macro “min” passed 3 arguments, but takes just 2

Affected hardware

This error is not due to ArduinoJson but to a bug in the Arduino core. Many cores fixed this bug, but it remains in the one for SAMD21, so the following boards are affected:

Other cores are probably affected as well; please let me know.

Reproducing the error

This error can come up when you include ArduinoJson.h, but can also occur on an empty sketch. For example, take the following program:

#include <string>

void setup() {
  // put your setup code here, to run once:

}

void loop() {
  // put your main code here, to run repeatedly:

}

If you try to compile this sketch for an mkr1000, you’ll get the following output:

In file included from ...\arm-none-eabi\include\c++\7.2.1\bits\char_traits.h:39:0,
                 from ...\arm-none-eabi\include\c++\7.2.1\string:40,
                 from mysketch.ino:1:
...\arm-none-eabi\include\c++\7.2.1\bits\stl_algobase.h:243:56: error: macro "min" passed 3 arguments, but takes just 2
     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
                                                        ^
...\arm-none-eabi\include\c++\7.2.1\bits\stl_algobase.h:265:56: error: macro "max" passed 3 arguments, but takes just 2
     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
                                                        ^
exit status 1
Error compiling for board Arduino MKR1000.

This error occurs as soon as you include a header from the C++ standard library (also known as “STL”), such as <string>.

Why does this error occur?

This error comes from a conflict between the min() macro defined in Arduino.h and the std::min() function declared in stl_algobase.h.

Here is the definition of the macro:

#define min(a,b) ((a)<(b)?(a):(b))

And here is the declaration of the overload of std::min() that triggers the error:

template< class T, class Compare >
const T& min(const T& a, const T& b, Compare comp);

When the preprocessor parses this declaration, it tries to expand the min() macro but fails because the macro expects 2 arguments not 3.

A Pull Request should fix this bug someday.

How to fix this error?

ArduinoJson triggers this error because it includes <string>. To prevent that, you simply need to disable the support for std::string in the library by defining ARDUINOJSON_ENABLE_STD_STRING to 0.

But it won’t be enough because ArduinoJson.h also includes <istream> and <ostream> to support standard C++ streams. To disable this feature, you must define ARDUINOJSON_ENABLE_STD_STREAM to 0 as well.

Finally, your code should look like that:

#define ARDUINOJSON_ENABLE_STD_STRING 0
#define ARDUINOJSON_ENABLE_STD_STREAM 0
#include <ArduinoJson.h>

Depending on your situation, they might be other ways to fix this error. For example, you could ensure that <string> is included before Arduino.h, possibly using the -include command line option of the compiler.

See also