Conversion to STL map<string, vector<int>> gives error

[pnth]

I tried to convert a json object to and from STL map<string, vector<int>>. The "from" direction works well:

map<string, vector<int> > c_map{ { "one", { 1, 2 } }, { "two", { 3, 4 } }, { "three", { 5, 6 } } };
json j_map(c_map);
cout << j_map.dump() << endl;
// Output: {"one":[1,2],"three":[5,6],"two":[3,4]}

But the "to" direction gives the following error:

map<string, vector<int> > v2 = j_map;
or
map<string, vector<int> > v2 = j_map.get<map<string, vector<int>>>();

/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/5.3.0/../../../../include/c++/5
.3.0/bits/stl_pair.h:125:22: error: call to constructor of 'std::vector<int, s
td::allocator<int> >' is ambiguous

Is there a way to get around this error?

The library does conversions fine for map<string, int>

map<string, int> c_map{ { "one", 1 }, { "two", 4 }, { "three", 6 } };
json j_map(c_map);
cout << j_map.dump() << endl;
map<string, int > v2 = j_map;

I also tried this but did not work either

map<string, vector<int> > v2;
v2["one"] = j_map["one"];
main.cpp:249:13: error: use of overloaded operator '=' is ambiguous (with oper
and types 'mapped_type' (aka 'std::vector<int, std::allocator<int> >') and 'va
lue_type' (aka 'nlohmann::basic_json<std::map, std::vector, std::__cxx11::basi
c_string<char>, bool, long, unsigned long, double, std::allocator>'))

This final one works but looks ugly:

map<string, vector<int> > v2;
vector<int> one = j_map["one"];
v2["one"] = one;
vector<int> two = j_map["two"];
v2["two"] = two;

Many thanks

[nlohmann]

This could be related to #176.

[nlohmann]

I can reproduce the error.

Clang:

./../src/json.hpp:2450:20: note: in instantiation of function template specialization 'std::__1::map<std::__1::basic_string<char>, std::__1::vector<int, std::__1::allocator<int> >, std::__1::less<std::__1::basic_string<char> >, std::__1::allocator<std::__1::pair<const std::__1::basic_string<char>, std::__1::vector<int, std::__1::allocator<int> > > > >::map<std::__1::__map_iterator<std::__1::__tree_iterator<std::__1::__value_type<std::__1::basic_string<char>, nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator> >, std::__1::__tree_node<std::__1::__value_type<std::__1::basic_string<char>, nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator> >, void *> *, long> > >' requested here
            return T(m_value.object->begin(), m_value.object->end());

GCC:

../src/json.hpp:2450:68:   required from 'T nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType>::get_impl(T*) const [with T = std::map<std::__cxx11::basic_string<char>, std::vector<int> >; typename std::enable_if<(std::is_convertible<typename ObjectType<StringType, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType>, std::less<StringType>, AllocatorType<std::pair<const StringType, nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType> > > >::key_type, typename T::key_type>::value && std::is_convertible<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType>, typename T::mapped_type>::value), int>::type <anonymous> = 0; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long long int; NumberUnsignedType = long long unsigned int; NumberFloatType = double; AllocatorType = std::allocator]'

[nlohmann]

json j = {1,2,3,4};
std::vector<int> v(j);

yields

issue220.cpp:15:22: error: call to constructor of 'std::vector<int>' is ambiguous
    std::vector<int> v(j);
                     ^ ~
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:489:40: note: candidate constructor
    _LIBCPP_INLINE_VISIBILITY explicit vector(const allocator_type& __a)
                                       ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:501:14: note: candidate constructor
    explicit vector(size_type __n);
             ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:537:5: note: candidate constructor
    vector(initializer_list<value_type> __il);
    ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:555:5: note: candidate constructor
    vector(vector&& __x)
    ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:549:5: note: candidate constructor
    vector(const vector& __x);
    ^
1 error generated.

This error message is similar to the one of

std::map<std::string, std::vector <int> > v2 = j_map;

which is

issue220.cpp:11:52: note: in instantiation of function template specialization 'nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator>::operator map<std::__1::map<std::__1::basic_string<char>, std::__1::vector<int, std::__1::allocator<int> >, std::__1::less<std::__1::basic_string<char> >, std::__1::allocator<std::__1::pair<const std::__1::basic_string<char>, std::__1::vector<int, std::__1::allocator<int> > > > >, 0>' requested here
    std::map<std::string, std::vector <int> > v2 = j_map;
                                                   ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:489:40: note: candidate constructor
    _LIBCPP_INLINE_VISIBILITY explicit vector(const allocator_type& __a)
                                       ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:501:14: note: candidate constructor
    explicit vector(size_type __n);
             ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:537:5: note: candidate constructor
    vector(initializer_list<value_type> __il);
    ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:555:5: note: candidate constructor
    vector(vector&& __x)
    ^
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:549:5: note: candidate constructor
    vector(const vector& __x);
    ^
1 error generated.

Note that

json j = {1,2,3,4};
std::vector<int> v(j);

fails, whereas

json j = {1,2,3,4};
std::vector<int> v = j;

works.

[nlohmann]

If I understand the problem correctly, std::vector<int> v(j); fails, because a json value can be implicitly converted to a lot of types, including size_type. I am not sure whether we can do anything about this.

[nlohmann]

It seems that no one has an idea about this.