Glaze

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

Glaze also supports binary messages via BEVE and CSV support. And, the library has many more useful features for building APIs.

Version 1.8.1 glz::meta reflection! For MSVC, GCC, and Clang!

In the past you were required to write out the key name along with the member object pointer in your glz::meta definition. As of 1.8.1 key names are entirely optional. You can individually include custom key names as needed, but otherwise simple reflection will use the name of your member variable.

Instead of writing:

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

You can now write:

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

This update has zero runtime performance overhead!
Every field can be individually named and customized as before
The reflection works on all types, non-aggregates, non-default constructible, non-constexpr etc.

NOTE: If you want comments in your glz::meta without providing key names, you must denote those comments explicitly. Glaze now has a comment type and a _c literal for denoting comments. These explicit comments are only needed if you remove the key name.
template <>
struct glz::meta<my_struct> {
   using T = my_struct;
   static constexpr auto value = object(
      &T::i, "i is an integer"_c,
      &T::d, comment("d is a double"),
      &T::hello, "hello is a string"_c,
      &T::arr, comment("this is an array of integers")
   );
};

Highlights

Glaze requires C++20, using concepts for cleaner code and more helpful errors.

Simple registration
Standard C++ library support
Direct to memory serialization/deserialization
Compile time maps with constant time lookups and perfect hashing
Nearly zero intermediate allocations
Pure compile time reflection for aggregate, constexpr structs in Clang
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
Tagged binary spec 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)
JSON-RPC 2.0 support
JSON Schema generation
CSV Reading/Writing
Much more!

Performance

Library	Roundtrip Time (s)	Write (MB/s)	Read (MB/s)
Glaze	1.21	988	1016
simdjson (on demand)	N/A	N/A	1134
yyjson	1.45	731	941
daw_json_link	2.73	360	547
RapidJSON	3.50	271	445
json_struct	5.31	167	330
Boost.JSON	5.28	182	276
nlohmann	14.66	83	81

Performance test code available here

Note: simdjson is great, but can 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 it must re-iterate through the document). And for large, nested objects, simdjson typically requires significantly more coding from the user.

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

Library	Read (MB/s)
Glaze	632
simdjson (on demand)	107

Binary Performance

Tagged binary specification: BEVE

Metric	Roundtrip Time (s)	Write (MB/s)	Read (MB/s)
Raw performance	0.41	3121	2366
Equivalent JSON data*	0.41	3409	2584

JSON message size: 616 bytes

Binary message size: 564 bytes

*Binary data packs more efficiently than JSON, so transporting the same amount of information is even faster.

Performance improvements and size savings are much more significant when testing large numeric arrays.

Compiler Support

Actions automatically build and test with Clang, MSVC, and GCC compilers on apple, windows, and linux.

Example

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

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

JSON Output/Input

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

Write JSON

my_struct s{};
std::string buffer = glz::write_json(s);
// buffer is now: {"i":287,"d":3.14,"hello":"Hello World","arr":[1,2,3]}

my_struct s{};
std::string buffer{};
glz::write_json(s, buffer);
// buffer is now: {"i":287,"d":3.14,"hello":"Hello World","arr":[1,2,3]}

Read JSON

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

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

Read/Write From File

auto ec = glz::read_file(obj, "./obj.json", std::string{}); // reads as JSON from the extension
auto ec = glz::read_file_json(obj, "./obj.txt", std::string{}); // reads some text file as JSON

auto ec = glz::write_file(obj, "./obj.json", std::string{}); // writes file based on extension
auto ec = glz::write_file_json(obj, "./obj.txt", std::string{}); // explicit JSON write

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

Glaze Conan recipe
Also included in Conan Center

find_package(glaze REQUIRED)

target_link_libraries(main PRIVATE glaze::glaze)

Arch Linux

AUR packages: glaze and glaze-git

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

See Wiki for Frequently Asked Questions

Local Glaze Meta

Glaze also supports metadata provided 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 };
  
  struct glaze {
     using T = my_struct;
     static constexpr auto value = glz::object(
        &T::i,
        &T::d,
        &T::hello,
        &T::arr
     );
  };
};

Template specialization of glz::meta is preferred when separating class definition from the serialization mapping. Local glaze metadata is helpful for working within the local namespace.

