How do we get this error?

If you try to compile the following program:

DynamicJsonDocument doc;

you’ll get this horrible error:

MyProgram.ino: In function 'void setup()':
MyProgram:21:23: error: no matching function for call to 'ArduinoJson6130_0000010::BasicJsonDocument<ArduinoJson6130_0000010::DefaultAllocator>::BasicJsonDocument()'
   DynamicJsonDocument doc;
                       ^~~
In file included from ArduinoJson/Document/DynamicJsonDocument.hpp:7:0,
                 from ArduinoJson.hpp:21,
                 from ArduinoJson.h:9,
                 from MyProgram.ino:9:
ArduinoJson/Document/BasicJsonDocument.hpp:56:3: note: candidate: ArduinoJson6130_0000010::BasicJsonDocument<TAllocator>::BasicJsonDocument(ArduinoJson6130_0000010::VariantRef) [with TAllocator = ArduinoJson6130_0000010::DefaultAllocator]
   BasicJsonDocument(VariantRef src)
   ^~~~~~~~~~~~~~~~~
ArduinoJson/Document/BasicJsonDocument.hpp:56:3: note:   candidate expects 1 argument, 0 provided
ArduinoJson/Document/BasicJsonDocument.hpp:49:3: note: candidate: template<class T> ArduinoJson6130_0000010::BasicJsonDocument<TAllocator>::BasicJsonDocument(const T&, typename ArduinoJson6130_0000010::enable_if<ArduinoJson6130_0000010::IsVisitable<T>::value>::type*)
   BasicJsonDocument(const T& src,
   ^~~~~~~~~~~~~~~~~
ArduinoJson/Document/BasicJsonDocument.hpp:49:3: note:   template argument deduction/substitution failed:
MyProgram.ino:21:23: note:   candidate expects 2 arguments, 0 provided
   DynamicJsonDocument doc;
                       ^~~
In file included from ArduinoJson/Document/DynamicJsonDocument.hpp:7:0,
                 from ArduinoJson.hpp:21,
                 from ArduinoJson.h:9,
                 from MyProgram.ino:9:
ArduinoJson/Document/BasicJsonDocument.hpp:42:3: note: candidate: ArduinoJson6130_0000010::BasicJsonDocument<TAllocator>::BasicJsonDocument(const ArduinoJson6130_0000010::BasicJsonDocument<TAllocator>&) [with TAllocator = ArduinoJson6130_0000010::DefaultAllocator]
   BasicJsonDocument(const BasicJsonDocument& src)
   ^~~~~~~~~~~~~~~~~
ArduinoJson/Document/BasicJsonDocument.hpp:42:3: note:   candidate expects 1 argument, 0 provided
ArduinoJson/Document/BasicJsonDocument.hpp:39:12: note: candidate: ArduinoJson6130_0000010::BasicJsonDocument<TAllocator>::BasicJsonDocument(size_t, TAllocator) [with TAllocator = ArduinoJson6130_0000010::DefaultAllocator; size_t = unsigned int]
   explicit BasicJsonDocument(size_t capa, TAllocator allocator = TAllocator())
            ^~~~~~~~~~~~~~~~~
ArduinoJson/Document/BasicJsonDocument.hpp:39:12: note:   candidate expects 2 arguments, 0 provided

Understanding the error

Extract the signal from the noise

It’s a long output, but it’s not that complicated. Once you remove all the noise (namespaces, template parameters, file paths…), it’s just:

error: no matching function for call to 'BasicJsonDocument::BasicJsonDocument()'
   DynamicJsonDocument doc;
                       ^~~

note: candidate: BasicJsonDocument::BasicJsonDocument(VariantRef)
   BasicJsonDocument(VariantRef src)
   ^~~~~~~~~~~~~~~~~
note:   candidate expects 1 argument, 0 provided

note: candidate: template<class T> BasicJsonDocument::BasicJsonDocument(const T&, typename enable_if<IsVisitable<T>::value>::type*)
   BasicJsonDocument(const T& src,
   ^~~~~~~~~~~~~~~~~
note:   template argument deduction/substitution failed:
note:   candidate expects 2 arguments, 0 provided

note: candidate: BasicJsonDocument::BasicJsonDocument(const BasicJsonDocument&)
   BasicJsonDocument(const BasicJsonDocument& src)
   ^~~~~~~~~~~~~~~~~
note:   candidate expects 1 argument, 0 provided

note: candidate: BasicJsonDocument::BasicJsonDocument(size_t, TAllocator)
   explicit BasicJsonDocument(size_t capa, TAllocator allocator = TAllocator())
            ^~~~~~~~~~~~~~~~~
note:   candidate expects 2 arguments, 0 provided

What does it mean?

What the compiler is trying to say is this: when compiling DynamicJsonDocument doc;, I’ve been looking for BasicJsonDocument::BasicJsonDocument() but I didn’t find it. Instead, I found:

  1. BasicJsonDocument(VariantRef src)
  2. BasicJsonDocument(const T&, typename enable_if<IsVisitable<T>::value>::type*)
  3. BasicJsonDocument(const BasicJsonDocument& src)
  4. BasicJsonDocument(size_t capa, TAllocator allocator = TAllocator())

So, the compiler tried to find a constructor with no parameter, but it didn’t find one. This means we forgot to pass an argument to the constructor.

How does this relate to DynamicJsonDocument?

DynamicJsonDocument is an alias of BasicJsonDocument; it’s defined like that:

typedef BasicJsonDocument<DefaultAllocator> DynamicJsonDocument;

That’s why the compiler says BasicJsonDocument when we use DynamicJsonDocument

How to solve the error?

To fix this error, you must pass an argument to the constructor of DynamicJsonDocument. It’s an integer that specifies the capacity of the memory pool; it’s a mandatory argument in ArduinoJson 6.

DynamicJsonDocument doc(capacity);

To choose the right capacity for your project, use the ArduinoJson Assistant, or read How to determine the capacity of the JsonDocument?