5.5 is a double, but none of your functions take a double argument. So, the compiler gets confused on whether to call the function with the int parameter, or the function with the float parameter. So, you get a an error saying it is ambiguous.
That is why when you changed the function to have a double parameter, the error no longer came, because now there is a function which can take a double argument, and thus there is ambiguity there.
You can also fix the problem by calling the function as
obj.add(5.5f);
Adding the f after a number makes it to a float.
Let's look at the C++ Standard
§ 2.13.4
1 A floating literal consists of an integer part, a decimal point, a
fraction part, an e or E, an optionally signed integer exponent, and
an optional type suffix. The integer and fraction parts both consist
of a sequence of decimal (base ten) digits. Optional separating single
quotes in a digit-sequence are ignored when determining its value. [
Example: The literals 1.602’176’565e-19 and 1.602176565e-19 have the
same value. —end example ] Either the integer part or the fraction
part (not both) can be omitted; either the decimal point or the letter
e (or E ) and the exponent (not both) can be omitted. The integer
part, the optional decimal point and the optional fraction part form
the significant part of the floating literal. The exponent, if
present, indicates the power of 10 by which the significant part is to
be scaled. If the scaled value is in the range of representable values
for its type, the result is the scaled value if representable, else
the larger or smaller representable value nearest the scaled value,
chosen in an implementation-defined manner. The type of a floating
literal is double unless explicitly specified by a suffix. The
suffixes f and F specify float, the suffixes l and L specify long
double. If the scaled value is not in the range of representable
values for its type, the program is ill-formed.
( Sorry for posting all of it, but you can learn more about floats this way )
