如何将 cin 和 cout 重定向到文件？

Question

如何将cin 重定向到in.txt 并将cout 重定向到out.txt？

Answer 1

这是您想要做的工作示例。阅读 cmets 以了解代码中每一行的作用。我已经在我的电脑上用 gcc 4.6.1 测试过它；它工作正常。

#include <iostream>
#include <fstream>
#include <string>

void f()
{
    std::string line;
    while(std::getline(std::cin, line))  //input from the file in.txt
    {
        std::cout << line << "\n";   //output to the file out.txt
    }
}
int main()
{
    std::ifstream in("in.txt");
    std::streambuf *cinbuf = std::cin.rdbuf(); //save old buf
    std::cin.rdbuf(in.rdbuf()); //redirect std::cin to in.txt!

    std::ofstream out("out.txt");
    std::streambuf *coutbuf = std::cout.rdbuf(); //save old buf
    std::cout.rdbuf(out.rdbuf()); //redirect std::cout to out.txt!

    std::string word;
    std::cin >> word;           //input from the file in.txt
    std::cout << word << "  ";  //output to the file out.txt

    f(); //call function


    std::cin.rdbuf(cinbuf);   //reset to standard input again
    std::cout.rdbuf(coutbuf); //reset to standard output again

    std::cin >> word;   //input from the standard input
    std::cout << word;  //output to the standard input
}

您可以在一行中保存和重定向：

auto cinbuf = std::cin.rdbuf(in.rdbuf()); //save and redirect

这里std::cin.rdbuf(in.rdbuf()) 将std::cin's 缓冲区设置为in.rdbuf()，然后返回与std::cin 关联的旧缓冲区。 std::cout 或任何 stream 也可以做到这一点。

希望对您有所帮助。

Answer 2

随便写

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

int main()
{
    freopen("output.txt","w",stdout);
    cout<<"write in file";
    return 0;
}

Answer 3

这里有一个短代码 sn-p 用于遮蔽 cin/cout，对编程竞赛很有用：

#include <bits/stdc++.h>

using namespace std;

int main() {
    ifstream cin("input.txt");
    ofstream cout("output.txt");

    int a, b;   
    cin >> a >> b;
    cout << a + b << endl;
}

这带来了额外的好处，即普通 fstream 比同步的 stdio 流更快。 但这仅适用于单个功能的范围。

全局cin/cout重定向可以写成：

#include <bits/stdc++.h>

using namespace std;

void func() {
    int a, b;
    std::cin >> a >> b;
    std::cout << a + b << endl;
}

int main() {
    ifstream cin("input.txt");
    ofstream cout("output.txt");

    // optional performance optimizations    
    ios_base::sync_with_stdio(false);
    std::cin.tie(0);

    std::cin.rdbuf(cin.rdbuf());
    std::cout.rdbuf(cout.rdbuf());

    func();
}

请注意，ios_base::sync_with_stdio 也会重置 std::cin.rdbuf。所以顺序很重要。

另见Significance of ios_base::sync_with_stdio(false); cin.tie(NULL);

Std io 流也可以很容易地在单个文件的范围内隐藏，这对于竞争性编程很有用：

#include <bits/stdc++.h>

using std::endl;

std::ifstream cin("input.txt");
std::ofstream cout("output.txt");

int a, b;

void read() {
    cin >> a >> b;
}

void write() {
    cout << a + b << endl;
}

int main() {
    read();
    write();
}

但在这种情况下，我们必须一一选择std 声明并避免using namespace std;，因为它会产生歧义错误：

error: reference to 'cin' is ambiguous
     cin >> a >> b;
     ^
note: candidates are: 
std::ifstream cin
    ifstream cin("input.txt");
             ^
    In file test.cpp
std::istream std::cin
    extern istream cin;  /// Linked to standard input
                   ^

另请参阅How do you properly use namespaces in C++?、Why is "using namespace std" considered bad practice? 和 How to resolve a name collision between a C++ namespace and a global function?

