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

Glaze isn't just a JSON library. Glaze also supports:

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

• For aggregate initializable structs
• Compile time reflection so we still get perfect hashing!

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

• Pure, compile time reflection for structs
• Standard C++ library support
• Header only
• Direct to memory serialization/deserialization
• Compile time maps with constant time lookups and perfect hashing
• Nearly zero intermediate allocations
• Reflection for member object pointers when customizing
• 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)
• Rapid error handling with short circuiting
• JSON-RPC 2.0 support
• JSON Schema generation
• CSV Reading/Writing
• Much more!

See DOCS for more documentation.

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:

Tagged binary specification: BEVE

JSON message size: 670 bytes

Binary message size: 611 bytes

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

Actions automatically build and test with Clang (14+), MSVC (2022), and GCC (11+) on apple, windows, and linux.

Glaze is only designed and tested for 64bit little-endian systems.

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 Output/Input (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);

or

my_struct s{};
std::string buffer{};
glz::write_json(s, buffer);

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 for error
{
  s.value(); // s.value() is a my_struct populated from JSON
}

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);
if (ec) {
  // handle error
}
// populates s from JSON

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

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)

• Glaze Conan recipe
• Also included in Conan Center

find_package(glaze REQUIRED)

target_link_libraries(main PRIVATE glaze::glaze)

• AUR packages: glaze and glaze-git

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
   );
};

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 };
  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
     );
  };
};

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.

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,
      "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:

• Wrappers
• Lambda functions

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]})");
   };
};

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 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 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 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");

Comments are supported with the specification defined here: JSONC

Comments may also be included in the glz::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")

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.

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\"");

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 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 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

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));

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; };
};

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).

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

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.

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

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);

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, 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.

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);

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(...)

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
  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
  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
  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)
};

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{}, &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

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(&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

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.

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);

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.

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.