Why cannot you use a 'f' suffix directly after an integer?

Question

While writing a C++ application, I have found out that I cannot use the f suffix (e.g. 3f) as Visual Studio shows me the following error: "Literal operator not found". The problem, of course, disappears when I use the .f suffix(e.g. 3.f). Why is that?

Answer 1

Here are the rules from the standard:

floating-point-literal:
    decimal-floating-point-literal
    hexadecimal-floating-point-literal

decimal-floating-point-literal:
    fractional-constant exponent-part-opt floating-point-suffix-opt
    digit-sequence exponent-part floating-point-suffix-opt

fractional-constant:
    digit-sequence-opt . digit-sequence
    digit-sequence .

exponent-part:
    e sign-opt digit-sequence
    E sign-opt digit-sequence

floating-point-suffix: one of
    f  l  F  L

As you can see, the . has to be always included in a decimal floating point constant, if there is no exponent part.

The K&R book explains this as:

Floating-point constants contain a decimal point ( 123.4 ) or an exponent ( 1e-2 ) or both; their type is double, unless suffixed. The suffixes f or F indicate a float constant; l or L indicate a long double .

So, the thinking is, that f or l modifies an already floating-point constant (double) to have the type of float or long double. The suffix itself doesn't make a non-floating-point constant to a floating-point constant.