Answer 4

假设您的编译程序名称是 x.exe $ 是系统 shell 或提示符

$ x <infile >outfile 

将从 infile 获取输入并输出到 outfile 。

Answer 5

试试这个把 cout 重定向到文件。

#include <iostream>
#include <fstream>

int main()
{
    /** backup cout buffer and redirect to out.txt **/
    std::ofstream out("out.txt");

    auto *coutbuf = std::cout.rdbuf();
    std::cout.rdbuf(out.rdbuf());

    std::cout << "This will be redirected to file out.txt" << std::endl;

    /** reset cout buffer **/
    std::cout.rdbuf(coutbuf);

    std::cout << "This will be printed on console" << std::endl;

    return 0;
}

阅读全文Use std::rdbuf to Redirect cin and cout

Answer 6

如果你的输入文件是in.txt，你可以使用freopen将stdin文件设置为in.txt

freopen("in.txt","r",stdin);

如果你想对你的输出做同样的事情：

freopen("out.txt","w",stdout);

这适用于 std::cin（如果使用 c++）、printf 等...

这也将帮助您在 clion、vscode 中调试代码

编辑
如果你想重置标准输入

fclose(stdin);
stdin = fdopen(0, "r"); //reopen: 0 is file descriptor of std input

并重置标准输出

fclose(stdout);
stdout = fdopen(1, "w"); //reopen: 1 is file descriptor of std output

Answer 7

接受的答案显示了重定向cin 和cout 的正确方法。您需要构造另一个生命周期超过cin 或cout 的流对象。如果你想写一个像freopen这样的函数，你可以为每个要重定向的流分配一个数组，以保存分配的流对象。

#include <iostream>
#include <string>
#include <fstream>
#include <vector>

using namespace std;

template<typename>
    struct fstream_traits { };
template<typename CharT, typename Traits>
    struct fstream_traits<basic_istream<CharT, Traits>> { using type = basic_ifstream<CharT, Traits>; };
template<typename CharT, typename Traits>
    struct fstream_traits<basic_ostream<CharT, Traits>> { using type = basic_ofstream<CharT, Traits>; };

template <typename Stream>
void redirect(Stream& str, string filename)
{
    using fstream_type = typename fstream_traits<Stream>::type;
    static int index = std::ios_base::xalloc();
    if (str.pword(index) == nullptr)
    {
        str.pword(index)= new vector<ios_base*>{};
        str.register_callback([](ios_base::event event, std::ios_base& stream, int index) {
            if (event == ios_base::erase_event)
            {
                for (auto fs : *(vector<ios_base*>*)stream.pword(index))
                    delete fs;
                delete (vector<ios_base*>*)stream.pword(index);
            }
        }, index);
    }
    vector<ios_base*>* list = (vector<ios_base*>*)str.pword(index);
    list->push_back(new fstream_type{filename});
    str.rdbuf(dynamic_cast<fstream_type*>(list->back())->rdbuf())->~basic_streambuf();
}

int main()
{
    redirect(cout, "out.txt");
    cout << "Redirected text!";
    return 0;
}

如果您明确使用istream/ostream 而不是Stream，则不需要模板和别名。

Answer 8

C++ 中的 I/O 重定向

https://www.geeksforgeeks.org/io-redirection-c/

// Cpp program to redirect cout to a file
#include <fstream>
#include <iostream>
#include <string>
 
using namespace std;
 
int main()
{
    fstream file;
    file.open("cout.txt", ios::out);
    string line;
 
    // Backup streambuffers of  cout
    streambuf* stream_buffer_cout = cout.rdbuf();
    streambuf* stream_buffer_cin = cin.rdbuf();
 
    // Get the streambuffer of the file
    streambuf* stream_buffer_file = file.rdbuf();
 
    // Redirect cout to file
    cout.rdbuf(stream_buffer_file);
 
    cout << "This line written to file" << endl;
 
    // Redirect cout back to screen
    cout.rdbuf(stream_buffer_cout);
    cout << "This line is written to screen" << endl;
 
    file.close();
    return 0;
}