can call simdjson::pad(std::string&) to pad the string with SIMDJSON_PADDING spaces: this function returns a simdjson::padding_string_view which can be be passed to the parser's iterator function:
{c++}
std::string json = "[1]";
dom::element doc = parser.parse(simdjson::pad(json));
The parsed document resulting from the parser.load and parser.parse calls depends on the parser instance. Thus the parser instance must remain in scope. Furthermore, you must have at most one parsed document in play per parser instance. You cannot copy a parser instance, you may only move it.
If you need to keep a document around long term, you can keep or move the parser instance. Note that moving a parser instance, or keeping one in a movable data structure like vector or map, can cause any outstanding element, object or array instances to be invalidated. The element, object or array instances are mere thin wrappers akin to an std::vector<int>::iterator: they are invalid when default constructed, they must be tied to a valid document instance. If you need to store a parser in a movable data structure, you should use a std::unique_ptr to avoid this invalidation(e.g., std::unique_ptr<dom::parser> parser(new dom::parser{})).
During theload or parse calls, neither the input file nor the input string are ever modified. After calling load or parse, the source (either a file or a string) can be safely discarded. All of the JSON data is stored in the parser instance. The parsed document is also immutable in simdjson: you do not modify it by accessing it.
For best performance, a parser instance should be reused over several files: otherwise you will needlessly reallocate memory, an expensive process. It is also possible to avoid entirely memory allocations during parsing when using simdjson. See our performance notes for details.
If you need a lower-level interface, you may call the function parser.parse(const char * p, size_t l) on a pointer p while specifying the length of your input l in bytes. To see how to get the very best performance from a low-level approach, you way want to read our performance notes on this topic (see the Padding and Temporary Copies section).
Windows-specific: Windows users who need to read files with non-ANSI characters in the name should set their code page to UTF-8 (65001). This should be the default with Windows 11 and better. Further, they may use the AreFileApisANSI function to determine whether the filename is interpreted using the ANSI or the system default OEM codepage, and they may call SetFileApisToOEM accordingly.
Using the Parsed JSON
Once you have an element, you can navigate it with idiomatic C++ iterators, operators and casts.
- Extracting Values (with exceptions): You can cast a JSON element to a native type:
double(element) or double x = json_element. This works for double, uint64_t, int64_t, bool, dom::object and dom::array. An exception (simdjson::simdjson_error) is thrown if the cast is not possible.
- Extracting Values (without exceptions): You can use a variant usage of
get() with error codes to avoid exceptions. You first declare the variable of the appropriate type (double, uint64_t, int64_t, bool, std::string_view, dom::object and dom::array) and pass it by reference to get() which gives you back an error code: e.g., {c++}
simdjson::error_code error;
simdjson::padded_string numberstring = "1.2"_padded; // our JSON input ("1.2")
simdjson::dom::parser parser;
double value; // variable where we store the value to be parsed
error = parser.parse(numberstring).get(value);
if (error) { std::cerr << error << std::endl; return EXIT_FAILURE; }
std::cout << "I parsed " << value << " from " << numberstring.data() << std::endl;
- Field Access: To get the value of the "foo" field in an object, use
object["foo"].
- Array Iteration: To iterate through an array, use
for (auto value : array) { ... }. If you know the type of the value, you can cast it right there, too! for (double value : array) { ... }
- Object Iteration: You can iterate through an object's fields, too:
for (auto [key, value] : object)
- Array Index: To get at an array value by index, use the at() method:
array.at(0) gets the first element. The at() method has linear-time complexity so it should not be used to iterate over the values of an array.
Note that array[0] does not compile, because implementing [] gives the impression indexing is a O(1) operation, which it is not presently in simdjson. Instead, you should iterate over the elements using a for-loop, as in our examples.
* Array and Object size Given an array or an object, you can get its size (number of elements or keys) with the size() method.
- Checking an Element Type: You can check an element's type with
element.type(). It returns an element_type with values such as simdjson::dom::element_type::ARRAY, simdjson::dom::element_type::OBJECT, simdjson::dom::element_type::INT64, simdjson::dom::element_type::UINT64,simdjson::dom::element_type::DOUBLE, simdjson::dom::element_type::STRING, simdjson::dom::element_type::BOOL or, simdjson::dom::element_type::NULL_VALUE.
- Output to streams and strings: Given a document or an element (or node) out of a JSON document, you can output a minified string version using the C++ stream idiom (
out << element). You can also request the construction of a minified string version (simdjson::minify(element)) or a prettified string version (simdjson::prettify(element)). Numbers are serialized as 64-bit floating-point numbers (double).
