Modern C++ JSON schema validator based on Niels Lohmann's JSON for Modern C++
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Bryan Gillespie and pboettch Add CMake options to disable extras and support install
- Adds support for the following CMake options:
- BUILD_TESTS (default: ON)
- BUILD_EXAMPLES (default: ON)
- CMAKE_INSTALL_PREFIX (both static and shared)
Latest commit e3d42e6 May 11, 2018

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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)

Numerical validation uses int64_t, uint64_t or double, depending on if the schema type is "integer" or "number". Bignum (i.e. arbitrary precision and range) is not supported at this time.

Unsigned integer validation will only take place if the following two conditions are true:

  • The nlohmann type() of the json object under validation is nlohmann::json::value_t::number_unsigned
  • The schema specifies a numerical minimum greater than or equal to 0

How to use

The current state of the build-system needs at least version 3.1.1 of NLohmann's JSON library. It is looking for the json.hpp within a nlohmann/-path.

When build the library you need to provide the path to the directory where the include-file is located as nlohmann/json.hpp.


Within a build-dir

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

As a subdirectory from within

# 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)

Building a shared library

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

In your initial call to cmake simply add:



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

#include <iostream>

#include "json-schema.hpp"

using nlohmann::json;
using nlohmann::json_uri;
using nlohmann::json_schema_draft4::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 */

    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
            std::cout << "Validation succeeded\n";
        } catch (const std::exception &e) {
            std::cerr << "Validation failed, here is why: " << e.what() << "\n";
    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.