Modern C++ JSON schema validator based on Niels Lohmann's JSON for Modern C++
C++ CMake Shell

Modern C++ JSON schema validator

What is it?

This is a C++ library for validating JSON documents based on a JSON Schema which itself should validate with draft-4 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.

The name is for the moment purely marketing, because there is, IMHO, not so much modern C++ inside. There is plenty of space to make it more modern.

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

Design goals

The main goal of this validator is to produce human-comprehensible error messages if a JSON-document/instance does not comply with its schema. This is done with exceptions thrown at the users with a helpful message telling what's wrong with the document while validating.

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


Schema-reference resolution is not recursivity-proven: If there is a nested cross-schema reference, it will not stop. (Though I haven't tested it)

How to use



git clone
cd json-schema-validator
mkdir build
cd build
cmake .. \
    -DNLOHMANN_JSON_DIR=<path/to/json.hpp> \
    -DJSON_SCHEMA_TEST_SUITE_PATH=<path/to/JSON-Schema-test-suite> # optional
make # install
ctest # if test-suite has been given

or from another CMakeLists.txt as a subdirectory:

# create an interface-target called json-hpp
add_library(json-hpp INTERFACE)

# set this path to schema-test-suite to get tests compiled - optional
set(JSON_SCHEMA_TEST_SUITE_PATH "path/to/json-schema-test-suite")
enable_testing() # if you want to inherit tests

add_subdirectory(path-to-this-project json-schema-validator)


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

#include "json-schema.hpp"

using nlohmann::json;
using nlohmann::json_uri;
using nlohmann::json_schema_draft4::json_validator;

static void loader(const json_uri &uri, json &schema)
    // get the schema from uri and feed it into schema
    // if not possible, otherwise, throw an excpetion

int main(void)
    json schema;

    /* json-parse the schema */

    json_validator validator(loader); // create validator with a loader-callback

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

    json document;

    /* json-parse the document */

    try {
        validator.validate(document); // validate the document
    } catch (const std::exception &e) {
        std::cerr << "Validation failed, here is why: " << e.what() << "\n";
        return EXIT_FAILURE;
    return EXIT_SUCCESS;


There is an application which can be used for testing the validator with the JSON-Schema-Test-Suite.

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.

12 optional tests of 305 total (required + optional) tests are failing:

  • 10 of them are format-strings which are not supported.
  • big numbers are not working (2)

Additional features

Default values

The goal is to create an empty document, based on schema-defined default-values, recursively populated.