error: no matching function for call to ‘nlohmann::basic_json<>::value(int, std::set<int>&)’

[leonardltk]

Hi, i have a hashmap that i want to serialize/deserialize to/from JSON.
here is my code snippet:

#include <iostream>
#include "nlohmann/json.hpp"
#include <unordered_map>
#include <set>
using namespace std;

using json = nlohmann::json;

int main(){

    unordered_map <int, set<int>> shash;
    shash[3] = { 1 };
    shash[3].insert(33);
    shash[4] = { 44 };
    /*
    shash = {
        3 : {1,33},
        4 : {44},
    }
    */

    json jdEmployees = shash;

    set<int> nullSet;
    nullSet.insert(0);
    set<int> ValueSet = jdEmployees.value(3, nullSet);

    return 0;
}

I compiled it like this:

g++ -o ./debug.out -std=c++11 test.cpp nlohmann/json.hpp && ./debug.out

However i get this error:

test.cpp: In function ‘int main()’:
test.cpp:26:53: error: no matching function for call to ‘nlohmann::basic_json<>::value(int, std::set<int>&)’
     set<int> ValueSet = jdEmployees.value(3, nullSet);
                                                         ^
In file included from test.cpp:2:0:
nlohmann/json.hpp:20279:15: note: candidate: template<class ValueType, typename std::enable_if<(nlohmann::detail::is_getable<nlohmann::basic_json<>, ValueType>::value && (! std::is_same<nlohmann::detail::value_t, ValueType>::value)), int>::type <anonymous> > ValueType nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::value(const typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::key_type&, const ValueType&) const [with ValueType = ValueType; typename std::enable_if<(nlohmann::detail::is_getable<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>, ValueType>::value && (! std::is_same<nlohmann::detail::value_t, ValueType>::value)), int>::type <anonymous> = <enumerator>; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]
     ValueType value(const typename object_t::key_type& key, const ValueType& default_value) const
               ^
nlohmann/json.hpp:20279:15: note:   template argument deduction/substitution failed:
test.cpp:50:57: note:   cannot convert ‘3’ (type ‘int’) to type ‘const key_type& {aka const std::__cxx11::basic_string<char>&}’
         set<int> ValueSet = jdEmployees.value(3, nullSet);
                                                         ^
In file included from test.cpp:2:0:
nlohmann/json.hpp:20301:14: note: candidate: nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::string_t nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::value(const typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::key_type&, const char*) const [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::string_t = std::__cxx11::basic_string<char>; typename nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::object_t::key_type = std::__cxx11::basic_string<char>]
     string_t value(const typename object_t::key_type& key, const char* default_value) const
              ^
nlohmann/json.hpp:20301:14: note:   no known conversion for argument 1 from ‘int’ to ‘const key_type& {aka const std::__cxx11::basic_string<char>&}’
nlohmann/json.hpp:20351:15: note: candidate: template<class ValueType, typename std::enable_if<nlohmann::detail::is_getable<nlohmann::basic_json<>, ValueType>::value, int>::type <anonymous> > ValueType nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::value(const json_pointer&, const ValueType&) const [with ValueType = ValueType; typename std::enable_if<nlohmann::detail::is_getable<nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>, ValueType>::value, int>::type <anonymous> = <enumerator>; ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>]
     ValueType value(const json_pointer& ptr, const ValueType& default_value) const
               ^
nlohmann/json.hpp:20351:15: note:   template argument deduction/substitution failed:
test.cpp:50:57: note:   cannot convert ‘3’ (type ‘int’) to type ‘const json_pointer& {aka const nlohmann::json_pointer<nlohmann::basic_json<> >&}’
         set<int> ValueSet = jdEmployees.value(3, nullSet);
                                                         ^
In file included from test.cpp:2:0:
nlohmann/json.hpp:20375:14: note: candidate: nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::string_t nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::value(const json_pointer&, const char*) const [with ObjectType = std::map; ArrayType = std::vector; StringType = std::__cxx11::basic_string<char>; BooleanType = bool; NumberIntegerType = long int; NumberUnsignedType = long unsigned int; NumberFloatType = double; AllocatorType = std::allocator; JSONSerializer = nlohmann::adl_serializer; BinaryType = std::vector<unsigned char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::string_t = std::__cxx11::basic_string<char>; nlohmann::basic_json<ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType>::json_pointer = nlohmann::json_pointer<nlohmann::basic_json<> >]
     string_t value(const json_pointer& ptr, const char* default_value) const
              ^
nlohmann/json.hpp:20375:14: note:   no known conversion for argument 1 from ‘int’ to ‘const json_pointer& {aka const nlohmann::json_pointer<nlohmann::basic_json<> >&}’

[venkatpvc24]

leonardltk,

I have just read the docs for json, there is no member function value, that takes a container as seocnd parameter.

value methods

set<int> ValueSet = jdEmployees.value(3, nullSet); // you have to change this line to below

set<int> ValueSet = jdEmployees.value(3, " a default value that you want return, if key not found" );

[nlohmann]

Please have a look at the documentation and examples of the value function. It is only applicable to JSON objects.

[leonardltk]

set<int> ValueSet = jdEmployees.value(3, " a default value that you want return, if key not found" );

a default value has to be an integer? so it means i cannot define set<int> ValueSet, but rather auto ValueSet ?

[nlohmann]

value takes an object key as first parameter and can only be called on JSON objects. The second parameter must be a JSON value - you cannot pass a std::set<int> there.

Example:

#include <iostream>
#include <set>
#include "json.hpp"

using json = nlohmann::json;

int main()
{
    json j;
    
    // create a JSON object and store an array with 4 integers at key "key"
    j["key"] = {1,2,3,4};
    
    // access call via []
    std::set<int> s1 = j["key"];
    
    // access via value
    std::set<int> nullset;
    std::set<int> s2 = j.value("key", json(nullset));
    std::set<int> s3 = j.value("nonexisting", json(nullset));
}

[nlohmann]

Do you need further assistance with this issue?

[blandrum2]

I am doing a simple assignment data["Name"] = { 'W', 'O', '1' }; in Visual Studio 2022. It won't compile, I get the error code C2678, because of the error: "No operator "=" matches these operands.

I used this line: using json = nlohmann::json;. To include the namespace.

The declaration statement for data is:

nlohmann::json data;

I also tried another line of code that has worked for other coders:

data["key"] = { 1, 2, 3 };

This too generates the same error.

Why am I seeing this error?

[gregmarr]

@blandrum2 That's because there is no assignment operator on basic_json that takes an initializer_list. You need to construct the sub-object first and then assign it, such as by wrapping it as data["key"] = json({1,2,3}).