Examples
The following code illustrates all of the above:
{c++}
auto cars_json = R"( [
{ "make": "Toyota", "model": "Camry", "year": 2018, "tire_pressure": [ 40.1, 39.9, 37.7, 40.4 ] },
{ "make": "Kia", "model": "Soul", "year": 2012, "tire_pressure": [ 30.1, 31.0, 28.6, 28.7 ] },
{ "make": "Toyota", "model": "Tercel", "year": 1999, "tire_pressure": [ 29.8, 30.0, 30.2, 30.5 ] }
] )"_padded;
dom::parser parser;
// Iterating through an array of objects
for (dom::object car : parser.parse(cars_json)) {
// Accessing a field by name
cout << "Make/Model: " << car["make"] << "/" << car["model"] << endl;
// Casting a JSON element to an integer
uint64_t year = car["year"];
cout << "- This car is " << 2020 - year << "years old." << endl;
// Iterating through an array of floats
double total_tire_pressure = 0;
for (double tire_pressure : car["tire_pressure"]) {
total_tire_pressure += tire_pressure;
}
cout << "- Average tire pressure: " << (total_tire_pressure / 4) << endl;
// Writing out all the information about the car
for (auto field : car) {
cout << "- " << field.key << ": " << field.value << endl;
}
}
Here is a different example illustrating the same ideas:
{C++}
auto abstract_json = R"( [
{ "12345" : {"a":12.34, "b":56.78, "c": 9998877} },
{ "12545" : {"a":11.44, "b":12.78, "c": 11111111} }
] )"_padded;
dom::parser parser;
// Parse and iterate through an array of objects
for (dom::object obj : parser.parse(abstract_json)) {
for(const auto key_value : obj) {
cout << "key: " << key_value.key << " : ";
dom::object innerobj = key_value.value;
cout << "a: " << double(innerobj["a"]) << ", ";
cout << "b: " << double(innerobj["b"]) << ", ";
cout << "c: " << int64_t(innerobj["c"]) << endl;
}
}
And another one:
{C++}
auto abstract_json = R"(
{ "str" : { "123" : {"abc" : 3.14 } } } )"_padded;
dom::parser parser;
double v = parser.parse(abstract_json)["str"]["123"]["abc"];
cout << "number: " << v << endl;
C++17 Support
While the simdjson library can be used in any project using C++ 11 and above, field iteration has special support C++ 17's destructuring syntax. For example:
{c++}
padded_string json = R"( { "foo": 1, "bar": 2 } )"_padded;
dom::parser parser;
dom::object object; // invalid until the get() succeeds
auto error = parser.parse(json).get(object);
if (error) { cerr << error << endl; return; }
for (auto [key, value] : object) {
cout << key << " = " << value << endl;
}
For comparison, here is the C++ 11 version of the same code:
{c++}
// C++ 11 version for comparison
padded_string json = R"( { "foo": 1, "bar": 2 } )"_padded;
dom::parser parser;
dom::object object; // invalid until the get() succeeds
auto error = parser.parse(json).get(object);
if (error) { cerr << error << endl; return; }
for (dom::key_value_pair field : object) {
cout << field.key << " = " << field.value << endl;
}
JSON Pointer
The simdjson library also supports JSON pointer through the at_pointer() method, letting you reach further down into the document in a single call:
{c++}
auto cars_json = R"( [
{ "make": "Toyota", "model": "Camry", "year": 2018, "tire_pressure": [ 40.1, 39.9, 37.7, 40.4 ] },
{ "make": "Kia", "model": "Soul", "year": 2012, "tire_pressure": [ 30.1, 31.0, 28.6, 28.7 ] },
{ "make": "Toyota", "model": "Tercel", "year": 1999, "tire_pressure": [ 29.8, 30.0, 30.2, 30.5 ] }
] )"_padded;
dom::parser parser;
dom::element cars = parser.parse(cars_json);
cout << cars.at_pointer("/0/tire_pressure/1") << endl; // Prints 39.9
A JSON Pointer expression is a sequence of segments each starting with the '/' character. Within arrays, an integer index allows you to select the indexed node. Within objects, the string value of the key allows you to select the value. If your keys contain the characters '/' or '~', they must be escaped as '~1' and '~0' respectively. An empty JSON Pointer expression refers to the whole document.
