When compiling this code (without any header)
template <typename T>
struct Temperature {
T temp;
explicit Temperature(T t)
: temp(t)
{}
};
Temperature<long double> operator "" _f (long double t)
{
return Temperature<long double>((t - 32) / 1.8);
}
int main()
{
auto t = 100.0_f;
t.temp;
100.0_f.temp; // ERROR AT THIS LINE
return 0;
}
The compilers (both g++ 4.8 and clang++ 3.4 on Ubuntu 14.04) will complain that
error: unable to find numeric literal operator ‘operator"" _f.temp’
100.0_f.temp;
^
It seems that the _f.temp is considered as a suffix there. Why do the compilers parse it like that, instead of stopping at the dot?
Preprocessing numbers are odd beasts, specified mostly to make the preprocessor easier to write.
pp-number:
digit
. digit
pp-number digit
pp-number identifier-nondigit
pp-number ' digit
pp-number ' nondigit
pp-number e sign
pp-number E sign
pp-number p sign
pp-number P sign
pp-number .
12 is a valid pp-number token, so is 0xe+foo (see the example in [lex.pptoken]/4), and so is .12.CA'TS_RULE..56.me+owp-urr. If the latter two make it past translation phase 6, then the program is ill-formed because it cannot be converted to a valid token in phase 7. Until then, however, it is valid, so maximal munch says we parse 0xe+foo or 100.0_f.temp as a single preprocessing token.
来源：https://stackoverflow.com/questions/38091427/compile-error-when-using-a-member-of-a-user-defined-literal