ios_base :: sync_with_stdio（false）在来自stdin的两个输入之间不起作用 [英] ios_base::sync_with_stdio(false) does not work between two inputs from stdin

问题描述

我想知道为什么以下代码无法按预期工作：

I am wondering why does the following code not work as expected:

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

int main(){
    int n;
    string s;
    //scanf("%d",&n);
    cin >> n;
    ios_base::sync_with_stdio(false);
    cin >> s;
    cout << n << " " << s;
    return 0;
}

输入：

10 
abcd 

输出： 10

字符串未打印！如果使用 scanf 输入整数，则结果相同。但是，如果将行 ios_base :: sync_with_stdio（false）; 放在第一个 cin （或者在 scanf 之前）。

The string is not getting printed! The result is same if I use scanf to input the integer. However, the code does work as expected if the line ios_base::sync_with_stdio(false); is placed before the first cin (or before the scanf) instead.

我非常感谢您提供任何帮助来澄清这种行为。

I would highly appreciate any help to clarify this behaviour.

编辑：
输入包含在文件 inp.txt 中，我正在使用< （重定向运算符）从文件中读取，并使用> 将结果输出到 out.txt ，例如：
a.out< inp.txt> out.txt

The inputs are contained in a file inp.txt and I am using < (re-direction operator) to read from the file and > to output the result to out.txt, like: a.out < inp.txt > out.txt

如果输入是直接从控制台给出的，则代码将提供预期的输出。对不起您的任何误解或困惑。

The code gives expected output if the inputs are given directly from console. Sorry for any misunderstanding or confusion.

推荐答案

调用 sync_with_stdio 执行完I / O导致实施定义的行为（比cppreference的有任何效果更强的措词）：

Calling sync_with_stdio after I/O has been performed leads to implementation-defined behavior (which is stronger wording than cppreference's "has any effect"):

效果：在调用之前使用标准流进行了输入或输出操作，效果是实现定义的
。否则，使用错误的参数调用它，它
允许标准流独立于标准C
流运行。

Effects: If any input or output operation has occurred using the standard streams prior to the call, the effect is implementation-defined. Otherwise, called with a false argument, it allows the standard streams to operate independently of the standard C streams.

在libc ++中，什么也没有发生，因为它所做的只是切换一个标志。在libstdc ++中，它实际上执行逻辑来切换流。使用该流的任何进一步尝试都会导致失败：

In libc++, nothing happens because all it does is toggle a flag. In libstdc++, it actually performs logic to switch the streams. Any further attempt to use the streams results in failure:

cin >> s;
cin.clear();
cin >> s;
std::cout << n << " " << s;

无论哪种方式， libstdc ++手册指出，您需要在执行I / O之前调用函数 。因此，您现在实际拥有的是未定义的行为。

Either way, the libstdc++ manual states you need to call the function before performing I/O. So what you actually have is undefined behavior now.

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