We also extend the JSON Pointer support to include relative paths. You can apply a JSON Pointer expression to any node and the path gets interpreted relatively, as if the current node were a whole JSON document.
Consider the following example:
{c++}
auto cars_json = R"( [
{ "make": "Toyota", "model": "Camry", "year": 2018, "tire_pressure": [ 40.1, 39.9, 37.7, 40.4 ] },
{ "make": "Kia", "model": "Soul", "year": 2012, "tire_pressure": [ 30.1, 31.0, 28.6, 28.7 ] },
{ "make": "Toyota", "model": "Tercel", "year": 1999, "tire_pressure": [ 29.8, 30.0, 30.2, 30.5 ] }
] )"_padded;
dom::parser parser;
dom::element cars = parser.parse(cars_json);
cout << cars.at_pointer("/0/tire_pressure/1") << endl; // Prints 39.9
for (dom::element car_element : cars) {
dom::object car; // invalid until the get() succeeds
simdjson::error_code error;
if ((error = car_element.get(car))) { std::cerr << error << std::endl; return; }
double x = car.at_pointer("/tire_pressure/1");
cout << x << endl; // Prints 39.9, 31 and 30
}
JSONPath
The simdjson library supports a subset of JSONPath through the at_path() method, allowing you to reach further into the document in a single call. The subset of JSONPath that is implemented is the subset that is trivially convertible into the JSON Pointer format, using . to access a field and [] to access a specific index.
Consider the following example:
{c++}
auto cars_json = R"( [
{ "make": "Toyota", "model": "Camry", "year": 2018, "tire_pressure": [ 40.1, 39.9, 37.7, 40.4 ] },
{ "make": "Kia", "model": "Soul", "year": 2012, "tire_pressure": [ 30.1, 31.0, 28.6, 28.7 ] },
{ "make": "Toyota", "model": "Tercel", "year": 1999, "tire_pressure": [ 29.8, 30.0, 30.2, 30.5 ] }
] )"_padded;
dom::parser parser;
dom::element doc;
auto error = parser.parse(cars_json).get(doc);
if(error) { /*won't happen*/ }
double p;
error = doc.at_path("[0].tire_pressure[1]").get(p);
if(error) { /*won't happen*/ }
cout << p << endl; // Prints 39.9
We also support the $ prefix. When you start a JSONPath expression with $, you are indicating that the path starts from the root of the JSON document. E.g.,
{c++}
auto json = R"( { "c" :{ "foo": { "a": [ 10, 20, 30 ] }}, "d": { "foo2": { "a": [ 10, 20, 30 ] }} , "e": 120 })"_padded;
dom::parser parser;
dom::element doc;
auto error = parser.parse(json).get(doc);
if(error) { /*won't happen*/ }
dom::object obj;
error = doc.get_object().get(obj);
if(error) { /*won't happen*/ }
int64_t x;
error = obj.at_path("$[3].foo.a[1]").get(x);
if(error) { /*won't happen*/ }
if(x != 20) { /*won't happen*/ }
x = obj.at_path("$.d.foo2.a.2");
if(error) { /*won't happen*/ }
Error Handling
All simdjson APIs that can fail return simdjson_result<T>, which is a <value, error_code> pair. You can retrieve the value with .get(), like so:
{c++}
dom::element doc;
auto error = parser.parse(json).get(doc);
if (error) { cerr << error << endl; exit(1); }
When there is no error, the error code simdjson::SUCCESS is returned: it evaluates as false as a Boolean. We have several error codes to indicate errors, they all evaluate to true as a Boolean: your software should not generally not depend on exact error codes. We may change the error codes in future releases and the exact error codes could vary depending on your system.
When you use the code without exceptions, it is your responsibility to check for error before using the result: if there is an error, the result value will not be valid and using it will caused undefined behavior.
We can write a "quick start" example where we attempt to parse the following JSON file and access some data, without triggering exceptions:
{
"statuses": [
{
"id": 505874924095815700
},
{
"id": 505874922023837700
}
],
"search_metadata": {
"count": 100
}
}
Our program loads the file, selects value corresponding to key "search_metadata" which expected to be an object, and then it selects the key "count" within that object.
