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JSON schema validator for JSON for Modern C++

What is it?

This is a C++ library for validating JSON documents based on a JSON Schema which itself should validate with draft-7 of JSON Schema Validation.

First a disclaimer: It is work in progress and contributions or hints or discussions are welcome.

Niels Lohmann et al develop a great JSON parser for C++ called JSON for Modern C++. This validator is based on this library, hence the name.

External documentation is missing as well. However the API of the validator is rather simple.

New in version 2

Although significant changes have been done for the 2nd version (a complete rewrite) the API is compatible with the 1.0.0 release. Except for the namespace which is now `nlohmann::json_schema.

Version 2 supports JSON schema draft 7, whereas 1 was supporting draft 4 only. Please update your schemas.

The primary change in 2 is the way a schema is used. While in version 1 the schema was kept as a JSON-document and used again and again during validation, in version 2 the schema is parsed into compiled C++ objects which are then used during validation. There are surely still optimizations to be done, but validation speed has improved by factor 100 or more.

Design goals

The main goal of this validator is to produce human-comprehensible error messages if a JSON-document/instance does not comply to its schema.

By default this is done with exceptions thrown at the users with a helpful message telling what's wrong with the document while validating.

Starting with 2.0.0 the user can pass a json_schema::basic_error_handler-derived object along with the instance to validate to receive a callback each time a validation error occurs and decide what to do (throwing, counting, collecting).

Another goal was to use Niels Lohmann's JSON-library. This is why the validator lives in his namespace.


Instance validation is thread-safe and the same validator-object can be used by different threads:

The validate method is const which indicates the object is not modified when being called:

	json json_validator::validate(const json &) const;

Validator-object creation however is not thread-safe. A validator has to be created in one (main?) thread once.


Numerical validation uses nlohmann-json's integer, unsigned and floating point types, depending on if the schema type is "integer" or "number". Bignum (i.e. arbitrary precision and range) is not supported at this time.


This library is based on Niels Lohmann's JSON-library and thus has a build-dependency to it.

Currently at least version 3.6.0 of NLohmann's JSON library is required.

Various methods using CMake can be used to build this project.

Build out-of-source

Do not run cmake inside the source-dir. Rather create a dedicated build-dir:

git clone
cd json-schema-validator
mkdir build
cd build
cmake [..]
make install # if needed
ctest # run unit, non-regression and test-suite tests

Building as shared library

By default a static library is built. Shared libraries can be generated by using the BUILD_SHARED_LIBS-cmake variable:

In your initial call to cmake simply add:

cmake [..] -DBUILD_SHARED_LIBS=ON [..]

nlohmann-json integration

As nlohmann-json is a dependency, this library tries find it.

The cmake-configuration first checks if nlohmann-json is available as a cmake-target. This may be the case, because it is used as a submodule in a super-project which already provides and uses nlohmann-json. Otherwise, it calls find_package for nlohmann-json and requires nlohmann-json to be installed on the system.

Building with Hunter package manager

To enable access to nlohmann json library, Hunter can be used. Just run with JSON_VALIDATOR_HUNTER=ON option. No further dependencies needed


Building as a CMake-subdirectory from within another project

Adding this library as a subdirectory to a parent project is one way of building it.

If the parent project already used find_package() to find the CMake-package of nlohmann_json or includes it as a submodule likewise.

Building directly, finding a CMake-package. (short)

When nlohmann-json has been installed, it provides files which allows CMake's find_package() to be used.

This library is using this mechanism if nlohmann_json::nlohmann_json-target does not exist.


Since version 2.1.0 this library can be installed and CMake-package-files will be created accordingly. If the installation of nlohmann-json and this library is done into default unix-system-paths CMake will be able to find this library by simply doing:

find_package(nlohmann_json_schema_validator REQUIRED)


target_link_libraries(<your-target> [..] nlohmann_json_schema_validator)

to build and link.


See also app/json-schema-validate.cpp.

