is<char*>() should return true for null

[gerikintercede]

There is no way to test for "key":null in a JsonVariant.
JsonVariant().is< char* >() returns false as expected, so is no use.
JsonVariant().asString() returns NULL, but it will also return NULL for "key":1, so is no use.

Needs something like:

inline bool JsonVariant::isNull() const {
  using namespace Internals;
  return (_type == JSON_UNPARSED && _content.asString &&
      !strcmp("null", _content.asString));
}

However, this also needs bool JsonVariant::is< void* >();, or possibly class JsonNull; bool JsonVariant::is< JsonNull >();

Also there are no tests in JsonVariant_As_Tests.cpp and JsonVariant_Is_Tests.cpp for "key":null .

[bblanchon]

When you write variant = (const char*)0, is<char*>() returns true, I think that's what most people expect.
So I think it would be better if is<char*>() returned true when the input contains a null.
In your case, you'd just have to write:

bool isNull(const JsonVariant& v) {
  return v.is<char*>() && !v.as<char*>();
}

is<void*>() won't do it because most implementations define NULL as an int.

The JsonNull will be confusing. Look:

char * s = 0;
JsonVariant v = s;
s.is<char*>();  // false or true ?
s.is<JsonNull>(); // true: it feels like the type has changed

In C++11, we could also leverage nullptr and nullptr_t, but I want to keep the library C++98 compliant for now.

[gerikintercede]

Not so.
A variant with from "key":null results in a JSON_UNPARSED type with _content.asString = “null”
Therefore is<char*>() returns false.

  template <typename T>
  typename TypeTraits::EnableIf<TypeTraits::IsSame<T, const char *>::value ||
                                    TypeTraits::IsSame<T, char *>::value,
                                bool>::type
  is() const {
    return isString();
  }
  bool isString() const {
    return _type == Internals::JSON_STRING;
  }

Perhaps you need to modify isString() to match the code in asString():

  bool isString() const {
    if (_type == JSON_UNPARSED && _content.asString &&
        !strcmp("null", _content.asString))
      return true;
    return _type == Internals::JSON_STRING;
  }

[bblanchon]

Sorry, my message was unclear.

When I said "it would be better if is<char*>() returned true", I meant that I see this as a bug in is<char*>().

I'll fix this ASAP.

[bblanchon]

Fixed in v5.6.5
Sorry for the long delay.

[milkpirate]

Sorry for my question but I really couldnt figure out what to do to check if "key":null is null.

So will

bool isNull(const JsonVariant& v) {
  return v.is<char*>() && !v.as<char*>();
}

if( isNull(root["key"]) ) {
   // yes, key == null 
}

work?

[bblanchon]

Hi @milkpirate,

Yes, this isNull() function would work, but it's probably possible to avoid this check.
Why do you need to know if the value is null?

Regards,
Benoit

[milkpirate]

I transmit settings from a web form via a JSON string as URL parameter (its easier to parse it that way instead of every setting by its own). If not all settings are changed I set them to null and discard them in further processing.