JSON schema validator for JSON for Modern C++

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.

Building

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 https://github.com/pboettch/json-schema-validator.git
cd json-schema-validator
mkdir build
cd build
cmake [..]
make
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

cmake [..] -DJSON_VALIDATOR_HUNTER=ON [..]

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.

Install

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)

and

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

to build and link.

Code

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": "http://json-schema.org/draft-07/schema#",
    "title": "A person",
    "properties": {
        "name": {
            "description": "Name",
            "type": "string"
        },
        "age": {
            "description": "Age of the person",
            "type": "number",
            "minimum": 2,
            "maximum": 200
        }
    },
    "required": [
                 "name",
                 "age"
                 ],
    "type": "object"
}

)"_json;

// 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";
        else
            std::cout << "Validation succeeded\n";
    }

    return EXIT_SUCCESS;
}

Compliance

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.

Format

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
                         nlohmann::json_schema::default_string_format_check);

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"(
{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "title": "A rectangle",
    "properties": {
        "width": {
            "$ref": "#/definitions/length",
            "default": 20
        },
        "height": {
            "$ref": "#/definitions/length"
        }
    },
    "definitions": {
        "length": {
            "type": "integer",
            "minimum": 1,
            "default": 10
        }
    }
})"_json;

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;
	}
	return EXIT_SUCCESS;
}

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).