Roundtripping null to nullptr does not work #1169

• What is the issue you have?

We use the nullptr to denote JSON's null value.

json j = nullptr;

But the reverse does not work:

std::nullptr_t n = j;

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

#include <iostream>
#include "json.hpp"

using json = nlohmann::json;

int main()
{
    json j = nullptr;
    std::nullptr_t n = j;
    std::cout << std::boolalpha << (n == nullptr) << std::endl;
}

• What is the expected behavior?

Output:

true

• And what is the actual behavior instead?

Error could not find from_json() method in T's namespace.

• Which compiler and operating system are you using? Is it a supported compiler?

Xcode Version 10.0 beta (10L176w).

• Did you use a released version of the library or the version from the develop branch?

develop.

Adding the following code to the library solves the issue:

template<typename BasicJsonType>
void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
{
    if (JSON_UNLIKELY(not j.is_null()))
    {
        JSON_THROW(type_error::create(302, "type must be null, but is " + std::string(j.type_name())));
    }
    n = nullptr;
}