What did the author mean with his comment on user type input?

Question

This is an example extracted from section 10.3.3. Entering custom types from the book "C ++ Programming Language", second edition, B. Stroustrup. The code is old, but still compiles with minor changes. Example:

#include <istream>
#include <complex>

using namespace std;

istream& operator>>(istream& s, complex<double>& a)
{
    // input formats for a complex; "f" indicates a float:
    //
    //   f
    //   (f)
    //   (f, f)

    double re = 0, im = 0;
    char c = 0;

    s >> c;
    if( c == '(' ) {
        s >> re >> c;
        if( c == ',' ) s >> im >> c;
        if( c != ')' ) s.clear(ios::badbit);  // set state
    }
    else {
        s.putback(c);
        s >> re;
    }

    if( s ) a = complex<double>(re, im);
    return s;
}

Despite the lack of error handling code, this will actually be the majority of types of errors. The local variable is cinitialized in order to avoid after the emergency operation << 22> the value of the'(' final check of the flow state ensures that the value of the argument achanges if everything goes well.

I could not understand the phrase underlined above.

Answer 1

s >> c , c .

c , if( c == '(' ). char undefined.

undefined.

---

char c = 0; , s.putback(c); , s good(). , :

char c;
s >> c;

if ( !s )
    return s;

, , , ; , , .

Answer 2

, , c:

char c;
s >> c;
if (c == '(') { ... }

, >> . , c == '(', :

• >> , '(' istream.
• >>, , c, , '('.

c 0, 2 : , , c 0... , '(' istream.