{C++}
#include <iostream>
#include "simdjson.h"
int main(void) {
simdjson::dom::parser parser;
simdjson::dom::element tweets; // invalid until the get() succeeds
auto error = parser.load("twitter.json").get(tweets);
if (error) { std::cerr << error << std::endl; return EXIT_FAILURE; }
simdjson::dom::element res; // invalid until the get() succeeds
if ((error = tweets["search_metadata"]["count"].get(res))) {
std::cerr << "could not access keys" << std::endl;
return EXIT_FAILURE;
}
std::cout << res << " results." << std::endl;
}
The following is a similar example where one wants to get the id of the first tweet without triggering exceptions. To do this, we use ["statuses"].at(0)["id"]. We break that expression down:
- Get the list of tweets (the
"statuses" key of the document) using ["statuses"]). The result is expected to be an array.
- Get the first tweet using
.at(0). The result is expected to be an object.
- Get the id of the tweet using ["id"]. We expect the value to be a non-negative integer.
Observe how we use the at method when querying an index into an array, and not the bracket operator.
{C++}
#include <iostream>
#include "simdjson.h"
int main(void) {
simdjson::dom::parser parser;
simdjson::dom::element tweets; // invalid until the get() succeeds
auto error = parser.load("twitter.json").get(tweets);
if (error) { std::cerr << error << std::endl; return EXIT_FAILURE; }
uint64_t identifier;
error = tweets["statuses"].at(0)["id"].get(identifier);
if (error) { std::cerr << error << std::endl; return EXIT_FAILURE; }
std::cout << identifier << std::endl;
return EXIT_SUCCESS;
}
The at() method has linear-time complexity: it should not be used to iterate over the content of an array.
Error Handling Example
This is how the example in "Using the Parsed JSON" could be written using only error code checking:
{c++}
auto cars_json = R"( [
{ "make": "Toyota", "model": "Camry", "year": 2018, "tire_pressure": [ 40.1, 39.9, 37.7, 40.4 ] },
{ "make": "Kia", "model": "Soul", "year": 2012, "tire_pressure": [ 30.1, 31.0, 28.6, 28.7 ] },
{ "make": "Toyota", "model": "Tercel", "year": 1999, "tire_pressure": [ 29.8, 30.0, 30.2, 30.5 ] }
] )"_padded;
dom::parser parser;
dom::array cars; // invalid until the get() succeeds
auto error = parser.parse(cars_json).get(cars);
if (error) { cerr << error << endl; exit(1); }
// Iterating through an array of objects
for (dom::element car_element : cars) {
dom::object car;
if ((error = car_element.get(car))) { cerr << error << endl; exit(1); }
// Accessing a field by name
std::string_view make, model;
if ((error = car["make"].get(make))) { cerr << error << endl; exit(1); }
if ((error = car["model"].get(model))) { cerr << error << endl; exit(1); }
cout << "Make/Model: " << make << "/" << model << endl;
// Casting a JSON element to an integer
uint64_t year;
if ((error = car["year"].get(year))) { cerr << error << endl; exit(1); }
cout << "- This car is " << 2020 - year << "years old." << endl;
// Iterating through an array of floats
double total_tire_pressure = 0;
dom::array tire_pressure_array;
if ((error = car["tire_pressure"].get(tire_pressure_array))) { cerr << error << endl; exit(1); }
for (dom::element tire_pressure_element : tire_pressure_array) {
double tire_pressure;
if ((error = tire_pressure_element.get(tire_pressure))) { cerr << error << endl; exit(1); }
total_tire_pressure += tire_pressure;
}
cout << "- Average tire pressure: " << (total_tire_pressure / 4) << endl;
// Writing out all the information about the car
for (auto field : car) {
cout << "- " << field.key << ": " << field.value << endl;
}
}
Here is another example:
{C++}
auto abstract_json = R"( [
{ "12345" : {"a":12.34, "b":56.78, "c": 9998877} },
{ "12545" : {"a":11.44, "b":12.78, "c": 11111111} }
] )"_padded;
dom::parser parser;
dom::array array; // invalid until after the next line
auto error = parser.parse(abstract_json).get(array);
if (error) { cerr << error << endl; exit(1); }
// Iterate through an array of objects
for (dom::element elem : array) {
dom::object obj;
if ((error = elem.get(obj))) { cerr << error << endl; exit(1); }
for (auto & key_value : obj) {
cout << "key: " << key_value.key << " : ";
dom::object innerobj;
if ((error = key_value.value.get(innerobj))) { cerr << error << endl; exit(1); }
double va, vb;
if ((error = innerobj["a"].get(va))) { cerr << error << endl; exit(1); }
cout << "a: " << va << ", ";
if ((error = innerobj["b"].get(vc))) { cerr << error << endl; exit(1); }
cout << "b: " << vb << ", ";
int64_t vc;
if ((error = innerobj["c"].get(vc))) { cerr << error << endl; exit(1); }
cout << "c: " << vc << endl;
}
}
And another one:
{C++}
auto abstract_json = R"(
{ "str" : { "123" : {"abc" : 3.14 } } } )"_padded;
dom::parser parser;
double v;
auto error = parser.parse(abstract_json)["str"]["123"]["abc"].get(v);
if (error) { cerr << error << endl; exit(1); }
cout << "number: " << v << endl;
Notice how we can string several operations (parser.parse(abstract_json)["str"]["123"]["abc"].get(v)) and only check for the error once, a strategy we call error chaining.
