为什么`s.clear（ios :: badbit）;`下面？为什么不`s.clear（ios :: failbit）;`？ [英] Why `s.clear(ios::badbit);` below? Why not `s.clear(ios::failbit);`?

问题描述

我在调查这个问题，我对提供的两个答案没有任何问题。但我不确定我理解了 s.clear（ios :: badbit）; 在下面突出显示的语句与 //设置状态。例如，为什么 s.clear（ios :: failbit）; 改为？

I was looking into this question and I have no problem understanding the two answers given to it. But I'm not sure I understood the s.clear(ios::badbit); in the statement highlighted below with the comment // set state. For instance, why not s.clear(ios::failbit); instead?

#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;
} 

推荐答案

您引用的书是在1991年发布的，在第一个ISO C ++标准发布之前的7年。不清楚为什么作者选择使用 ios :: badbit ，因为在本书的第2版或第3版中没有提供理由。

The book you're quoting was published in 1991, 7 years before the first ISO C++ standard was even published. It's not clear why the author chose to use ios::badbit because no rationale was provided in either the 2nd or 3rd editions of the book.

在C ++ 98中，为<$ cc添加了 operator>> 的非成员重载$ c> complex 。这要求在输入不正确而不是 badbit 的情况下设置 failbit 。

In C++98, a non-member overload for operator>> was added for complex. This mandates that failbit is required to be set in the case of bad input rather than badbit.

N1905 26.2 / 13

N1905 26.2/13

template < class T , class charT , class traits >
basic_istream < charT , traits >&
operator > >( basic_istream < charT , traits >& is , complex <T >& x );

需要： 。

Requires: The input values be convertible to T.

如果遇到错误输入，则调用 is.setstate（ios： ：failbit）（其中
可以抛出 ios :: failure （27.4.4.3）。

If bad input is encountered, calls is.setstate(ios::failbit) (which may throw ios::failure (27.4.4.3).

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