问题
As I understand, the extraction operator skips the whitespace in the beginning and stops upon encountering a whitespace or end of stream. noskipws can be used to stop ignoring the leading whitespaces.
I have the following program where I have used noskipws.
#include <iostream>
using namespace std;
int main()
{
char name[128];
cout<<"Enter a name ";
cin>>noskipws>>name;
cout<<"You entered "<<name<<"\n";
cout<<"Enter another name ";
cin>>name;
cout<<"You entered "<<(int)name[0]<<"\n";
return 0;
}
My queries are:
If I enter "John" as the first input, then the second cin>> operation does not wait for input and does not copy anything to the destination i.e. the name array. I expected second cin>> to transfer at-least a newline or end of stream, instead of just setting the destination string to empty. Why is this happening ?
The same thing is observed when I enter "John Smith" as the input for first cin>> statement. Why doesn't the second cin>> statement copy the space or "Smith" to the destination variable ?
Following is the output of the program:
Enter a name John
You entered John
Enter another name You entered 0
Enter a name John Smith
You entered John
Enter another name You entered 0
Thanks!!!
回答1:
The basic algorithm for >> of a string is:
skip whitespace
read and extract until next whitespace
If you use noskipws, then the first step is skipped. After the first read, you are positionned on a whitespace, so the next (and all following) reads will stop immediatly, extracting nothing.
>> to a string will never put whitespace into the string. More generally, using >> with noskipws is problematic, since whitespace is always a separator for >>; it may make sense to use it punctually, but it should generally be reset immediately after it has been used. (The once case where it might make sense is when using >> to a char. In this case, the stream always extracts one character.)
来源：https://stackoverflow.com/questions/10946771/effect-of-noskipws-on-cin