c++ - istream_iterator 行为

Question

我有两段代码。它们在 main() 中单独使用时可以正常工作。

vector<int> v;

cout << "Enter sequance of integers "<< "(press q to quit) : ";
istream_iterator<int> start_cin(cin);
istream_iterator<int> end_of_cin;
copy(start_cin,end_of_cin,back_inserter(v));

for ( vector<int>::iterator It = v.begin();It != v.end(); It++ )
    cout << *It << "\t";
cout << endl;

和

vector<string> vS;
cout << "Enter three strings : ";
for ( int i = 0; i < 3; i++ )
    vS.push_back(*istream_iterator<string>(cin));

ostream_iterator<string> sIt(cout,", ");
copy(vS.begin(),vS.end(),sIt);
cout << endl;

当这两部分一起使用时，即

#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
#include <string>
using namespace std;

int main ()
{
    // first part

    vector<int> v;

    cout << "Enter sequance of integers "<< "(press q to quit) : ";
    istream_iterator<int> start_cin(cin);
    istream_iterator<int> end_of_cin;
    copy(start_cin,end_of_cin,back_inserter(v));

    for ( vector<int>::iterator It = v.begin();It != v.end(); It++ )
        cout << *It << " \t";
    cout << endl;


    vector<string> vS;
    cout << "Enter three strings : ";
    for ( int i = 0; i < 3; i++ )
        vS.push_back(*istream_iterator<string>(cin));

    ostream_iterator<string> sIt(cout,", ");
    copy(vS.begin(),vS.end(),sIt);

    cout << endl;

    return 0;
}

此处第一部分有效，但第二部分给出输出:Enter Three Strings : , , ,。我想知道这种行为背后的原因是什么？

谢谢。

Answer 1

copy()完成后，cin会处于不可读状态(!cin.good())，由于读取“整数”q 失败。这意味着后续的 for 循环将无法读取任何内容。

添加:

cin.clear();
cin.ignore(); // To skip the unread "q"

在 for 循环之前。

编辑:

正如 James Kanze 评论的那样, 检查以确保 "q" 是 copy() 终止的原因:

...

cin.clear();
string int_read_terminator;
cin >> int_read_terminator;
if ("q" != int_read_terminator)
{
    cerr << "Integer copy() failure: " << int_read_terminator << "\n";
}
else
{
    ...