Why do user-defined string literals and integer literals have different behavior?

Question

I'm learning about user-defined literals, and confused with the following test code:

std::chrono::seconds operator"" _s(unsigned long long s) {
    return std::chrono::seconds(s);
}

std::string operator"" _str(const char *s, std::size_t len) {
    return std::string(s, len);
}

int main() {
    auto str = "xxxxx"_str;
    std::cout << str.size() << std::endl;    // works

    auto sec = 4_s;
    std::cout << sec.count() << std::endl;   // works

    std::cout << "xxxxx"_str.size() << std::endl;   // works

    std::cout << 4_s.count() << std::endl;   // does **NOT** work!

    return 0;
}

The compiler gives the following error message:

error: no matching literal operator for call to 'operator""_s.count' with argument of type 'unsigned long long' or 'const char *', and no matching literal operator template
cout << 4_s.count() << endl;

It seems that it takes _s.count as a user-defined literal. Also, a floating-point literal behaves like an integer literal.

Why do user-defined integer literals and string literals have different behavior?

(4_s).count works. You CANNOT have ( or ) in the ud-suffix. — for_stack
@for_stack It seems weird, . could not be used in ud-suffix either; can't get why _s.count is parsed as the whole ud-suffix. — songyuanyao
@songyuanyao Good to see you. It's not weird if an explanation can be found. — iBug
4_s.count is parsed as a preprocessing number token, which is tolerated by the preprocessor and is converted to a token after preprocessing but before syntactic and semantic analysis, but it can't be converted to a valid token (e.g. interger literal or floating literal), and thus violates [lex.pptoken]/1. Admittedly the compiler error is totally unhelpful in this case. — cpplearner

iBug iBug · Accepted Answer · 2017-11-24T06:24:33

That's how floating point literals work!!

Add a pair of parentheses and it should work:

std::cout << (4_s).count();

Or alternatively, separate them (to stop the compiler from interpreting it as an ill-formed fractional constant floating point literal):

std::cout << 4_s .count();
//              ^ Space here!

Reference: CppReference.com

In the Notes section of the reference above,

Due to maximal munch, user-defined integer and floating point literals ending in [p, P, (since C++17)] e and E, when followed by the operators + or -, must be separated from the operator with whitespace in the source:
long double operator""_E(long double);
long double operator""_a(long double);
int operator""_p(unsigned long long);

auto x = 1.0_E+2.0;  // error
auto y = 1.0_a+2.0;  // OK
auto z = 1.0_E +2.0; // OK
auto w = 1_p+2;      // error
auto u = 1_p +2;     // OK

So when it comes to dot, which is used as decimal point, it must be separated from anything behind, or it'll be treated as part of the floating point number.

I have tested the example above from CppReference and got a very similar error message:

test.cpp:19:10: error: unable to find numeric literal
operator 'operator""_E+2.0'
                    ^^^^^^
 auto x = 1.0_E+2.0;  // error

Got the point how _E+2.0 is considered as a whole ud-suffix?

My original explanation attempt can be found in the revision history of this post.

Why do user-defined string literals and integer literals have different behavior?

1 Answers