输入流的std :: setfill和std :: setw吗？

Question

Consider this code:

int xx;
std::cin >> std::setfill('0') >> std::setw(4) >> xx;

When sending 12 to the standard input I am expecting the value of xx to be 1200 and when sending 12345 I am expecting it to be 1234 .

However, it seems std::setfill and std::setw have no effect and I am getting 12 and 12345 respectively.

Is this a bug or it is according to the standard? Is there a good way to get the expected functionality?

Also note, when I change the type of xx to be std::string the std::setw takes effect while std::setfill still doesn't.

My compiler is gcc-7.0.1 .

Answer 1

According to C standard, setfill pertains to output stream. As for the setw , it works for input stream when used together with char* or string . For example, following program outputs abcd for input string abcdef (and 1234 for 123456 ):

string a;
cin >> setw(4) >> a;
cout << a;

Answer 2

setw and setfill are not applied so universally.

It sounds like you want to imitate the effect of formatting the given input in a fixed-width column, then re-reading it. The library does provide tools for exactly that:

int widen_as_field( int in, int width, char fill ) {
    std::stringstream field;
    field << std::setw( width ) << std::setfill( fill );
    field << std::setiosflags( std::ios::left );
    field << in;
    int ret;
    field >> ret;
    return ret;
}

Demo .

This function will not trim 12345 to 1234 , though. That would take another conversion through string .