Compilation error with std::map<std::string, std::string> on vs 2015

[leozzyzheng]

using json = nlohmann::json;
json j;
std::map<std::string, std::string> m;
m = j.get<decltype(m)>();

it says error C2665: 'std::pair<const _Kty,_Ty>::pair': none of the 2 overloads could convert all the argument types

if I change to

m = j;

it says 'operator =' is ambiguous

Did I miss anything about converting map data to/from json ?

[nlohmann]

I can run your above example without problems with Xcode

#include "json.hpp"

using json = nlohmann::json;

int main(int argc, char const *argv[])
{
    json j = {{"key", "value"}};
    std::map<std::string, std::string> m;
    m = j.get<decltype(m)>();
}

I can confirm the error with m = j;. There was a similar ticket, but I can't find it right now. Maybe @theodelrieu remembers it.

[leozzyzheng]

oh, sorry forget to tell that I faced the problem on vs2015.

[nlohmann]

Just to make sure: Can you compile and run the test suite without problems?

[leozzyzheng]

no, I'm using the code

#include "json.hpp"

using json = nlohmann::json;

int main(int argc, char const *argv[])
{
    json j;
    std::map<std::string, std::string> m;
    m = j.get<decltype(m)>();
}

I think there is no difference between your code and mine. :)

[nlohmann]

Do you run at least Microsoft Visual C++ 2015 / Build Tools 14.0.25123.0?

[leozzyzheng]

vs2015 with update 3, I need tomorrow to check the version number.

[leozzyzheng]

I have check that I'm using 14.0.25420.1

I think it should be without problems.

[leozzyzheng]

And I have switched to vs 2017, still has problem.

[leozzyzheng]

The last call stack in json.hpp is here:

obj = CompatibleObjectType(begin(*inner_object), end(*inner_object));
It calls

according to the error call stack. I can see the template _Iter is std::_Tree_const_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const std::string,nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>>>>>.

And this type is used to construct map at here

which needs

'std::pair<const _Kty,_Ty>::pair(std::pair<const _Kty,_Ty> &&)'
with
[
     _Kty=std::string,
    _Ty=std::string
]
or       'std::pair<const _Kty,_Ty>::pair(const std::pair<const _Kty,_Ty> &)'
with
[
    _Kty=std::string,
    _Ty=std::string
]

But _Args is const std::pair<const StringType,nlohmann::basic_json<std::map,std::vector,StringType,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>>

I can see the second type of std::pair is not the std::string, so I think this is the problem. I'm not very familiar with template, so maybe the error is caused by another reason. :)

[leozzyzheng]

I found if I change std::map<std::string, std::string> to std::map<std::string, int>, then it works without problems.

[nlohmann]

Sorry for not checking back earlier. Can I close this issue or is there still anything wrong with the library?

[leozzyzheng]

I still cannot convert this type into json using vs2015, so I write the special convert code for this type. It's fine to close it and I wish anyone had same problem could give me some advise.