Hi, 

Thanks for the example. 

Arduino and visual micro have to auto generate prototypes for functions if they do not exist. In the not too distant past arduino has changed to make a better job at automatic prototype insertion, although still not perfect. 

This is one of a couple of areas that visual micro differs from Arduino, but will shortly change/improve.  With visual micro (and c++) if you have your own types mixed in with your code you should insert prototypes at the location of that you choose.

A prototype is simply the exact same signature as the function in your code followed by a semi-colon ;

typedef struct RGB
{
	union
	{
		byte r, g, b;
		byte arr[3];
	};
} RGB;


/ * insert c++ prototypes before they are used */

RGB rgb(byte r, byte g, byte b);


/* main program code below this line  */

RGB rgb(byte r, byte g, byte b)
{
	RGB color;
	color.r = r;
	color.g = g;
	color.b = b;
	return color;
}
 

For complex prototypes it is better to switch off "auto prototype generation" which can be done in the F4 project properties tool window.

tip: If you declare user defined types in #include files or add your ow prototypes then your .ino code will move more easily to .cpp files as the project grows

If you have functions in .ino that include user defined types in their signature then you should put the forward declarations where they make sense. The arduino ide will see you have done that and not attempt to duplicate the declarations.

It's an anomaly of c++ in arduino .ino. Alternatively declare your types in header files so they can more easily be re-used.

I expect the future will be more forgiving so not to worry if it works now it's fine.