Glaze

One of the fastest JSON libraries in the world. Glaze reads and writes from object memory, simplifying interfaces and offering incredible performance.

Glaze also supports:

• BEVE (binary efficient versatile encoding)
• CSV (comma separated value)

With compile time reflection for MSVC, Clang, and GCC!

• Read/write aggregate initializable structs without writing any metadata or macros!
• See example on Compiler Explorer

Highlights

• Pure, compile time reflection for structs

  • Powerful meta specialization system for custom names and behavior

• JSON RFC 8259 compliance with UTF-8 validation

• Standard C++ library support

• Header only

• Direct to memory serialization/deserialization

• Compile time maps with constant time lookups and perfect hashing

• Powerful wrappers to modify read/write behavior (Wrappers)

• Use your own custom read/write functions (Custom Read/Write)

• Handle unknown keys in a fast and flexible manner

• Direct memory access through JSON pointer syntax

• Binary data through the same API for maximum performance

• No exceptions (compiles with -fno-exceptions)

  • If you desire helpers that throw for cleaner syntax see Glaze Exceptions

• No runtime type information necessary (compiles with -fno-rtti)

• Rapid error handling with short circuiting

• JSON-RPC 2.0 support

• JSON Schema generation

• Extremely portable, uses carefully optimized SWAR (SIMD Within A Register) for broad compatibility

• Partial Read and Partial Write support

• CSV Reading/Writing

• Much more!

See DOCS for more documentation.

Performance

Library	Roundtrip Time (s)	Write (MB/s)	Read (MB/s)
Glaze	1.04	1366	1224
simdjson (on demand)	N/A	N/A	1198
yyjson	1.23	1005	1107
daw_json_link	2.93	365	553
RapidJSON	3.65	290	450
Boost.JSON (direct)	4.76	199	447
json_struct	5.50	182	326
nlohmann	15.71	84	80

Performance test code available here

Performance caveats: simdjson and yyjson are great, but they experience major performance losses when the data is not in the expected sequence or any keys are missing (the problem grows as the file size increases, as they must re-iterate through the document).

Also, simdjson and yyjson do not support automatic escaped string handling, so if any of the currently non-escaped strings in this benchmark were to contain an escape, the escapes would not be handled.

ABC Test shows how simdjson has poor performance when keys are not in the expected sequence:

Library	Read (MB/s)
Glaze	678
simdjson (on demand)	93

Binary Performance

Tagged binary specification: BEVE

Metric	Roundtrip Time (s)	Write (MB/s)	Read (MB/s)
Raw performance	0.42	3235	2468
Equivalent JSON data*	0.42	3547	2706

JSON size: 670 bytes

BEVE size: 611 bytes

*BEVE packs more efficiently than JSON, so transporting the same data is even faster.

Example

Your struct will automatically get reflected! No metadata is required by the user.

struct my_struct
{
  int i = 287;
  double d = 3.14;
  std::string hello = "Hello World";
  std::array<uint64_t, 3> arr = { 1, 2, 3 };
  std::map<std::string, int> map{{"one", 1}, {"two", 2}};
};

JSON (prettified)

{
   "i": 287,
   "d": 3.14,
   "hello": "Hello World",
   "arr": [
      1,
      2,
      3
   ],
   "map": {
      "one": 1,
      "two": 2
   }
}

Write JSON

my_struct s{};
std::string buffer = glz::write_json(s).value_or("error");

or

my_struct s{};
std::string buffer{};
auto ec = glz::write_json(s, buffer);
if (ec) {
  // handle error
}

Read JSON

std::string buffer = R"({"i":287,"d":3.14,"hello":"Hello World","arr":[1,2,3],"map":{"one":1,"two":2}})";
auto s = glz::read_json<my_struct>(buffer);
if (s) // check std::expected
{
  s.value(); // s.value() is a my_struct populated from buffer
}

or

std::string buffer = R"({"i":287,"d":3.14,"hello":"Hello World","arr":[1,2,3],"map":{"one":1,"two":2}})";
my_struct s{};
auto ec = glz::read_json(s, buffer); // populates s from buffer
if (ec) {
  // handle error
}

Read/Write From File

auto ec = glz::read_file_json(obj, "./obj.json", std::string{});
auto ec = glz::write_file_json(obj, "./obj.json", std::string{});

Important

The file name (2nd argument), must be null terminated.

