Question

在istream中，我使用cin.getline（）接受一些字符并输入EOF信号（因为我的操作系统是MAC，我按下控制+ D）然后第二个cin.getline（）接受其余的流。但是，我测试cin.eof（）的值，在第一个cin.getline（）之前，第一个和第二个cin.getline（）之间，以及最后。在这个程序中，我都使用EOF信号来终止这三个cin.getline（）;

代码：

#include<iostream>
using namespace std;
int main()
{
    cout<<"now EOF:"<<cin.eof()<<endl;
    char character[10];
    cout<<"input ten digit character,with '!' in the middle,end with EOF(ctrl+d):"<<endl;
    cin.getline(character, 10,'!');
    cout<<endl;
    cout<<"get the character:"<<endl;
    cout<<character<<endl;
    char character2[10];
    cout<<"now EOF:"<<cin.eof()<<endl;
    cout<<"press(ctrl+d):"<<endl;
    cin.getline(character2, 10,'!');
    //cin>>character2;
    cout<<endl;
    cout<<character2<<endl;
    cout<<"now EOF:"<<cin.eof()<<endl;


}

结果在这里：

now EOF:0
input ten digit character,with '!' in the middle,end with EOF(ctrl+d):
123!456

get the character:
123
now EOF:0
press(ctrl+d):

456

now EOF:1

但是当我用注释的部分cin＆gt;＆gt; character2代替cin.getline（character2,10，＆＃39;！＆＃39;）时：

结果是：

now EOF:0
input ten digit character,with '!' in the middle,end with EOF(ctrl+d):
123!456

get the character:
123
now EOF:0
press(ctrl+d):

456
now EOF:0

我想知道，为什么会发生这种情况以及cin.eof（）值如何变化。谢谢！

Answer 1

在你的第二个例子中，新行字符仍可供阅读 - 你已经读过一个字符串，而不是一行。

在输出中查看缺少的额外换行符？这就是线索。

但更重要的是，请勿使用eof。见Why is “while ( !feof (file) )” always wrong?

Answer 2

如果<div class="quotation"> <img><img src="/assets/quote.png" alt="Quote sign" /></img> <blockquote>The tipping point is that magic moment when an idea, trend, or social behavior crosses a threshold, tips, and spreads like wildfire.<p>Malcolm Gladwell</p></blockquote> </div>返回menuData = { "menu": [ { "id": 0, "name": "DashBoard", "image":"/images/dashboard.gif", "link": "blah.html", "subMenu": [ { "id": 10, "name": "Running", "image": null, "link": "blah.html", "subMenu": null }, { "id": 11, "name": "Alarms", "image": null, "link": "blah.html", "subMenu": null }, { "id": 12, "name": "Moisture", "image": null, "link": "blah.html", "subMenu": [ { "id": 20, "name": "Overview", "image": null, "link": "blah.html", "subMenu": null }, { "id": 21, "name": "M. Details", "image": null, "link": "blah.html", "subMenu": null } ] } ] }, { "id": 1, "name": "Programs", "image":"/images/programs_on.gif", "subMenu": [ { "id": 30, "name": "Intro", "image": null, "link": "programs.html", "subMenu": null }, { "id": 31, "name": "Pg View", "image": null, "link": "programs.html", "subMenu": null }, { "id": 32, "name": "Programs", "image": null, "link": "programs.html", "subMenu": [ { "id": 40, "name": "Program #1", "image": null, "link": "programs.html", "subMenu": [ { "id": 50, "name": "Start Time", "image": null, "link": "programs.html", "subMenu": null }, { "id": 51, "name": "Station Time", "image": null, "link": "programs.html", "subMenu": null }, { "id": 52, "name": "Program Setting", "image": null, "link": "programs.html", "subMenu": null } ] }, { "id": 41, "name": "Program #2", "image": null, "link": "programs.html", "subMenu": [ { "id": 60, "name": "Start Time", "image": null, "link": "programs.html", "subMenu": null }, { "id": 61, "name": "Station Time", "image": null, "link": "programs.html", "subMenu": null }, { "id": 62, "name": "Program Setting", "image": null, "link": "programs.html", "subMenu": null } ] } ] } ] } ]}; Object.prototype.getNestedValue = function(...a) { return a.length > 1 ? (this[a[0]] !== void 0 && this[a[0]].getNestedValue(...a.slice(1))) : this[a[0]]; }; document.write(menuData.getNestedValue("menu",1,"subMenu",2,"subMenu",0,"subMenu",1,"id"));，则表示上一次输入操作失败，因为该流位于文件的末尾。这就是为什么你不能用它来控制输入循环的原因：当它真实时，输入已经失败了。这就是为什么输入操作本身可以告诉你它们是否成功的原因：只要输入成功，你就可以循环。这段代码

istream::eof()

每次true调用读取一行时，

都会执行循环。当while (cin.getline(whatever)) { do_something(whatever); }到达输入的末尾时，getline调用将失败，并且循环不会被执行。这段代码：

cin

即使对getline的调用失败，

也会调用while (!cin.eof()) { cin.getline(whatever); do_something(whatever); }。在之后调用失败，在下一次循环中，do_something将返回true并且循环将退出。

C ++ cin.eof（）的值代表什么？

2 个答案: