freopen() 等效于 C++ 流
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freopen() equivalent for c++ streams
提问by flight
When programming with c-style i/o I sometimes use freopen() to reopen stdin for testing purposes so that I don't have to retype the input over and over. I was wondering if there is an equivalent for c++ i/o streams. Also, I know that I can use pipes to redirect it on the command line/terminal/whateveritis but I was wondering if there was a way to do it inside my code (because as you can see, I'm not very knowledgeable about the cl/t/w).
当使用 c 风格的 i/o 编程时，我有时会使用 freopen() 重新打开 stdin 以进行测试，这样我就不必一遍又一遍地重新输入输入。我想知道 c++ i/o 流是否有等价物。另外，我知道我可以使用管道在命令行/终端/任何东西上重定向它，但我想知道是否有办法在我的代码中做到这一点（因为如你所见，我对升/吨/瓦）。
回答by UmmaGumma
freopenalso works with cinand cout. No need to search for something new.
freopen也适用于cin和cout。无需寻找新的东西。
freopen("input.txt", "r", stdin); // redirects standard input
freopen("output.txt", "w", stdout); // redirects standard output
int x;
cin >> x; // reads from input.txt
cout << x << endl; // writes to output.txt
Edit:From C++ standard 27.3.1:
编辑：来自 C++ 标准 27.3.1：
The object cin controls input from a stream buffer associated with the object stdin, declared in
<cstdio>.
对象 cin 控制来自与对象 stdin 关联的流缓冲区的输入，在 中声明
<cstdio>。
So according to the standard, if we redirect stdinit will also redirect cin. Vice versa for cout.
所以根据标准，如果我们重定向stdin它也会重定向cin. 反之亦然cout。
回答by Rob?
#include <iostream>
#include <fstream>
int main() {
// Read one line from stdin
std::string line;
std::getline(std::cin, line);
std::cout << line << "\n";
// Read a line from /etc/issue
std::ifstream issue("/etc/issue");
std::streambuf* issue_buf = issue.rdbuf();
std::streambuf* cin_buf = std::cin.rdbuf(issue_buf);
std::getline(std::cin, line);
std::cout << line << "\n";
// Restore sanity and read a line from stdin
std::cin.rdbuf(cin_buf);
std::getline(std::cin, line);
std::cout << line << "\n";
}
回答by Lightness Races in Orbit
Thisnewsgroup posting explores your options.
此新闻组帖子探讨了您的选择。
This is system dependent and the poster didn't indicate the system, but cin.clear() should work. I have tested the attached program on a UNIX system with AT&T version's of iostreams.
这是系统相关的，海报没有指出系统，但 cin.clear() 应该可以工作。我已经在带有 AT&T 版本的 iostreams 的 UNIX 系统上测试了附加的程序。
#include <iostream.h>
int main()
{
for(;;) {
if ( cin.eof() ) {
cout << "EOF" << endl;
cin.clear();
}
char c ;
if ( cin.get(c) ) cout.put(c) ;
}
}
Yes, that works okay in cfront and TC++. In g++ where the problem first arose an additional action is required:
是的，这在 cfront 和 TC++ 中工作正常。在首先出现问题的 g++ 中，需要额外的操作：
cin.clear();
rewind ( _iob ); // Seems quite out of place, doesn't it?
// cfront also accepts but doesn't
// require this rewind.
Though I note that this was in 1991, it should still work. Remember to use the now-standard iostreamheader, not iostream.h.
虽然我注意到这是在 1991 年，但它应该仍然有效。请记住使用现在标准的iostream标头，而不是iostream.h.
(BTW I found that post with the Google search terms "reopen cin c++", second result.)
（顺便说一句，我发现该帖子带有 Google 搜索词“reopen cin c++”，第二个结果。）
Let us know how you get on. You could also just use freopen.
让我们知道您的身体情况如何。你也可以只使用freopen.
