c++ - 重定向 std::cout

Question

我需要一个在其对象的生命周期内将一个 ostream 重定向到另一个 ostream 的类。经过一番修补，我想出了这个:

#include <iostream>
#include <fstream>


class ScopedRedirect
{
public:
    ScopedRedirect(std::ostream & inOriginal, std::ostream & inRedirect) :
        mOriginal(inOriginal),
        mRedirect(inRedirect)
    {
        mOriginal.rdbuf(mRedirect.rdbuf(mOriginal.rdbuf()));
    }

    ~ScopedRedirect()
    {
        mOriginal.rdbuf(mRedirect.rdbuf(mOriginal.rdbuf()));
    }    

private:
    ScopedRedirect(const ScopedRedirect&);
    ScopedRedirect& operator=(const ScopedRedirect&);

    std::ostream & mOriginal;
    std::ostream & mRedirect;
};


int main()
{
    std::cout << "Before redirect." << std::endl;
    std::ofstream filestream("redirected.txt");
    {
        ScopedRedirect redirect(std::cout, filestream);
        std::cout << "During redirect." << std::endl;
    }
    std::cout << "After redirect." << std::endl;

    return 0;
}

它似乎工作正常。然而，奇怪的是，在 构造函数和析构函数中都重复了以下行:

mOriginal.rdbuf(mRedirect.rdbuf(mOriginal.rdbuf()));

我认为这是正确的，但我想与 SO 社区进行验证。你能在这段代码中发现任何错误或危险吗？

编辑

使不可复制。

Answer 1

这些行相同的原因是因为您所做的是交换 缓冲区。 (也就是说，您通过将原始缓冲区与重定向缓冲区交换来“重定向”；恢复就是交换回来。)

虽然这可能会给您关于输出流的预期效果，但它是不正确的，因为重定向流现在输出到其他地方。 To redirect 表示取一个流并将其输出到其他地方；请注意，这不会影响“其他地方”。

您的类(class)不是重定向；实际上，它应该被命名为 ScopedStreamSwap。例如，试试这个:

#include <iostream>
#include <fstream>

class ScopedRedirect
{
public:
    ScopedRedirect(std::ostream & inOriginal, std::ostream & inRedirect) :
        mOriginal(inOriginal),
        mRedirect(inRedirect)
    {
        mOriginal.rdbuf(mRedirect.rdbuf(mOriginal.rdbuf()));
    }

    ~ScopedRedirect()
    {
        mOriginal.rdbuf(mRedirect.rdbuf(mOriginal.rdbuf()));
    }    

private:
    ScopedRedirect(const ScopedRedirect&);
    ScopedRedirect& operator=(const ScopedRedirect&);

    std::ostream & mOriginal;
    std::ostream & mRedirect;
};


int main()
{
    std::cout << "Before redirect." << std::endl;
    std::ofstream filestream("redirected.txt");
    {
        ScopedRedirect redirect(std::cout, filestream);
        std::cout << "During redirect." << std::endl;

        // oops:
        filestream << "also to the file, right?...nope" << std::endl;
        filestream << "ah, why am i on the screen?!" << std::endl;
    }
    std::cout << "After redirect." << std::endl;

    // in main, return 0 is implicit, if there is no return statement;
    // helpful to keep in mind in snippets and short things
}

你想要的是这个:

#include <iostream>
#include <fstream>

class ScopedRedirect
{
public:
    ScopedRedirect(std::ostream & inOriginal, std::ostream & inRedirect) :
        mOriginal(inOriginal),
        mOldBuffer(inOriginal.rdbuf(inRedirect.rdbuf()))
    { }

    ~ScopedRedirect()
    {
        mOriginal.rdbuf(mOldBuffer);
    }    

private:
    ScopedRedirect(const ScopedRedirect&);
    ScopedRedirect& operator=(const ScopedRedirect&);

    std::ostream & mOriginal;
    std::streambuf * mOldBuffer;
};


int main()
{
    std::cout << "Before redirect." << std::endl;
    std::ofstream filestream("redirected.txt");
    {
        ScopedRedirect redirect(std::cout, filestream);
        std::cout << "During redirect." << std::endl;

        // yay:
        filestream << "also to the file, right?...yes" << std::endl;
        filestream << "i am not on the screen" << std::endl;
    }
    std::cout << "After redirect." << std::endl;

    return 0;
}