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json::value_t can't be a map's key type in VC++ 2015 #486

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json::value_t can't be a map's key type in VC++ 2015#486
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nlohmann
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platform: visual studiorelated to MSVC
Milestone
Release 3.0.0

Description

@Getfree
Getfree
opened on Mar 7, 2017

The following code does not compile in Visual Studio 2015, but compiles fine in GCC.

#include <string>
#include <map>
#include <iostream>

#include "json.hpp"
using JSON = nlohmann::json ;

int main (){
	using namespace std ;
	
	map<JSON::value_t,string> jsonTypes ;

	jsonTypes[JSON::value_t::array] = "array" ;

	cout << jsonTypes[ JSON({"val1", 123, false}) ] << endl ;
}

Wandbox: http://melpon.org/wandbox/permlink/Izdt8DW4UGpVQdUd

Activity

  1. nlohmann
    added
    platform: visual studiorelated to MSVC
    on Mar 7, 2017

  2. nlohmann commented on Mar 7, 2017

    @nlohmann
    nlohmann
    on Mar 7, 2017
    Owner

    Can you please provide the error messages from MSVC?

  3. Getfree commented on Mar 7, 2017

    @Getfree
    Getfree
    on Mar 7, 2017
    Author

    No problem. Here:

    Z:\>cl /EHsc test3.cpp
test3.cpp
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstddef(239): error C2593: 'operator <' is ambiguous
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(436): note: could be 'bool std::operator <(const std::error_condition &,const std::error_condition &) noexcept'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(427): note: or       'bool std::operator <(const std::error_code &,const std::error_code &) noexcept'
z:\json.hpp(171): note: or       'bool nlohmann::detail::operator <(const nlohmann::detail::value_t,const nlohmann::detail::value_t) noexcept' [found using argument-dependent lookup]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstddef(239): note: while trying to match the argument list '(const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t, const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t)'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xstddef(238): note: while compiling class template member function 'bool std::less<_Kty>::operator ()(const _Ty &,const _Ty &) const'
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\map(210): note: see reference to function template instantiation 'bool std::less<_Kty>::operator ()(const _Ty &,const _Ty &) const' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\type_traits(421): note: see reference to class template instantiation 'std::less<_Kty>' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xutility(265): note: see reference to class template instantiation 'std::is_empty<_Ty1>' being compiled
        with
        [
            _Ty1=std::less<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t>
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xtree(957): note: see reference to class template instantiation 'std::_Tree_comp_alloc<_Traits>' being compiled
        with
        [
            _Traits=std::_Tmap_traits<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string,std::less<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t>,std::allocator<std::pair<const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string>>,false>
        ]
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\map(73): note: see reference to class template instantiation 'std::_Tree<std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,false>>' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=std::string,
            _Pr=std::less<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t>,
            _Alloc=std::allocator<std::pair<const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string>>
        ]
test3.cpp(11): note: see reference to class template instantiation 'std::map<nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,std::string,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>' being compiled
        with
        [
            _Kty=nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t,
            _Ty=std::string
        ]

  4. nlohmann commented on Mar 7, 2017

    @nlohmann
    nlohmann
    on Mar 7, 2017
    Owner

    Could you try explicitly to pass nlohmann::detail::operator< as Compare template argument to the map?

  5. Getfree commented on Mar 7, 2017

    @Getfree
    Getfree
    on Mar 7, 2017
    Author

    I don't know how to pass the operator itself, but I tried passing a functor, like this:

    #include <string>
#include <map>
#include <iostream>

#include "json.hpp"
using JSON = nlohmann::json ;

int main (){
	using namespace std ;

	struct value_tCompare {
		bool operator()(const JSON::value_t& a, const JSON::value_t& b) const{
			return a < b ;
		}
	} ;
	
	map<JSON::value_t, string, value_tCompare > jsonTypes ;

	jsonTypes[JSON::value_t::array] = "array" ;

	cout << jsonTypes[ JSON({"val1", 123, false}) ] << endl ;
}

    And the result was this:

    Z:\>cl /EHsc test3.cpp
test3.cpp
test3.cpp(13): error C2593: 'operator <' is ambiguous
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(436): note: could be 'bool std::operator <(const std::error_condition &,const std::error_condition &) noexcept'
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\system_error(427): note: or       'bool std::operator <(const std::error_code &,const std::error_code &) noexcept'
z:\json.hpp(171): note: or       'bool nlohmann::detail::operator <(const nlohmann::detail::value_t,const nlohmann::detail::value_t) noexcept' [found using argument-dependent lookup]
test3.cpp(13): note: while trying to match the argument list '(const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t, const nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::value_t)'

    Then I tried using nlohmann::detail::operator< explicitly inside the functor, and it compiled successfully.