Compiler/System Support

• Requires C++23
• Tested for both 64bit and 32bit
• Only supports little-endian systems

Actions build and test with Clang (17+), MSVC (2022), and GCC (12+) on apple, windows, and linux.

Glaze seeks to maintain compatibility with the latest three versions of GCC and Clang, as well as the latest version of MSVC and Apple Clang.

MSVC Compiler Flags

Glaze requires a C++ standard conformant pre-processor, which requires the /Zc:preprocessor flag when building with MSVC.

SIMD CMake Options

The CMake has the option glaze_ENABLE_AVX2. This will attempt to use AVX2 SIMD instructions in some cases to improve performance, as long as the system you are configuring on supports it. Set this option to OFF to disable the AVX2 instruction set, such as if you are cross-compiling for Arm. If you aren't using CMake the macro GLZ_USE_AVX2 enables the feature if defined.

How To Use Glaze

FetchContent

include(FetchContent)

FetchContent_Declare(
  glaze
  GIT_REPOSITORY https://github.com/stephenberry/glaze.git
  GIT_TAG main
  GIT_SHALLOW TRUE
)

FetchContent_MakeAvailable(glaze)

target_link_libraries(${PROJECT_NAME} PRIVATE glaze::glaze)

Conan

• Included in Conan Center

find_package(glaze REQUIRED)

target_link_libraries(main PRIVATE glaze::glaze)

build2

• Available on cppget

import libs = libglaze%lib{glaze}

Arch Linux

• AUR packages: glaze and glaze-git

See this Example Repository for how to use Glaze in a new project

---

See FAQ for Frequently Asked Questions

Explicit Metadata

If you want to specialize your reflection then you can optionally write the code below:

This metadata is also necessary for non-aggregate initializable structs.

template <>
struct glz::meta<my_struct> {
   using T = my_struct;
   static constexpr auto value = object(
      &T::i,
      &T::d,
      &T::hello,
      &T::arr,
      &T::map
   );
};

Local Glaze Meta

Glaze also supports metadata within its associated class:

struct my_struct
{
  int i = 287;
  double d = 3.14;
  std::string hello = "Hello World";
  std::array<uint64_t, 3> arr = { 1, 2, 3 };
  std::map<std::string, int> map{{"one", 1}, {"two", 2}};
  
  struct glaze {
     using T = my_struct;
     static constexpr auto value = glz::object(
        &T::i,
        &T::d,
        &T::hello,
        &T::arr,
        &T::map
     );
  };
};

Custom Key Names or Unnamed Types

When you define Glaze metadata, objects will automatically reflect the non-static names of your member object pointers. However, if you want custom names or you register lambda functions or wrappers that do not provide names for your fields, you can optionally add field names in your metadata.

Example of custom names:

template <>
struct glz::meta<my_struct> {
   using T = my_struct;
   static constexpr auto value = object(
      "integer", &T::i,
      "double", &T::d,
      "string", &T::hello,
      "array", &T::arr,
      "my map", &T::map
   );
};

Each of these strings is optional and can be removed for individual fields if you want the name to be reflected.

Names are required for:

• static constexpr member variables
• Wrappers
• Lambda functions

Reflection API

Glaze provides a compile time reflection API that can be modified via glz::meta specializations. This reflection API uses pure reflection unless a glz::meta specialization is provided, in which case the default behavior is overridden by the developer.

static_assert(glz::reflect<my_struct>::size == 5); // Number of fields
static_assert(glz::reflect<my_struct>::keys[0] == "i"); // Access keys

Warning

The glz::reflect fields described above have been formalized and are unlikely to change. Other fields within the glz::reflect struct may evolve as we continue to formalize the spec. Therefore, breaking changes may occur for undocumented fields in the future.

Custom Read/Write

Custom reading and writing can be achieved through the powerful to/from specialization approach, which is described here: custom-serialization.md. However, this only works for user defined types.

