C2664 (C++/CLR) cannot convert 'nullptr' to 'nullptr &&'

[freanux]

I'm working on a C++ to C# wrapper using C++/CLR and MS VS Community 2015 (Version 14.0.25431.01 Update 3).

Compiling the single header json.hpp fails with described error in the title. The json.hpp version in 3.1.1.

I uploaded a tiny test project to reproduce the compile error:
TestLibrary.zip

For me a possible workaround is to encapsulate the json object and return the dump()ed string to pass to the C++/CLR and pass that string to the C# world instead of passing the dump()ed string from my C++/CLR class to the C# environment.

Thanks in advance.

[nlohmann]

Can you compile and run the unit tests (https://github.com/nlohmann/json#execute-unit-tests)?

[freanux]

I'll give that a try. I should note that compiling json.hpp in a native C++ project in the same Visual Studio is successful.

[freanux]

First of all, i forgot to disable using precompiled headers. With the provided TestLibrary.zip you was not able compiling to reproduce the error. Here is the corrected one:
TestLibrary.zip

The error output of that project:

1>------ Rebuild All started: Project: TestLibrary, Configuration: Debug x64 ------
1>  AssemblyInfo.cpp
1>  TestLibrary.cpp
1>c:\prg\test\testlibrary\testlibrary\json.hpp(1808): error C2664: 'std::shared_ptr<nlohmann::detail::input_buffer_adapter> std::make_shared<nlohmann::detail::input_buffer_adapter,nullptr,const size_t&>(nullptr &&,const size_t &)': cannot convert argument 1 from 'nullptr' to 'nullptr &&'
1>  c:\prg\test\testlibrary\testlibrary\json.hpp(1808): note: nullptr can only be converted to pointer or handle types
1>  c:\prg\test\testlibrary\testlibrary\json.hpp(15582): note: see reference to function template instantiation 'nlohmann::detail::input_adapter::input_adapter<IteratorType,0>(IteratorType,IteratorType)' being compiled
1>          with
1>          [
1>              IteratorType=const char *
1>          ]
1>  c:\prg\test\testlibrary\testlibrary\json.hpp(17144): note: see reference to function template instantiation 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer> nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::parse<const char*,0>(IteratorType,IteratorType,const std::function<bool (int,nlohmann::detail::parser<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>>::parse_event_t,BasicJsonType &)>,const bool)' being compiled
1>          with
1>          [
1>              IteratorType=const char *,
1>              BasicJsonType=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>
1>          ]
1>  c:\prg\test\testlibrary\testlibrary\json.hpp(17144): note: see reference to function template instantiation 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer> nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::parse<const char*,0>(IteratorType,IteratorType,const std::function<bool (int,nlohmann::detail::parser<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>>::parse_event_t,BasicJsonType &)>,const bool)' being compiled
1>          with
1>          [
1>              IteratorType=const char *,
1>              BasicJsonType=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>
1>          ]
1>  Generating Code...
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

Here is the result of the unit test:

