The compiler attempts to save space in the executable by not including the float-to-string format conversion routines unless they are necessary, and sometimes does not recognize some code that does require it. Taking the address of a float in an argument list of a function call seems to trigger it, but taking the address of a float element of a struct may fool the compiler. A "%f" in a printf/scanf format string doesn't trigger it because format strings aren't examined at compile-time.
You can fix it by (1) using <iostream.h> instead of <stdio.h>, or (2) by including the following function definition somewhere in your compilation (but don't call it!):