#include <iostream>
#include <iomanip>

#include <nlohmann/json-schema.hpp>

using nlohmann::json;
using nlohmann::json_schema::json_validator;

// The schema is defined based upon a string literal
static json person_schema = R"(
    "$schema": "",
    "title": "A person",
    "properties": {
        "name": {
            "description": "Name",
            "type": "string"
        "age": {
            "description": "Age of the person",
            "type": "number",
            "minimum": 2,
            "maximum": 200
    "required": [
    "type": "object"


// The people are defined with brace initialization
static json bad_person = {{"age", 42}};
static json good_person = {{"name", "Albert"}, {"age", 42}};

int main()
    /* json-parse the schema */

    json_validator validator; // create validator

    try {
        validator.set_root_schema(person_schema); // insert root-schema
    } catch (const std::exception &e) {
        std::cerr << "Validation of schema failed, here is why: " << e.what() << "\n";
        return EXIT_FAILURE;

    /* json-parse the people - API of 1.0.0, default throwing error handler */

    for (auto &person : {bad_person, good_person}) {
        std::cout << "About to validate this person:\n"
                  << std::setw(2) << person << std::endl;
        try {
            validator.validate(person); // validate the document - uses the default throwing error-handler
            std::cout << "Validation succeeded\n";
        } catch (const std::exception &e) {
            std::cerr << "Validation failed, here is why: " << e.what() << "\n";

    /* json-parse the people - with custom error handler */
    class custom_error_handler : public nlohmann::json_schema::basic_error_handler
        void error(const nlohmann::json_pointer<nlohmann::basic_json<>> &pointer, const json &instance,
            const std::string &message) override
            nlohmann::json_schema::basic_error_handler::error(pointer, instance, message);
            std::cerr << "ERROR: '" << pointer << "' - '" << instance << "': " << message << "\n";

    for (auto &person : {bad_person, good_person}) {
        std::cout << "About to validate this person:\n"
                  << std::setw(2) << person << std::endl;

        custom_error_handler err;
        validator.validate(person, err); // validate the document

        if (err)
            std::cerr << "Validation failed\n";
            std::cout << "Validation succeeded\n";

    return EXIT_SUCCESS;


There is an application which can be used for testing the validator with the JSON-Schema-Test-Suite. In order to simplify the testing, the test-suite is included in the repository.

If you have cloned this repository providing a path the repository-root via the cmake-variable JSON_SCHEMA_TEST_SUITE_PATH will enable the test-target(s).

All required tests are OK.


Optionally JSON-schema-validator can validate predefined or user-defined formats. Therefore a format-checker-function can be provided by the user which is called by the validator when a format-check is required (ie. the schema contains a format-field).

This is how the prototype looks like and how it can be passed to the validation-instance:

static void my_format_checker(const std::string &format, const std::string &value)
	if (format == "something") {
		if (!check_value_for_something(value))
			throw std::invalid_argument("value is not a good something");
	} else
		throw std::logic_error("Don't know how to validate " + format);

// when creating the validator

json_validator validator(nullptr, // or loader-callback
                         my_format_checker); // create validator

Default Checker

The library contains a default-checker, which does some checks. It needs to be provided manually to the constructor of the validator:

json_validator validator(loader, // or nullptr for no loader

Supported formats: date-time, date, time, email, hostname, ipv4, ipv6, uuid, regex

More formats can be added in src/string-format-check.cpp. Please contribute implementions for missing json schema draft formats.

Default value processing

As a result of the validation, the library returns a json patch including the default values of the specified schema.

#include <iostream>
#include <nlohmann/json-schema.hpp>

using nlohmann::json;
using nlohmann::json_schema::json_validator;

static const json rectangle_schema = R"(
    "$schema": "",
    "title": "A rectangle",
    "properties": {
        "width": {
            "$ref": "#/definitions/length",
            "default": 20
        "height": {
            "$ref": "#/definitions/length"
    "definitions": {
        "length": {
            "type": "integer",
            "minimum": 1,
            "default": 10

int main()
	try {
		json_validator validator{rectangle_schema};
		/* validate empty json -> will be expanded by the default values defined in the schema */
		json rectangle = "{}"_json;
		const auto default_patch = validator.validate(rectangle);
		rectangle = rectangle.patch(default_patch);
		std::cout << rectangle.dump() << std::endl; // {"height":10,"width":20}
	} catch (const std::exception &e) {
		std::cerr << "Validation of schema failed: " << e.what() << "\n";
		return EXIT_FAILURE;

The example above will output the specified default values {"height":10,"width":20} to stdout.

Note that the default value specified in a $ref may be overridden by the current instance location. Also note that this behavior will break draft-7, but it is compliant to newer drafts (e.g. 2019-09 or 2020-12).


Before opening a pull request, please apply the coding style given in the .clang-format by running clang-format from the git top-level for all touched files:

git diff master --name-only | grep '\.[ch]pp$' | xargs -P 3 -I{} clang-format -i {}