C:\prg\Test\json\build>ctest --output-on-failure -C Debug
Test project C:/prg/Test/json/build
      Start  1: test-algorithms_default
 1/76 Test  #1: test-algorithms_default .............   Passed    0.10 sec
      Start  2: test-algorithms_all
 2/76 Test  #2: test-algorithms_all .................   Passed    0.02 sec
      Start  3: test-allocator_default
 3/76 Test  #3: test-allocator_default ..............   Passed    0.03 sec
      Start  4: test-allocator_all
 4/76 Test  #4: test-allocator_all ..................   Passed    0.01 sec
      Start  5: test-capacity_default
 5/76 Test  #5: test-capacity_default ...............   Passed    0.05 sec
      Start  6: test-capacity_all
 6/76 Test  #6: test-capacity_all ...................   Passed    0.03 sec
      Start  7: test-cbor_default
 7/76 Test  #7: test-cbor_default ...................   Passed   17.71 sec
      Start  8: test-cbor_all
 8/76 Test  #8: test-cbor_all .......................   Passed  164.93 sec
      Start  9: test-class_const_iterator_default
 9/76 Test  #9: test-class_const_iterator_default ...   Passed    0.05 sec
      Start 10: test-class_const_iterator_all
10/76 Test #10: test-class_const_iterator_all .......   Passed    0.02 sec
      Start 11: test-class_iterator_default
11/76 Test #11: test-class_iterator_default .........   Passed    0.04 sec
      Start 12: test-class_iterator_all
12/76 Test #12: test-class_iterator_all .............   Passed    0.03 sec
      Start 13: test-class_lexer_default
13/76 Test #13: test-class_lexer_default ............   Passed    0.04 sec
      Start 14: test-class_lexer_all
14/76 Test #14: test-class_lexer_all ................   Passed    0.02 sec
      Start 15: test-class_parser_default
15/76 Test #15: test-class_parser_default ...........   Passed    0.24 sec
      Start 16: test-class_parser_all
16/76 Test #16: test-class_parser_all ...............   Passed    0.22 sec
      Start 17: test-comparison_default
17/76 Test #17: test-comparison_default .............   Passed    0.15 sec
      Start 18: test-comparison_all
18/76 Test #18: test-comparison_all .................   Passed    0.14 sec
      Start 19: test-concepts_default
19/76 Test #19: test-concepts_default ...............***Failed    0.04 sec

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-concepts.exe is a Catch v1.9.7 host application.
Run with -? for options

-------------------------------------------------------------------------------
concepts
  class iterator
  CopyConstructible
-------------------------------------------------------------------------------
C:\prg\Test\json\test\src\unit-concepts.cpp(129)
...............................................................................

C:\prg\Test\json\test\src\unit-concepts.cpp(132): FAILED:
  CHECK( std::is_nothrow_copy_constructible<json::const_iterator>::value )
with expansion:
  false

===============================================================================
test cases:  1 |  0 passed | 1 failed
assertions: 31 | 30 passed | 1 failed


      Start 20: test-concepts_all
20/76 Test #20: test-concepts_all ...................***Failed    0.02 sec

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-concepts.exe is a Catch v1.9.7 host application.
Run with -? for options

-------------------------------------------------------------------------------
concepts
  class iterator
  CopyConstructible
-------------------------------------------------------------------------------
C:\prg\Test\json\test\src\unit-concepts.cpp(129)
...............................................................................

C:\prg\Test\json\test\src\unit-concepts.cpp(132): FAILED:
  CHECK( std::is_nothrow_copy_constructible<json::const_iterator>::value )
with expansion:
  false

===============================================================================
test cases:  1 |  0 passed | 1 failed
assertions: 31 | 30 passed | 1 failed



