Fixes #2728

includes some macros to be defined for using without file io.

include/nlohmann/detail/output/output_adapters.hpp

@@ -50,6 +50,7 @@ class output_vector_adapter : public output_adapter_protocol<CharType>
     std::vector<CharType>& v;
 };
 
+#ifndef JSON_NO_IO
 /// output adapter for output streams
 template<typename CharType>
 class output_stream_adapter : public output_adapter_protocol<CharType>
@@ -73,6 +74,7 @@ class output_stream_adapter : public output_adapter_protocol<CharType>
   private:
     std::basic_ostream<CharType>& stream;
 };
+#endif  // JSON_NO_IO
 
 /// output adapter for basic_string
 template<typename CharType, typename StringType = std::basic_string<CharType>>
@@ -105,8 +107,10 @@ class output_adapter
     output_adapter(std::vector<CharType>& vec)
         : oa(std::make_shared<output_vector_adapter<CharType>>(vec)) {}
 
+#ifndef JSON_NO_IO
     output_adapter(std::basic_ostream<CharType>& s)
         : oa(std::make_shared<output_stream_adapter<CharType>>(s)) {}
+#endif  // JSON_NO_IO
 
     output_adapter(StringType& s)
         : oa(std::make_shared<output_string_adapter<CharType, StringType>>(s)) {}