Json: compiling with gcc 7 -> error on bool operator <

[buffos]

when compiling i get

json.hpp:6395:66: error: wrong number of template arguments (1, should be 2) return *lhs.m_value.array < *rhs.m_value.array;

and

/usr/include/c++/7/array:94:12: note: provided for ‘template<class _Tp, long unsigned int _Nm> struct std::array’ struct array

[nlohmann]

Could you provide more information: what do you compile? Are there more error messages with other file locations?

[buffos]

https://github.com/buffos/carnd_projects/tree/master/10.project.mpc

It has few dependencies though.

[buffos]

here is the full output of the compilation

kostas@kostaslinuxlaptop:~/Code/carnd/CarND-MPC/build$ cmake .. && make
-- Configuring done
-- Generating done
-- Build files have been written to: /home/kostas/Code/carnd/CarND-MPC/build
Scanning dependencies of target mpc
[ 33%] Building CXX object CMakeFiles/mpc.dir/src/MPC.cpp.o
[ 66%] Building CXX object CMakeFiles/mpc.dir/src/main.cpp.o
In file included from /home/kostas/Code/carnd/CarND-MPC/src/main.cpp:10:0:
/home/kostas/Code/carnd/CarND-MPC/src/json.hpp: In function ‘bool nlohmann::operator<(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::const_reference, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::const_reference)’:
/home/kostas/Code/carnd/CarND-MPC/src/json.hpp:6395:66: error: wrong number of template arguments (1, should be 2)
                         return *lhs.m_value.array < *rhs.m_value.array;
                                                                  ^~~~~
In file included from /usr/include/c++/7/tuple:39:0,
                 from /usr/include/c++/7/mutex:38,
                 from /usr/include/uWS/Epoll.h:11,
                 from /usr/include/uWS/Backend.h:12,
                 from /usr/include/uWS/Networking.h:69,
                 from /usr/include/uWS/WebSocketProtocol.h:5,
                 from /usr/include/uWS/WebSocket.h:4,
                 from /usr/include/uWS/Group.h:4,
                 from /usr/include/uWS/Hub.h:4,
                 from /usr/include/uWS/uWS.h:4,
                 from /home/kostas/Code/carnd/CarND-MPC/src/main.cpp:2:
/usr/include/c++/7/array:94:12: note: provided for ‘template<class _Tp, long unsigned int _Nm> struct std::array’
     struct array
            ^~~~~
CMakeFiles/mpc.dir/build.make:86: recipe for target 'CMakeFiles/mpc.dir/src/main.cpp.o' failed
make[2]: *** [CMakeFiles/mpc.dir/src/main.cpp.o] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/mpc.dir/all' failed
make[1]: *** [CMakeFiles/mpc.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

[nlohmann]

Could you paste the complete error message? I compiled the test suite of this library with GCC7 without problems.

[buffos]

I just did above :)

[nlohmann]

It seems that the compiler tries to parse array as std::array. I can't install all the dependencies right now - can you try to create a smaller example with fewer dependencies?

To rule out a problem with your compiler: can you try to compile the unit tests for nlohmann/json (by just calling cmake/make after cloning nlohmann/json?

[buffos]

I tried cmake -DCMAKE_C_COMPILER="/usr/bin/gcc-6" -DCMAKE_CXX_COMPILER="/usr/bin/g++-6" .. && make and it builds fine
So its definitely an issue with the compiler. Probably due to dependencies.

[nlohmann]

So GCC6 works with your code, but GCC7 does not? This is strange. Then again, can you reproduce the error which fewer dependencies?

[buffos]

I will.

[whenov]

I met the same problem, here's one way to produce it:

a.cc:

#include <string>
using namespace std;

#include "json.hpp"

Compiling it with g++ -c a.cc will produce the error.

[nlohmann]

I cannot reproduce this issue with

• g++-7 (Homebrew GCC 7.1.0) 7.1.0
• g++ (GCC) 8.0.0 20170601 (experimental)

I can compile and run the complete test suite from the develop branch.

@whenov What is the OS/version of your compiler, and can you provide the error message?

