An overview of what you need to know to use simdjson, with examples.
Requirements
------------------
- A recent compiler (LLVM clang6 or better, GNU GCC 7 or better) on a 64-bit (ARM or x64 Intel/AMD) POSIX systems such as macOS, freeBSD or Linux. We require that the compiler supports the C++11 standard or better.
- Visual Studio 2017 or better under 64-bit Windows. Users should target a 64-bit build (x64) instead of a 32-bit build (x86). We support the LLVM clang compiler under Visual Studio (clangcl) as well as as the regular Visual Studio compiler.
Including simdjson
------------------
To include simdjson, copy the simdjson.h and simdjson.cpp files from the singleheader directory
into your project. Then include the header file in your project with:
- Users on macOS and other platforms were default compilers do not provide C++11 compliant by default should request it with the appropriate flag (e.g., `c++ -std=c++17 myproject.cpp simdjson.cpp`).
- Visual Studio users should compile with the `_CRT_SECURE_NO_WARNINGS` flag to avoid warnings with respect to our use of standard C functions such as `fopen`.
You can install the simdjson library on your system or in your project using multiple package managers such as MSYS2, the conan package manager, vcpkg, brew, the apt package manager (debian-based Linux systems), the FreeBSD package manager (FreeBSD), and so on. [Visit our wiki for more details](https://github.com/simdjson/simdjson/wiki/Installing-simdjson-with-a-package-manager).
You should replace `GIT_TAG v0.5.0` by the version you need. If you omit `GIT_TAG v0.5.0`, you will work from the main branch of simdjson: we recommend that if you are working on production code,
See [our CMake demonstration](https://github.com/simdjson/cmake_demo_single_file). It works under Linux, FreeBSD, macOS and Windows (including Visual Studio).
The CMake build in simdjson can be taylored with a few variables. You can see the available variables and their default values by entering the `cmake -LA` command.
The simdjson library offers a simple DOM tree API, which you can access by creating a
`dom::parser` and calling the `load()` method:
```
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()`:
```
dom::parser parser;
dom::element doc = parser.parse("[1,2,3]"_padded); // parse a string
```
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.
During the`load` 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.
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 is thrown if the cast is not possible.
* **Extracting Values (without expceptions):** 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`,
`dom::object` and `dom::array`) and pass it by reference to `get()` which gives you back an error code: e.g.,
* **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`).Youcanalsorequesttheconstructionofaminifiedstringversion(`simdjson::minify(element)`).
double v = parser.parse(abstract_json)["str"]["123"]["abc"];
cout << "number: " <<v<<endl;
```
C++11 Support and string_view
-------------
The simdjson library builds on compilers supporting the [C++11 standard](https://en.wikipedia.org/wiki/C%2B%2B11). It is also a strict requirement: we have no plan to support older C++ compilers.
We represent parsed strings in simdjson using the `std::string_view` class. It avoids
the need to copy the data, as would be necessary with the `std::string` class. It also
avoids the pitfalls of null-terminated C strings.
The `std::string_view` class has become standard as part of C++17 but it is not always available
on compilers which only supports C++11. When we detect that `string_view` is natively
available, we define the macro `SIMDJSON_HAS_STRING_VIEW`.
When we detect that it is unavailable,
we use [string-view-lite](https://github.com/martinmoene/string-view-lite) as a
substitute. In such cases, we use the type alias `using string_view = nonstd::string_view;` to
offer the same API, irrespective of the compiler and standard library. The macro
`SIMDJSON_HAS_STRING_VIEW` will be *undefined* to indicate that we emulate `string_view`.
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:
In some cases, you may have valid JSON strings that you do not wish to parse but that you wish to minify. That is, you wish to remove all unnecessary spaces. We have a fast function for this purpose (`simdjson::minify(const char * input, size_t length, const char * output, size_t& new_length)`). This function does not validate your content, and it does not parse it. It is much faster than parsing the string and re-serializing it in minified form (`simdjson::minify(parser.parse())`). Usage is relatively simple. You must pass an input pointer with a length parameter, as well as an output pointer and an output length parameter (by reference). The output length parameter is not read, but written to. The output pointer should point to a valid memory region that is as large as the original string length. The input pointer and input length are read, but not written to.
size_t new_length{}; // It will receive the minified length.
auto error = simdjson::minify(some_string, length, buffer.get(), new_length);
// The buffer variable now has "[1,2,3,4]" and new_length has value 9.
```
Though it does not validate the JSON input, it will detect when the document ends with an unterminated string. E.g., it would refuse to minify the string `"this string is not terminated` because of the missing final quote.
UTF-8 validation (alone)
----------------------
The simdjson library has fast functions to validate UTF-8 strings. They are many times faster than most functions commonly found in libraries. You can use our fast functions, even if you do not care about JSON.
The UTF-8 validation function merely checks that the input is valid UTF-8: it works with strings in general, not just JSON strings.
Your input string does not need any padding. Any string will do. The `validate_utf8` function does not do any memory allocation on the heap, and it does not throw exceptions.
JSON Pointer
------------
The simdjson library also supports [JSON pointer](https://tools.ietf.org/html/rfc6901) through the
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.
Unlike `parser.parse`, both `parser.load_many(filename)` and `parser.parse_many(string)` may parse
"on demand" (lazily). That is, no parsing may have been done before you enter the loop
`for (dom::element doc : docs) {` and you should expect the parser to only ever fully parse one JSON
document at a time.
1. When calling `parser.load_many(filename)`, the file's content is loaded up in a memory buffer owned by the `parser`'s instance. Thus the file can be safely deleted after calling `parser.load_many(filename)` as the parser instance owns all of the data.
2. When calling `parser.parse_many(string)`, no copy is made of the provided string input. The provided memory buffer may be accessed each time a JSON document is parsed. Calling `parser.parse_many(string)` on a temporary string buffer (e.g., `docs = parser.parse_many("[1,2,3]"_padded)`) is unsafe (and will not compile) because the `document_stream` instance needs access to the buffer to return the JSON documents. In constrast, calling `doc = parser.parse("[1,2,3]"_padded)` is safe because `parser.parse` eagerly parses the input.
Both `load_many` and `parse_many` take an optional parameter `size_t batch_size` which defines the window processing size. It is set by default to a large value (`1000000` corresponding to 1 MB). None of your JSON documents should exceed this window size, or else you will get the error `simdjson::CAPACITY`. You cannot set this window size larger than 4 GB: you will get the error `simdjson::CAPACITY`. The smaller the window size is, the less memory the function will use. Setting the window size too small (e.g., less than 100 kB) may also impact performance negatively. Leaving it to 1 MB is expected to be a good choice, unless you have some larger documents.
Thread Safety
-------------
We built simdjson with thread safety in mind.
The simdjson library is single-threaded except for `parse_many` which may use secondary threads under its control when the library is compiled with thread support.
We recommend using one `dom::parser` object per thread in which case the library is thread-safe.
It is unsafe to reuse a `dom::parser` object between different threads.
The parsed results (`dom::document`, `dom::element`, `array`, `object`) depend on the `dom::parser`, etc. therefore it is also potentially unsafe to use the result of the parsing between different threads.
The CPU detection, which runs the first time parsing is attempted and switches to the fastest
parser for your CPU, is transparent and thread-safe.
Backwards Compatibility
-----------------------
The only header file supported by simdjson is `simdjson.h`. Older versions of simdjson published a
number of other include files such as `document.h` or `ParsedJson.h` alongside `simdjson.h`; these headers
