Hi,

this simple program does not compile:

class Foo{};
void bar(Foo f) {}

void setup() {}
void loop() {} 

The weird error message is: 2:10: error: 'Foo' was not declared in this scope.

The reason is the translation from .ino to .cpp into the build folder before compilation. Here the corresponding .cpp file:

#include <arduino.h>
#line 1 "C:\\Users\\Ulli\\Documents\\Arduino\\CompileTest\\CompileTest.ino"
#line 1 "C:\\Users\\Ulli\\Documents\\Arduino\\CompileTest\\CompileTest.ino"

void bar(Foo f);
//
//

#line 1 "C:\\Users\\Ulli\\Documents\\Arduino\\CompileTest\\CompileTest.ino"
class Foo{};
void bar(Foo f) {}

void setup() {}
void loop() {} 

A prototype for 'bar' ist inserted before the class definition. The problem is the misleading error message. In a bigger project it took me several hours to find out the real problem.

Is it really necessary to generate these prototypes?

Thanks for the hint. I did not consider the special behavior of an .ino file.

It is okay to put the prototype anywhere in the .ino file.