For common use cases or cases where a specific member variable should have special reading and writing, you can use glz::custom to register read/write member functions, std::functions, or lambda functions.

See example:

struct custom_encoding
{
   uint64_t x{};
   std::string y{};
   std::array<uint32_t, 3> z{};
   
   void read_x(const std::string& s) {
      x = std::stoi(s);
   }
   
   uint64_t write_x() {
      return x;
   }
   
   void read_y(const std::string& s) {
      y = "hello" + s;
   }
   
   auto& write_z() {
      z[0] = 5;
      return z;
   }
};

template <>
struct glz::meta<custom_encoding>
{
   using T = custom_encoding;
   static constexpr auto value = object("x", custom<&T::read_x, &T::write_x>, //
                                        "y", custom<&T::read_y, &T::y>, //
                                        "z", custom<&T::z, &T::write_z>);
};

suite custom_encoding_test = [] {
   "custom_reading"_test = [] {
      custom_encoding obj{};
      std::string s = R"({"x":"3","y":"world","z":[1,2,3]})";
      expect(!glz::read_json(obj, s));
      expect(obj.x == 3);
      expect(obj.y == "helloworld");
      expect(obj.z == std::array<uint32_t, 3>{1, 2, 3});
   };
   
   "custom_writing"_test = [] {
      custom_encoding obj{};
      std::string s = R"({"x":"3","y":"world","z":[1,2,3]})";
      expect(!glz::read_json(obj, s));
      std::string out{};
      expect(not glz::write_json(obj, out));
      expect(out == R"({"x":3,"y":"helloworld","z":[5,2,3]})");
   };
};

Object Mapping

When using member pointers (e.g. &T::a) the C++ class structures must match the JSON interface. It may be desirable to map C++ classes with differing layouts to the same object interface. This is accomplished through registering lambda functions instead of member pointers.

template <>
struct glz::meta<Thing> {
   static constexpr auto value = object(
      "i", [](auto&& self) -> auto& { return self.subclass.i; }
   );
};

The value self passed to the lambda function will be a Thing object, and the lambda function allows us to make the subclass invisible to the object interface.

Lambda functions by default copy returns, therefore the auto& return type is typically required in order for glaze to write to memory.

Note that remapping can also be achieved through pointers/references, as glaze treats values, pointers, and references in the same manner when writing/reading.

Value Types

A class can be treated as an underlying value as follows:

struct S {
  int x{};
};

template <>
struct glz::meta<S> {
  static constexpr auto value{ &S::x };
};

or using a lambda:

template <>
struct glz::meta<S> {
  static constexpr auto value = [](auto& self) -> auto& { return self.x; };
};

Error Handling

Glaze is safe to use with untrusted messages. Errors are returned as error codes, typically within a glz::expected, which behaves just like a std::expected.

Glaze works to short circuit error handling, which means the parsing exits very rapidly if an error is encountered.

To generate more helpful error messages, call format_error:

auto pe = glz::read_json(obj, buffer);
if (pe) {
  std::string descriptive_error = glz::format_error(pe, buffer);
}

This test case:

{"Hello":"World"x, "color": "red"}

Produces this error:

1:17: expected_comma
   {"Hello":"World"x, "color": "red"}
                   ^

Denoting that x is invalid here.

Input Buffer (Null) Termination

A non-const std::string is recommended for input buffers, as this allows Glaze to improve performance with temporary padding and the buffer will be null terminated.

JSON

By default the option null_terminated is set to true and null-terminated buffers must be used when parsing JSON. The option can be turned off with a small loss in performance, which allows non-null terminated buffers:

constexpr glz::opts options{.null_terminated = false};
auto ec = glz::read<options>(value, buffer); // read in a non-null terminated buffer

BEVE

Null-termination is not required when parsing BEVE (binary). It makes no difference in performance.

CSV

Warning

Currently, null_terminated = false is not valid for CSV parsing and buffers must be null terminated.

Type Support

Array Types

Array types logically convert to JSON array values. Concepts are used to allow various containers and even user containers if they match standard library interfaces.