      Start 21: test-constructor1_default
21/76 Test #21: test-constructor1_default ...........   Passed    0.16 sec
      Start 22: test-constructor1_all
22/76 Test #22: test-constructor1_all ...............   Passed    0.13 sec
      Start 23: test-constructor2_default
23/76 Test #23: test-constructor2_default ...........   Passed    0.03 sec
      Start 24: test-constructor2_all
24/76 Test #24: test-constructor2_all ...............   Passed    0.02 sec
      Start 25: test-convenience_default
25/76 Test #25: test-convenience_default ............   Passed    0.03 sec
      Start 26: test-convenience_all
26/76 Test #26: test-convenience_all ................   Passed    0.01 sec
      Start 27: test-conversions_default
27/76 Test #27: test-conversions_default ............   Passed    0.15 sec
      Start 28: test-conversions_all
28/76 Test #28: test-conversions_all ................   Passed    0.14 sec
      Start 29: test-deserialization_default
29/76 Test #29: test-deserialization_default ........   Passed    0.07 sec
      Start 30: test-deserialization_all
30/76 Test #30: test-deserialization_all ............   Passed    0.06 sec
      Start 31: test-element_access1_default
31/76 Test #31: test-element_access1_default ........   Passed    0.09 sec
      Start 32: test-element_access1_all
32/76 Test #32: test-element_access1_all ............   Passed    0.06 sec
      Start 33: test-element_access2_default
33/76 Test #33: test-element_access2_default ........   Passed    0.12 sec
      Start 34: test-element_access2_all
34/76 Test #34: test-element_access2_all ............   Passed    0.07 sec
      Start 35: test-inspection_default
35/76 Test #35: test-inspection_default .............   Passed   30.10 sec
      Start 36: test-inspection_all
36/76 Test #36: test-inspection_all .................   Passed   30.68 sec
      Start 37: test-items_default
37/76 Test #37: test-items_default ..................   Passed    0.08 sec
      Start 38: test-items_all
38/76 Test #38: test-items_all ......................   Passed    0.04 sec
      Start 39: test-iterators1_default
39/76 Test #39: test-iterators1_default .............   Passed    0.18 sec
      Start 40: test-iterators1_all
40/76 Test #40: test-iterators1_all .................   Passed    0.11 sec
      Start 41: test-iterators2_default
41/76 Test #41: test-iterators2_default .............   Passed    0.12 sec
      Start 42: test-iterators2_all
42/76 Test #42: test-iterators2_all .................   Passed    0.10 sec
      Start 43: test-json_patch_default
43/76 Test #43: test-json_patch_default .............   Passed    0.13 sec
      Start 44: test-json_patch_all
44/76 Test #44: test-json_patch_all .................   Passed    0.11 sec
      Start 45: test-json_pointer_default
45/76 Test #45: test-json_pointer_default ...........   Passed    0.05 sec
      Start 46: test-json_pointer_all
46/76 Test #46: test-json_pointer_all ...............   Passed    0.02 sec
      Start 47: test-merge_patch_default
47/76 Test #47: test-merge_patch_default ............   Passed    0.05 sec
      Start 48: test-merge_patch_all
48/76 Test #48: test-merge_patch_all ................   Passed    0.02 sec
      Start 49: test-meta_default
49/76 Test #49: test-meta_default ...................   Passed    0.03 sec
      Start 50: test-meta_all
50/76 Test #50: test-meta_all .......................   Passed    0.01 sec
      Start 51: test-modifiers_default
51/76 Test #51: test-modifiers_default ..............   Passed    0.08 sec
      Start 52: test-modifiers_all
52/76 Test #52: test-modifiers_all ..................   Passed    0.05 sec
      Start 53: test-msgpack_default
53/76 Test #53: test-msgpack_default ................   Passed   17.54 sec
      Start 54: test-msgpack_all
54/76 Test #54: test-msgpack_all ....................   Passed  157.68 sec
      Start 55: test-noexcept_default
55/76 Test #55: test-noexcept_default ...............   Passed    0.03 sec
      Start 56: test-noexcept_all
56/76 Test #56: test-noexcept_all ...................   Passed    0.01 sec
      Start 57: test-pointer_access_default
57/76 Test #57: test-pointer_access_default .........   Passed    0.03 sec
      Start 58: test-pointer_access_all
58/76 Test #58: test-pointer_access_all .............   Passed    0.02 sec
      Start 59: test-readme_default
59/76 Test #59: test-readme_default .................   Passed    0.03 sec
      Start 60: test-readme_all
60/76 Test #60: test-readme_all .....................   Passed    0.02 sec
      Start 61: test-reference_access_default
61/76 Test #61: test-reference_access_default .......   Passed    0.04 sec
      Start 62: test-reference_access_all
62/76 Test #62: test-reference_access_all ...........   Passed    0.02 sec
      Start 63: test-regression_default
63/76 Test #63: test-regression_default .............   Passed   30.19 sec
      Start 64: test-regression_all
64/76 Test #64: test-regression_all .................   Passed   29.04 sec
      Start 65: test-serialization_default
65/76 Test #65: test-serialization_default ..........   Passed    0.04 sec
      Start 66: test-serialization_all
66/76 Test #66: test-serialization_all ..............   Passed    0.01 sec
      Start 67: test-testsuites_default
67/76 Test #67: test-testsuites_default .............   Passed    0.34 sec
      Start 68: test-testsuites_all
68/76 Test #68: test-testsuites_all .................   Passed    0.36 sec
      Start 69: test-to_chars_default
69/76 Test #69: test-to_chars_default ...............   Passed    0.05 sec
      Start 70: test-to_chars_all
70/76 Test #70: test-to_chars_all ...................   Passed    0.03 sec
      Start 71: test-ubjson_default
71/76 Test #71: test-ubjson_default .................   Passed    8.30 sec
      Start 72: test-ubjson_all
72/76 Test #72: test-ubjson_all .....................   Passed   66.80 sec
      Start 73: test-udt_default
73/76 Test #73: test-udt_default ....................   Passed    0.04 sec
      Start 74: test-udt_all
74/76 Test #74: test-udt_all ........................   Passed    0.02 sec
      Start 75: test-unicode_default
75/76 Test #75: test-unicode_default ................   Passed    0.06 sec
      Start 76: test-unicode_all
76/76 Test #76: test-unicode_all ....................***Timeout 1500.00 sec
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96% tests passed, 3 tests failed out of 76

