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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. Even though a 2.0.0 release is imminent.

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 [..]

Providing access to nlohmann-json

The CMake-file of this libraries tries several ways to ultimately include nlohmann/json.hpp

During the cmake-configurate-step there are 3 tries done trying to access nlohmann-json:

  1. link with a nlohmann_json::nlohmann_json-target,
  2. find the nlohmann_json-cmake-package and link with nlohmann_json::nlohmann_json-target or
  3. find path to nlohmann/json.hpp.

1 is there to make it work when this project is added as a sub-directory (via add_subdirectory()), 2 and 3 can be assisted by setting the nlohmann_json_DIR-variable.

Building with Hunter package manager

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

cmake [..] -DHUNTER_ENABLED=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, method 1 will work.
  • uses the git-repo of nlohmann_json as a subdirectory, method 1 will work.
  • sets nlohmann_json_DIR, method 2 or 3 will work.

Afterwards a target called nlohmann_json_schema_validator is available in order to compile and link.

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.

The variable nlohmann_json_DIR can be used to help find_package() find this package.

Building directly: provide a path to where to find json.hpp

The last method before fataling out is by providing a path where the file json.hpp can be found.

The variable nlohmann_json_DIR has to be used to point to the path where json.hpp is found in a subdirectory called nlohmann, e.g.:

json.hpp is located in /path/to/nlohmann/json.hpp. The cmake-command has to be run as follows:

cmake -Dnlohmann_json_DIR=/path/to [..]

Method 1 - long version

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 REQUIRED)
find_package(nlohmann_json_schema_validator REQUIRED)


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

to build and link.

If a custom path has been used to install this library (and nlohmann-json), find_package() needs a hint for where to find the package-files, it can be provided by setting the following variables

cmake .. \
    -Dnlohmann_json_DIR=<path/to/>lib/cmake/nlohmann_json \

Note that if the this library is used as cmake-package, nlohmann-json also has to be used a cmake-package.


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

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


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 {}
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