• glz::array (compile time mixed types)
• std::tuple (compile time mixed types)
• std::array
• std::vector
• std::deque
• std::list
• std::forward_list
• std::span
• std::set
• std::unordered_set

Object Types

Object types logically convert to JSON object values, such as maps. Like JSON, Glaze treats object definitions as unordered maps. Therefore the order of an object layout does not have to match the same binary sequence in C++.

• glz::object (compile time mixed types)
• std::map
• std::unordered_map
• std::pair (enables dynamic keys in stack storage)

std::pair is handled as an object with a single key and value, but when std::pair is used in an array, Glaze concatenates the pairs into a single object. std::vector<std::pair<...>> will serialize as a single object. If you don't want this behavior set the compile time option .concatenate = false.

Variants

• std::variant

See Variant Handling for more information.

Nullable Types

• std::unique_ptr
• std::shared_ptr
• std::optional

Nullable types may be allocated by valid input or nullified by the null keyword.

std::unique_ptr<int> ptr{};
std::string buffer{};
expect(not glz::write_json(ptr, buffer));
expect(buffer == "null");

expect(not glz::read_json(ptr, "5"));
expect(*ptr == 5);
buffer.clear();
expect(not glz::write_json(ptr, buffer));
expect(buffer == "5");

expect(not glz::read_json(ptr, "null"));
expect(!bool(ptr));

Enums

By default enums will be written and read in integer form. No glz::meta is necessary if this is the desired behavior.

However, if you prefer to use enums as strings in JSON, they can be registered in the glz::meta as follows:

enum class Color { Red, Green, Blue };

template <>
struct glz::meta<Color> {
   using enum Color;
   static constexpr auto value = enumerate(Red,
                                           Green,
                                           Blue
   );
};

In use:

Color color = Color::Red;
std::string buffer{};
glz::write_json(color, buffer);
expect(buffer == "\"Red\"");

JSON With Comments (JSONC)

Comments are supported with the specification defined here: JSONC

Read support for comments is provided with glz::read_jsonc or glz::read<glz::opts{.comments = true}>(...).

Prettify JSON

Formatted JSON can be written out directly via a compile time option:

auto ec = glz::write<glz::opts{.prettify = true}>(obj, buffer);

Or, JSON text can be formatted with the glz::prettify_json function:

std::string buffer = R"({"i":287,"d":3.14,"hello":"Hello World","arr":[1,2,3]})");
auto beautiful = glz::prettify_json(buffer);

beautiful is now:

{
   "i": 287,
   "d": 3.14,
   "hello": "Hello World",
   "arr": [
      1,
      2,
      3
   ]
}

Minify JSON

To write minified JSON:

auto ec = glz::write_json(obj, buffer); // default is minified

To minify JSON text call:

std::string minified = glz::minify_json(buffer);

Minified JSON Reading

If you wish require minified JSON or know your input will always be minified, then you can gain a little more performance by using the compile time option .minified = true.

auto ec = glz::read<glz::opts{.minified = true}>(obj, buffer);

Boolean Flags

Glaze supports registering a set of boolean flags that behave as an array of string options:

struct flags_t {
   bool x{ true };
   bool y{};
   bool z{ true };
};

template <>
struct glz::meta<flags_t> {
   using T = flags_t;
   static constexpr auto value = flags("x", &T::x, "y", &T::y, "z", &T::z);
};

Example:

flags_t s{};
expect(glz::write_json(s) == R"(["x","z"])");

Only "x" and "z" are written out, because they are true. Reading in the buffer will set the appropriate booleans.

When writing BEVE, flags only use one bit per boolean (byte aligned).

Logging JSON

Sometimes you just want to write out JSON structures on the fly as efficiently as possible. Glaze provides tuple-like structures that allow you to stack allocate structures to write out JSON with high speed. These structures are named glz::obj for objects and glz::arr for arrays.

Below is an example of building an object, which also contains an array, and writing it out.

auto obj = glz::obj{"pi", 3.14, "happy", true, "name", "Stephen", "arr", glz::arr{"Hello", "World", 2}};

std::string s{};
expect(not glz::write_json(obj, s));
expect(s == R"({"pi":3.14,"happy":true,"name":"Stephen","arr":["Hello","World",2]})");

This approach is significantly faster than glz::json_t for generic JSON. But, may not be suitable for all contexts.