The next two functions will take as input a JSON document containing an array with a single element, either a string or a number. They return true upon success.
{C++}
simdjson::dom::parser parser{};
bool parse_double(const char *j, double &d) {
auto error = parser.parse(j, std::strlen(j))
.at(0)
.get(d, error);
if (error) { return false; }
return true;
}
bool parse_string(const char *j, std::string &s) {
std::string_view answer;
auto error = parser.parse(j, strlen(j))
.at(0)
.get(answer, error);
if (error) { return false; }
s.assign(answer.data(), answer.size());
return true;
}
To ensure you don't write any code that uses exceptions, compile with SIMDJSON_EXCEPTIONS=OFF. For example, if including the project via cmake:
target_compile_definitions(simdjson PUBLIC SIMDJSON_EXCEPTIONS=OFF)
Exceptions
Users more comfortable with an exception flow may choose to directly cast the simdjson_result<T> to the desired type:
{c++}
dom::element doc = parser.parse(json); // Throws an exception if there was an error!
When used this way, a simdjson_error exception will be thrown if an error occurs, preventing the program from continuing if there was an error.
If one is willing to trigger exceptions, it is possible to write simpler code:
{C++}
#include <iostream>
#include "simdjson.h"
int main(void) {
simdjson::dom::parser parser;
simdjson::dom::element tweets = parser.load("twitter.json");
std::cout << "ID: " << tweets["statuses"].at(0)["id"] << std::endl;
return EXIT_SUCCESS;
}
Tree Walking and JSON Element Types
Sometimes you don't necessarily have a document with a known type, and are trying to generically inspect or walk over JSON elements. To do that, you can use iterators and the type() method. For example, here's a quick and dirty recursive function that verbosely prints the JSON document as JSON (* ignoring nuances like trailing commas and escaping strings, for brevity's sake):
{c++}
void print_json(dom::element element) {
switch (element.type()) {
case dom::element_type::ARRAY:
cout << "[";
for (dom::element child : dom::array(element)) {
print_json(child);
cout << ",";
}
cout << "]";
break;
case dom::element_type::OBJECT:
cout << "{";
for (dom::key_value_pair field : dom::object(element)) {
cout << "\"" << field.key << "\": ";
print_json(field.value);
}
cout << "}";
break;
case dom::element_type::INT64:
cout << int64_t(element) << endl;
break;
case dom::element_type::UINT64:
cout << uint64_t(element) << endl;
break;
case dom::element_type::DOUBLE:
cout << double(element) << endl;
break;
case dom::element_type::STRING:
cout << std::string_view(element) << endl;
break;
case dom::element_type::BOOL:
cout << bool(element) << endl;
break;
case dom::element_type::NULL_VALUE:
cout << "null" << endl;
break;
}
}
void basics_treewalk_1() {
dom::parser parser;
print_json(parser.load("twitter.json"));
}
Reusing the parser for maximum efficiency
If you're using simdjson to parse multiple documents, or in a loop, you should make a parser once and reuse it. The simdjson library will allocate and retain internal buffers between parses, keeping buffers hot in cache and keeping memory allocation and initialization to a minimum. In this manner, you can parse terabytes of JSON data without doing any new allocation.
{c++}
dom::parser parser;
// This initializes buffers and a document big enough to handle this JSON.
dom::element doc = parser.parse("[ true, false ]"_padded);
cout << doc << endl;
// This reuses the existing buffers, and reuses and *overwrites* the old document
doc = parser.parse("[1, 2, 3]"_padded);
cout << doc << endl;
// This also reuses the existing buffers, and reuses and *overwrites* the old document
dom::element doc2 = parser.parse("true"_padded);
// Even if you keep the old reference around, doc and doc2 refer to the same document.
cout << doc << endl;
cout << doc2 << endl;
It's not just internal buffers though. The simdjson library reuses the document itself. The dom::element, dom::object and dom::array instances are references to the internal document. You are only borrowing the document from simdjson, which purposely reuses and overwrites it each time you call parse. This prevent wasteful and unnecessary memory allocation in 99% of cases where JSON is just read, used, and converted to native values or thrown away.
