Unexpected behaviour of is_null - Part II

[phyz777]

The following code produces a core dump with this error message: Assertion `m_value.object->find(key) != m_value.object->end()' failed

int main() {
    nlohmann::json j;
    j["subobject"] = {{"key1", "value1"}, {"key2", "value2"}};

    const nlohmann::json &subobject = j["subobject"];

    if (!subobject["key1"].is_null()) {
        std::cout << "CHECK" << std::endl;
    } else {
        std::cout << "FAIL" << std::endl;
    }

    if (!subobject["key3"].is_null()) {
        std::cout << "CHECK" << std::endl;
    } else {
        std::cout << "FAIL" << std::endl;
    }

    return 0;
}

This is because the check inserts {"key3", null} into the object.

I'm not sure whether this behaviour is intentionally, it might at least become a pitfall for some, so I wanted to bring this to your attention.

[nlohmann]

For const json objects, operator[] has an undefined semantics if the given key is not present, see https://nlohmann.github.io/json/classnlohmann_1_1basic__json_ab2318780e5ae692039e816b6ac32c91e.html#ab2318780e5ae692039e816b6ac32c91e

[nlohmann]

More details:

• subobject is const
• operator[] const cannot add keys to an object, so using it with an unknown key such as "key3" has undefined behavior
• when assertions are switched on, this undefined behavior triggers an assertion
• always use at if you need a runtime check

[phyz777]

For my purposes, it was sufficient to simply remove the const specifier.
But I'm sure all these additional information will come in handy soon or later, both for me and for others.
Thanks a lot!

[nlohmann]

Ok, thanks for checking back!

[p-i-]

json j = {
	{ "foo", 1 }
};

std::cout << j["bar"].is_null() << std::endl;
// 1

std::cout << j << std::endl;
// {"bar":null,"foo":1}

This is very unexpected behaviour.

[gregmarr]

Where is the unexpected behavior here?

[nlohmann]

When you have a non-const JSON value and use operator[] to access a non-existing key, a value is default-constructed at that key. This is the same behavior as std::map.