Custom Key Names or Unnamed Types

When you define Glaze metadata, objects will automatically reflect the names of your member object pointers. However, if you want custom names or your 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
   );
};

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

Struct Registration Macros

GLZ_META and GLZ_LOCAL_META will be deprecated in the future now that we have reflection support for external and local Glaze metadata. Current documentation is here: Struct Registration Macros

Custom Read/Write

Custom reading and writing can be achieved through the powerful to_json/from_json 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 an 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{};
      glz::write_json(obj, out);
      expect(out == R"({"x":3,"y":"helloworld","z":[5,2,3]})");
   };
};

JSON Pointer Syntax

Link to simple JSON pointer syntax explanation

Glaze supports JSON pointer syntax access in a C++ context. This is extremely helpful for building generic APIs, which allows components of complex arguments to be accessed without needed know the encapsulating class.

my_struct s{};
auto d = glz::get<double>(s, "/d");
// d.value() is a std::reference_wrapper to d in the structure s

my_struct s{};
glz::set(s, "/d", 42.0);
// d is now 42.0

JSON pointer syntax works with deeply nested objects and anything serializable.

// Tuple Example
auto tuple = std::make_tuple(3, 2.7, std::string("curry"));
glz::set(tuple, "/0", 5);
expect(std::get<0>(tuple) == 5.0);

read_as

read_as allows you to read into an object from a JSON pointer and an input buffer.

Thing thing{};
glz::read_as_json(thing, "/vec3", "[7.6, 1292.1, 0.333]");
expect(thing.vec3.x == 7.6 && thing.vec3.y == 1292.1 &&
thing.vec3.z == 0.333);

glz::read_as_json(thing, "/vec3/2", "999.9");
expect(thing.vec3.z == 999.9);

get_as_json

get_as_json allows you to get a targeted value from within an input buffer. This is especially useful if you need to change how an object is parsed based on a value within the object.

std::string s = R"({"obj":{"x":5.5}})";
auto z = glz::get_as_json<double, "/obj/x">(s);
expect(z == 5.5);

get_sv_json

get_sv_json allows you to get a std::string_view to a targeted value within an input buffer. This can be more efficient to check values and handle custom parsing than constructing a new value with get_as_json.

std::string s = R"({"obj":{"x":5.5}})";
auto view = glz::get_sv_json<"/obj/x">(s);
expect(view == "5.5");

JSON With Comments (JSONC)

Comments are supported with the specification defined here: JSONC

Comments may also be included in the glaze::meta description for your types. These comments can be written out to provide a description of your JSON interface. Calling write_jsonc as opposed to write_json will write out any comments included in the meta description.

struct thing {
  double x{5.0};
  int y{7};
};

template <>
struct glz::meta<thing> {
   using T = thing;
   static constexpr auto value = object(
      &T::x, "x is a double"_c,
      &T::y, "y is an int"_c
   );
};

Prettified output:

{
  "x": 5 /*x is a double*/,
  "y": 7 /*y is an int*/
}

The _c is necessary if member object pointer names are reflected. You can also write comment("x is a double")

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.

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", Red,
                                           "Green", Green,
                                           "Blue", Blue
   );
};

In use:

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

Prettify

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

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

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

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

beautiful is now:

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

Simplified prettify definition below, which allows the use of tabs or changing the number of spaces per indent.

string prettify(auto& in, bool tabs = false, uint32_t indent_size = 3)

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
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 mach the same binary sequence in C++ (hence the tagged specification).

glz::object (compile time mixed types)
std::map
std::unordered_map

Nullable Types

Glaze supports std::unique_ptr, std::shared_ptr, and std::optional as nullable types. Nullable types can be allocated by JSON input or nullified by the null keyword.

std::unique_ptr<int> ptr{};
std::string buffer{};
glz::write_json(ptr, buffer);
expect(buffer == "null");

glz::read_json(ptr, "5");
expect(*ptr == 5);
buffer.clear();
glz::write_json(ptr, buffer);
expect(buffer == "5");

glz::read_json(ptr, "null");
expect(!bool(ptr));

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; };
};

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 binary, flags only uses one bit per boolean (byte aligned).

Variant Handling and Type Deduction

See Variant Handling for details on std::variant support.

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{};
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);

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.

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, s);
}

This test case:

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

Produces this error:

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

Denoting that x is invalid here.

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());
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::binary}>(...) -> glz::read_binary(...)
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; // Write out 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
      char indentation_char = ' '; // Prettified JSON indentation char
      uint8_t indentation_width = 3; // Prettified JSON indentation size
      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 force_conformance = false; // Do not allow invalid json normally accepted such as comments, nan, inf.
      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
      uint32_t layout = rowwise; // CSV row wise output/input
      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
};

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.

struct S {
  int i{};
};

template <>
struct glz::meta<S> {
  static constexpr auto value = object("key_to_skip", skip{}, "x", &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 access.

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("i", &T::i,  //
                                        "d", &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.

NDJSON Support

Glaze supports 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);
glz::read_ndjson(x, s);

Extensions

See the ext directory for extensions.

Eigen. Supports reading and writing from Eigen Vector types.
JSON-RPC documentation. Glaze wrapper supporting JSON-RPC 2.0 specification.

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.