Merge

glz::merge allows the user to merge multiple JSON object types into a single object.

glz::obj o{"pi", 3.141};
std::map<std::string_view, int> map = {{"a", 1}, {"b", 2}, {"c", 3}};
auto merged = glz::merge{o, map};
std::string s{};
glz::write_json(merged, s); // will write out a single, merged object
// s is now: {"pi":3.141,"a":0,"b":2,"c":3}

glz::merge stores references to lvalues to avoid copies

Generic JSON

See Generic JSON for glz::json_t.

glz::json_t json{};
std::string buffer = R"([5,"Hello World",{"pi":3.14}])";
glz::read_json(json, buffer);
assert(json[2]["pi"].get<double>() == 3.14);

Raw Buffer Performance

Glaze is just about as fast writing to a std::string as it is writing to a raw char buffer. If you have sufficiently allocated space in your buffer you can write to the raw buffer, as shown below, but it is not recommended.

glz::read_json(obj, buffer);
const auto n = glz::write_json(obj, buffer.data()).value_or(0);
buffer.resize(n);

Compile Time Options

The glz::opts struct defines compile time optional settings for reading/writing.

Instead of calling glz::read_json(...), you can call glz::read<glz::opts{}>(...) and customize the options.

For example: glz::read<glz::opts{.error_on_unknown_keys = false}>(...) will turn off erroring on unknown keys and simple skip the items.

glz::opts can also switch between formats:

• glz::read<glz::opts{.format = glz::BEVE}>(...) -> glz::read_beve(...)
• glz::read<glz::opts{.format = glz::JSON}>(...) -> glz::read_json(...)

Available Compile Time Options

The struct below shows the available options and the default behavior.

struct opts {
  uint32_t format = json;
  bool comments = false; // Support reading in JSONC style comments
  bool error_on_unknown_keys = true; // Error when an unknown key is encountered
  bool skip_null_members = true; // Skip writing out params in an object if the value is null
  bool use_hash_comparison = true; // Will replace some string equality checks with hash checks
  bool prettify = false; // Write out prettified JSON
  bool minified = false; // Require minified input for JSON, which results in faster read performance
  char indentation_char = ' '; // Prettified JSON indentation char
  uint8_t indentation_width = 3; // Prettified JSON indentation size
  bool new_lines_in_arrays = true; // Whether prettified arrays should have new lines for each element
  bool shrink_to_fit = false; // Shrinks dynamic containers to new size to save memory
  bool write_type_info = true; // Write type info for meta objects in variants
  bool error_on_missing_keys = false; // Require all non nullable keys to be present in the object. Use
                                        // skip_null_members = false to require nullable members
  bool error_on_const_read =
     false; // Error if attempt is made to read into a const value, by default the value is skipped without error
  bool validate_skipped = false; // If full validation should be performed on skipped values
  bool validate_trailing_whitespace =
     false; // If, after parsing a value, we want to validate the trailing whitespace

  uint8_t layout = rowwise; // CSV row wise output/input

  // The maximum precision type used for writing floats, higher precision floats will be cast down to this precision
  float_precision float_max_write_precision{};

  bool bools_as_numbers = false; // Read and write booleans with 1's and 0's

  bool quoted_num = false; // treat numbers as quoted or array-like types as having quoted numbers
  bool number = false; // read numbers as strings and write these string as numbers
  bool raw = false; // write out string like values without quotes
  bool raw_string =
     false; // do not decode/encode escaped characters for strings (improves read/write performance)
  bool structs_as_arrays = false; // Handle structs (reading/writing) without keys, which applies
  bool allow_conversions = true; // Whether conversions between convertible types are
  // allowed in binary, e.g. double -> float

  bool partial_read =
     false; // Reads into only existing fields and elements and then exits without parsing the rest of the input

  // glaze_object_t concepts
  bool partial_read_nested = false; // Advance the partially read struct to the end of the struct
  bool concatenate = true; // Concatenates ranges of std::pair into single objects when writing

  bool hide_non_invocable =
     true; // Hides non-invocable members from the cli_menu (may be applied elsewhere in the future)
};

Many of these compile time options have wrappers to apply the option to only a single field. See Wrappers for more details.