    #include <string>
#include <map>
#include <iostream>

#include "json.hpp"
using JSON = nlohmann::json ;

int main (){
	using namespace std ;

	struct value_tCompare {
		bool operator()(const JSON::value_t& a, const JSON::value_t& b) const{
			return nlohmann::detail::operator<(a,b) ;
		}
	} ;
	
	map<JSON::value_t, string, value_tCompare > jsonTypes ;

	jsonTypes[JSON::value_t::array] = "array" ;

	cout << jsonTypes[ JSON({"val1", 123, false}) ] << endl ;
}

    This seems to be an argument-dependent lookup issue. For some reason the compiler is not prioritizing nlohmann::detail::operator< over the others.

    Hopefully there is a way to solve it, so we don't need this workaround.

  6. nlohmann commented on Mar 7, 2017

    @nlohmann
    nlohmann
    on Mar 7, 2017
    Owner

    This seems to be an issue where @theodelrieu may know more.

  7. theodelrieu commented on Mar 7, 2017

    @theodelrieu
    theodelrieu
    on Mar 7, 2017
    Contributor

    I will take a look tomorrow, this looks like a MSVC-specific weirdness at first sight

  8. theodelrieu commented on Mar 8, 2017

    @theodelrieu
    theodelrieu
    on Mar 8, 2017
    Contributor

    There is a workaround mentioned in this StackOverflow thread
    Should we add this to the library?

    namespace std {
template <>
struct less<::nlohmann::detail::value_t>
{
  bool operator()(::nlohmann::detail::value_t lhs, ::nlohmann::detail::value_t rhs) const noexcept
  {
    return ::nlohmann::detail::operator<(lhs, rhs);
  }
};
}

  9. nlohmann commented on Mar 8, 2017

    @nlohmann
    nlohmann
    on Mar 8, 2017
    Owner

    Is it safe to add this to the std namespace? @gregmarr ?

  10. theodelrieu commented on Mar 8, 2017

    @theodelrieu
    theodelrieu
    on Mar 8, 2017
    Contributor

    That's a template specialization, it's allowed and safe.

    Plus, this is what the default std::less<nlohmann::detail::value_t> looks like on the other platforms.

  11. nlohmann
    self-assigned this
    on Mar 11, 2017
  12. nlohmann
    added this to the Release 3.0.0 milestone on Mar 11, 2017
  13. nlohmann
    added a commit that references this issue on Mar 11, 2017

    ✨ added overload for std::less<value_t> #486

    f4126e4

  14. nlohmann commented on Mar 11, 2017

    @nlohmann
    nlohmann
    on Mar 11, 2017
    Owner

    The following code now compiles with MSVC 2015 (see https://ci.appveyor.com/project/nlohmann/json/build/1765):

    std::map<json::value_t, std::string> jsonTypes ;
jsonTypes[json::value_t::array] = "array";

    @Getfree Can you try this version of the code please: f4126e4

  15. nlohmann
    added
    solution: proposed fixa fix for the issue has been proposed and waits for confirmation
    on Mar 11, 2017

  16. Getfree commented on Mar 11, 2017

    @Getfree
    Getfree
    on Mar 11, 2017
    Author

    I added the fixed version to my project and it compiled successfully.

    Many thanks.

  17. nlohmann
    added a commit that references this issue on Mar 12, 2017

    🚑 fix for #486

    4d7c299
  18. nlohmann
    removed
    solution: proposed fixa fix for the issue has been proposed and waits for confirmation
    on Mar 12, 2017
  19. nlohmann
    closed this as completedon Mar 12, 2017
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