[riccardolardi]

I'm having the exact same issue when compiling any OpenFrameworks app on Arch Linux since updating gcc to 7.1.1

/home/thinkpad2000/Downloads/of_v20170517_linux64_release/libs/json/include/json.hpp:5002:62: error: wrong number of template arguments (1, should be 2)
                   return *lhs.m_value.array < *rhs.m_value.array;
                                                            ^~~~~
In file included from /usr/include/c++/7.1.1/tuple:39:0,
               from /usr/include/c++/7.1.1/bits/unique_ptr.h:37,
               from /usr/include/c++/7.1.1/bits/locale_conv.h:41,
               from /usr/include/c++/7.1.1/locale:43,
               from /usr/include/c++/7.1.1/iomanip:43
...

[jebrosen]

I tried to help @alberto2000 track this down in IRC earlier. In https://github.com/openframeworks/openFrameworks/blob/master/libs/openFrameworks/utils/ofConstants.h#L435 -- 'using namespace std;' is present and indirectly ahead of the include for json.hpp, which seems to trigger the issue.

@buffos -- your code also has a 'using namespace std;' indirectly before json.hpp is included

@nlohmann -- I did the same as @whenov and had the same error message about template arguments, which goes away if 'using namespace std;' is commented out. The test suite built and ran fine.

[whenov]

@nlohmann
OS: Arch Linux
Compiler: gcc version 7.1.1 20170516 (GCC)
Error message:

json.hpp: In function ‘bool nlohmann::operator<(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::const_reference, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::const_reference)’:
json.hpp:6057:62: error: wrong number of template arguments (1, should be 2)
                     return *lhs.m_value.array < *rhs.m_value.array;
                                                              ^~~~~
In file included from json.hpp:33:0,
                 from a.cc:4:
/usr/include/c++/7.1.1/array:94:12: note: provided for ‘template<class _Tp, long unsigned int _Nm> struct std::array’
     struct array
            ^~~~~

[nlohmann]

Ok, now I can also reproduce the error with the following code and GCC-7:

using namespace std;
#include <json.hpp>

int main()
{}

The error is

In file included from std.cpp:2:0:
src/json.hpp: In function 'bool nlohmann::operator<(nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::const_reference, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer>::const_reference)':
src/json.hpp:6384:62: error: wrong number of template arguments (1, should be 2)
                     return *lhs.m_value.array < *rhs.m_value.array;
                                                              ^~~~~
In file included from src/json.hpp:33:0,
                 from std.cpp:2:
/usr/local/Cellar/gcc/7.1.0/include/c++/7.1.0/array:94:12: note: provided for 'template<class _Tp, long unsigned int _Nm> struct std::array'
     struct array
            ^~~~~
make: *** [std] Error 1

It is very strange that the name array is mistaken as std::array. I wonder if this is a bug in GCC.

[jebrosen]

Changing the line in json.hpp to read

return (*lhs.m_value.array) < *rhs.m_value.array;

seems to fix the error using that simple test case. It's quite possible this is a GCC bug; I can't understand why .array< should be interpreted as template parameters to array with that . preceding it. Furthermore, clang compiles the snippet without issue in C++11 and in C++14 mode.

[jaredgrubb]

Yes, definitely seems like a compiler bug, especially if it worked fine in other GCC7 versions, then it's probably a regression.

[nlohmann]

@jebrosen Thanks for the fix. I shall implement it. I further try to get a GCC7 working in Travis so such errors can be detected automatically.

As for the bug - it would be great to boil the problem down to a size that is "digestible" by StackOverflow. Any help would be greatly appreciated.

[nlohmann]

I opened an issue on StackOverflow, see https://stackoverflow.com/questions/44367859/using-stdarray-and-using-array-as-name

It is not sure yet whether this is an issue in the language or the compilers. Nevertheless, adding parentheses fixes the problem, so I close this issue.

[niklas88]

This issue seems to have come back with the 3.6.0 release. I guess this was caused by some automatic reformatting: Here's the line that previously had parenthesis

https://github.com/nlohmann/json/blob/release/3.6.0/include/nlohmann/json.hpp#L5731

Also this problem still appears with GCC 8.2.1

[nlohmann]

Oh, that's a pity! I use GCC 7 and 8 in the CI, but did not catch this issue. Is it just the array comparison?

[niklas88]

@nlohmann I haven't tested the commit you linked but adding the parenthesis by hand locally fixes it for me.