JSON Conformance

By default Glaze is strictly conformant with the latest JSON standard except in two cases with associated options:

• validate_skipped This option does full JSON validation for skipped values when parsing. This is not set by default because values are typically skipped when the user is unconcerned with them, and Glaze still validates for major issues. But, this makes skipping faster by not caring if the skipped values are exactly JSON conformant. For example, by default Glaze will ensure skipped numbers have all valid numerical characters, but it will not validate for issues like leading zeros in skipped numbers unless validate_skipped is on. Wherever Glaze parses a value to be used it is fully validated.
• validate_trailing_whitespace This option validates the trailing whitespace in a parsed document. Because Glaze parses C++ structs, there is typically no need to continue parsing after the object of interest has been read. Turn on this option if you want to ensure that the rest of the document has valid whitespace, otherwise Glaze will just ignore the content after the content of interest has been parsed.

Note

Glaze does not automatically unicode escape control characters (e.g. "\x1f" to "\u001f"), as this poses a risk of embedding null characters and other invisible characters in strings. A compile time option will be added to enable these conversions (open issue: unicode escaped write), but it will not be the default behavior.

Skip

It can be useful to acknowledge a keys existence in an object to prevent errors, and yet the value may not be needed or exist in C++. These cases are handled by registering a glz::skip type with the meta data.

See example:

struct S {
  int i{};
};

template <>
struct glz::meta<S> {
  static constexpr auto value = object("key_to_skip", skip{}, &S::i);
};

std::string buffer = R"({"key_to_skip": [1,2,3], "i": 7})";
S s{};
glz::read_json(s, buffer);
// The value [1,2,3] will be skipped
expect(s.i == 7); // only the value i will be read into

Hide

Glaze is designed to help with building generic APIs. Sometimes a value needs to be exposed to the API, but it is not desirable to read in or write out the value in JSON. This is the use case for glz::hide.

glz::hide hides the value from JSON output while still allowing API (and JSON pointer) access.

See example:

struct hide_struct {
  int i = 287;
  double d = 3.14;
  std::string hello = "Hello World";
};

template <>
struct glz::meta<hide_struct> {
   using T = hide_struct;
   static constexpr auto value = object(&T::i,  //
                                        &T::d, //
                                        "hello", hide{&T::hello});
};

hide_struct s{};
auto b = glz::write_json(s);
expect(b == R"({"i":287,"d":3.14})"); // notice that "hello" is hidden from the output

Quoted Numbers

You can parse quoted JSON numbers directly to types like double, int, etc. by utilizing the glz::quoted wrapper.

struct A {
   double x;
   std::vector<uint32_t> y;
};

template <>
struct glz::meta<A> {
   static constexpr auto value = object("x", glz::quoted_num<&A::x>, "y", glz::quoted_num<&A::y>;
};

{
  "x": "3.14",
  "y": ["1", "2", "3"]
}

The quoted JSON numbers will be parsed directly into the double and std::vector<uint32_t>. The glz::quoted function works for nested objects and arrays as well.

JSON Lines (NDJSON) Support

Glaze supports JSON Lines (or Newline Delimited JSON) for array-like types (e.g. std::vector and std::tuple).

std::vector<std::string> x = { "Hello", "World", "Ice", "Cream" };
std::string s = glz::write_ndjson(x).value_or("error");
auto ec = glz::read_ndjson(x, s);

More Features

Data Recorder

Command Line Interface Menu

JSON Include System

JSON Pointer Syntax

JSON-RPC 2.0

JSON Schema

Shared Library API

Tagged Binary Messages

Thread Pool

Time Trace Profiling

• Output performance profiles to JSON and visualize using Perfetto

Wrappers

Extensions

See the ext directory for extensions.

• Eigen
• JSON-RPC 2.0
• Command Line Interface Menu (cli_menu)

License

Glaze is distributed under the MIT license with an exception for embedded forms:

--- Optional exception to the license ---

As an exception, if, as a result of your compiling your source code, portions of this Software are embedded into a machine-executable object form of such source code, you may redistribute such embedded portions in such object form without including the copyright and permission notices.