You are only borrowing the document from the simdjson parser. Don't keep it long term!
This is key: don't keep the document&, dom::element, dom::array, dom::object or string_view objects you get back from the API. Convert them to C++ native values, structs and arrays that you own.
Server Loops: Long-Running Processes and Memory Capacity
The simdjson library automatically expands its memory capacity when larger documents are parsed, so that you don't unexpectedly fail. In a short process that reads a bunch of files and then exits, this works pretty flawlessly.
Server loops, though, are long-running processes that will keep the parser around forever. This means that if you encounter a really, really large document, simdjson will not resize back down. The simdjson library lets you adjust your allocation strategy to prevent your server from growing without bound:
- You can set a max capacity when constructing a parser:
{c++}
dom::parser parser(1000*1000); // Never grow past documents > 1MB
for (web_request request : listen()) {
dom::element doc;
auto error = parser.parse(request.body).get(doc);
// If the document was above our limit, emit 413 = payload too large
if (error == CAPACITY) { request.respond(413); continue; }
// ...
}
This parser will grow normally as it encounters larger documents, but will never pass 1MB.
- You can set a fixed capacity that never grows, as well, which can be excellent for predictability and reliability, since simdjson will never call malloc after startup!
{c++}
dom::parser parser(0); // This parser will refuse to automatically grow capacity
auto error = parser.allocate(1000*1000); // This allocates enough capacity to handle documents <= 1MB
if (error) { cerr << error << endl; exit(1); }
for (web_request request : listen()) {
dom::element doc;
error = parser.parse(request.body).get(doc);
// If the document was above our limit, emit 413 = payload too large
if (error == CAPACITY) { request.respond(413); continue; }
// ...
}
Best Use of the DOM API
The simdjson API provides access to the JSON DOM (document-object-model) content as a tree of dom::element instances, each representing an object, an array or an atomic type (null, true, false, number). These dom::element instances are lightweight objects (e.g., spanning 16 bytes) and it might be advantageous to pass them by value, as opposed to passing them by reference or by pointer.
Padding and Temporary Copies
The simdjson function parser.parse reads data from a padded buffer, containing SIMDJSON_PADDING extra bytes added at the end. If you are passing a padded_string to parser.parse or loading the JSON directly from disk (parser.load), padding is automatically handled. When calling parser.parse on a pointer (e.g., parser.parse(my_char_pointer, my_length_in_bytes)) a temporary copy is made by default with adequate padding and you, again, do not need to be concerned with padding.
Some users may not be able use our padded_string class or to load the data directly from disk (parser.load). They may need to pass data pointers to the library. If these users wish to avoid temporary copies and corresponding temporary memory allocations, they may want to call parser.parse with the realloc_if_needed parameter set to false (e.g., parser.parse(my_char_pointer, my_length_in_bytes, false)). In such cases, they need to ensure that there are at least SIMDJSON_PADDING extra bytes at the end that can be safely accessed and read. They do not need to initialize the padded bytes to any value in particular. The following example is safe:
```C++ const char *json = R"({"key":"value"})"; const size_t json_len = std::strlen(json); std::unique_ptr<char[]> padded_json_copy{new char[json_len + SIMDJSON_PADDING]}; memcpy(padded_json_copy.get(), json, json_len); memset(padded_json_copy.get() + json_len, 0, SIMDJSON_PADDING); simdjson::dom::parser parser; simdjson::dom::element element = parser.parse(padded_json_copy.get(), json_len, false); ````
Setting the realloc_if_needed parameter false in this manner may lead to better performance since copies are avoided, but it requires that the user takes more responsibilities: the simdjson library cannot verify that the input buffer was padded with SIMDJSON_PADDING extra bytes.
Performance Tips
- For release builds, we recommend setting
NDEBUG pre-processor directive when compiling the simdjson library. Importantly, using the optimization flags -O2 or -O3 under GCC and LLVM clang does not set the NDEBUG directrive, you must set it manually (e.g., -DNDEBUG).
- For long streams of JSON documents, consider `iterate_many` and `parse_many` for better performance.
1.9.1