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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](http://json-schema.org/) which itself should validate with
[draft-4 of JSON Schema Validation](http://json-schema.org/schema).

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++](https://github.com/nlohmann/json). 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.

# Weaknesses

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`.

## Build

### Within a build-dir

```Bash
git clone https://github.com/pboettch/json-schema-validator.git
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

```CMake
# create an interface-target called json-hpp
add_library(json-hpp INTERFACE)
target_include_directories(json-hpp
    INTERFACE
        path/to/nlohmann/json.hpp)

# 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:
```bash
cmake -DBUILD_SHARED_LIBS=ON
```

## Code

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

```C++
#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": "http://json-schema.org/draft-04/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 */

    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;
}

```

# Compliance

There is an application which can be used for testing the validator with the
[JSON-Schema-Test-Suite](https://github.com/json-schema-org/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.