pure virtual conversion operator

[lzheng5]

Consider the following code.

class ToJsonBase
{
protected:
  std::string b;

public:
  ToJsonBase(const std::string& ab) : b(ab){}

  virtual operator json() = 0;
};

class ToJson : public ToJsonBase
{
  std::string d;

public:
  ToJson(const std::string& ad)
    :ToJsonBase("Base String"), d(ad){}
  operator json()
  {
    json j;
    j["base"] = b;
    j["derived"] = d;
    return j;
  }
};

int main()
{
  ToJsonBase* p = new ToJson("derived string");
  json j = (json)*p; // error here
}

When you have the conversion operator as a pure virtual function, it will give me the following error.

In file included from /usr/include/c++/6.1.1/bits/uniform_int_dist.h:35:0,
                 from /usr/include/c++/6.1.1/bits/stl_algo.h:66,
                 from /usr/include/c++/6.1.1/algorithm:62,
                 from /home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/./include/net/json.hpp:32,
                 from /home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/tools/json-test/main.cpp:1:
/usr/include/c++/6.1.1/limits: In instantiation of ‘struct std::numeric_limits<ToJsonBase>’:
/home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/./include/net/json.hpp:1418:94:   required by substitution of ‘template<class CompatibleNumberIntegerType, typename std::enable_if<((std::is_constructible<long long int, CompatibleNumberIntegerType>::value && std::numeric_limits<_Tp>::is_integer) && std::numeric_limits<_Tp>::is_signed), CompatibleNumberIntegerType>::type <anonymous> > nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType>::basic_json(CompatibleNumberIntegerType) [with CompatibleNumberIntegerType = ToJsonBase; typename std::enable_if<((std::is_constructible<long long int, CompatibleNumberIntegerType>::value && std::numeric_limits<_Tp>::is_integer) && std::numeric_limits<_Tp>::is_signed), CompatibleNumberIntegerType>::type <anonymous> = <missing>]’
/home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/tools/json-test/main.cpp:78:16:   required from here
/usr/include/c++/6.1.1/limits:320:7: error: invalid abstract return type ‘ToJsonBase’
       min() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }
       ^~~
/home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/tools/json-test/main.cpp:11:7: note:   because the following virtual functions are pure within ‘ToJsonBase’:
 class ToJsonBase
       ^~~~~~~~~~
/home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/tools/json-test/main.cpp:19:11: note:  virtual ToJsonBase::operator nlohmann::json()
   virtual operator json() = 0;
           ^~~~~~~~
In file included from /usr/include/c++/6.1.1/bits/uniform_int_dist.h:35:0,
                 from /usr/include/c++/6.1.1/bits/stl_algo.h:66,
                 from /usr/include/c++/6.1.1/algorithm:62,
                 from /home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/./include/net/json.hpp:32,
                 from /home/lzheng5/Dropbox/home/ncsu/Spring2016/ECE484/IotFaceRec/tools/json-test/main.cpp:1:
/usr/include/c++/6.1.1/limits:324:7: error: invalid abstract return type ‘ToJsonBase’
       max() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }
       ^~~
/usr/include/c++/6.1.1/limits:330:7: error: invalid abstract return type ‘ToJsonBase’
       lowest() noexcept { return _Tp(); }
       ^~~~~~
/usr/include/c++/6.1.1/limits:336:7: error: invalid abstract return type ‘ToJsonBase’
       epsilon() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }
       ^~~~~~~
/usr/include/c++/6.1.1/limits:340:7: error: invalid abstract return type ‘ToJsonBase’
       round_error() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }
       ^~~~~~~~~~~
/usr/include/c++/6.1.1/limits:344:7: error: invalid abstract return type ‘ToJsonBase’
       infinity() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }
       ^~~~~~~~
/usr/include/c++/6.1.1/limits:349:7: error: invalid abstract return type ‘ToJsonBase’
       quiet_NaN() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }
       ^~~~~~~~~
/usr/include/c++/6.1.1/limits:354:7: error: invalid abstract return type ‘ToJsonBase’
       signaling_NaN() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }
       ^~~~~~~~~~~~~
/usr/include/c++/6.1.1/limits:360:7: error: invalid abstract return type ‘ToJsonBase’
       denorm_min() _GLIBCXX_USE_NOEXCEPT { return _Tp(); }

[nlohmann]

I cannot reproduce the issue with Clang 3.5-3.8 as well as Apple's clang-800.0.42.1.
I can reproduce the issue with GCC 5.4.0 and GCC 6.2.0.

Not sure what to do about this.

[boguscoder]

FYI, its interesting one however I could not solve it, so decided to ask here with minimal example http://stackoverflow.com/questions/40442956/clang-vs-gcc-in-abstract-class-handling-in-compile-time

[boguscoder]

it's not really json library issue at all, though it helps to surface the difference in compiler behavior

[nlohmann]

Thanks for reporting and opening the thread at SO. There is already an answer, so I close this issue here. If anything related to the library comes up, please let me know.