Label Time Summary:
all        = 1951.09 sec*proc (38 tests)
default    = 106.59 sec*proc (38 tests)

Total Test time (real) = 2064.65 sec

The following tests FAILED:
         19 - test-concepts_default (Failed)
         20 - test-concepts_all (Failed)
         76 - test-unicode_all (Timeout)
Errors while running CTest

[freanux]

new information:

editing line 1808 from
ia = std::make_shared<input_buffer_adapter>(nullptr, len);
to
ia = std::make_shared<input_buffer_adapter>(__nullptr, len);
fixes the problem.

to check, if compiler is a C++/CLI compiler the predefined macro __cplusplus_cli is set.

my workaround so far is to add following lines somewhere at the beginning of json.hpp:

#if defined(__cplusplus_cli)
    #define JSON_NULLPTR __nullptr
#else
    #define JSON_NULLPTR nullptr
#endif

and change the mentioned line to:

 ia = std::make_shared<input_buffer_adapter>(JSON_NULLPTR, len);

for further information:
https://stackoverflow.com/questions/7978006/nullptr-vs-nullptr

[nlohmann]

Thanks for the feedback! I wonder whether it is sufficient to make the change only at that position.

[freanux]

I think it should, because the keyword nullptr was introduced in C++/CLI for the managed side way before C++11. Now, with both side (C++/CLI and C++11) having the keyword nullptr, the language has to differentiate between unmanaged nullptr and managed nullptr. The new unmanaged nullptr became __nullptr.

The plain C++ code on Windows is working with my workaround, on my Embedded Linux environment with GCC/G++ also. Now I'm implementing the C# wrapper, then I'll see if that is working as well.

[OvermindDL1]

__nullptr

Uh, doesn't that mean that C++/CLI is not following C++ standards then? It should be using the C++ names for everything, else how would they expect to be able to compile C++ code?

Better yet, why didn't they put their weird .NET nullptr in a namespace, because it DEFINITELY should not be a keyword as the C++ spec reserves all keywords...

[freanux]

I think exactly the same. Here is Microsoft's statement:
https://msdn.microsoft.com/en-us/library/jj651642.aspx
or here:
https://msdn.microsoft.com/en-us/library/4ex65770.aspx

[OvermindDL1]

Also, why try to change nullptr in projects all around the world to fit it, why not instead just #define nullptr __nullptr then include the source files in a wrapper file?

Some wrapper cpp:

#define nullptr __nullptr
#include "../path/out/of/source/json.cpp"
#undef nullptr

And similar for headers too. This requires no changes to whatever number of libraries one may wish to use in the broken C++/CLR.

I'm surprised the compiler itself doesn't have a method to do that itself to regain some sense of standards?

[freanux]

That should work. I will try that out tomorrow at work. But yes, Microsoft's compiler should be smart enough to fit the keyword nullptr into C++ or into C++/CLI, because the compiler complains about using the "wrong" nullptr.

[OvermindDL1]

because the compiler complains about using the "wrong" nullptr.

You'd think that alone would show that the compiler could test both versions and use whichever is appropriate. ;-)

But eh, extend, embrace, then break compat, that's their way...

[nlohmann]

If the snippet from #987 (comment) fixes the the problem, I would close this issue.

[freanux]

But eh, extend, embrace, then break compat, that's their way...

Unfortunately, this is true.