Is this a bug with getline(), or am I doing something wrong. Right way to use getline()?

问题

It might not be a bug, but I don't know what is going wrong. My first entry is repeated for str1 on 2nd iteration, and is same way from then. Only first iteration goes good.

#include <iostream>
#include <string>
using namespace std;

int main () {

cout << " \n Enter two words. \n " ;
char c = 'y';
string str;
string str1;
while (c == 'y'){

    getline(cin,str);

    getline (cin,str1);

    cout << " \n\n str : " << str << " str1 : " << str1 ;
    cout << " \n Continue ? \n " ;
    cin >> c;
}

return 0;
}

The output is :

 Enter two words.
 hello world
this is mr


 str : hello world str1 : this is mr
 Continue ?
 y
hello world


 str :  str1 : hello world
 Continue ?
 n

回答1:

Add

cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );

after your

cin >> c;

Consider the following input:

    dog
    cat
    y
    owl
    fish
    n

If we examine the characters that are present in the input stream individually, we'll see:

    d o g \n c a t \n y \n o w l \n f i s h \n n \n

The first call to getline consumes dog\n; the second consumes cat\n, leaving this:

    y \n o w l \n f i s h \n n \n

The first call to cin >> c consumes only y but not the subsequent newline, leaving this:

    \n o w l \n f i s h \n n \n

Now, the fun begins: What happens during the next call to getline? Why it reads up to the next newline, of course. So the next call to getline returns an empty line, and leaves owl... in the input stream.

The solution, as I outlined above, is to consume the remainder of the (now useless) input line.

回答2:

As rob says.

But an alternative fix that looks nicer:

// change
char c = 'y';
....
while (c == 'y'){
....
    cin >> c;

// Into
std::string c = "y";
....
while (c == "y"){
....
    std::getline(cin, c);

When dealing with manual user input you should be careful of using the >> operator as this will always leave the '\n' on the input. Which means you can either use a method that retrieves the '\n' character (getline()) or you can manually remove it afterwords (ignore()).

来源：https://stackoverflow.com/questions/11144834/is-this-a-bug-with-getline-or-am-i-doing-something-wrong-right-way-to-use-ge

标签

c++

istream