我们写程序的时候，为了方便，经常需要修改流的方向，或者将默认的输出流重定向到我们的文件或者是内存中，而不是系统默认的终端。参考代码如下：

#include <iostream>
#include <fstream>

int main(int argc, char* argv[])
{
    std::ofstream StreamOut( "out.txt");
    std::streambuf * OldBuf = std::cout.rdbuf( streamOut.rdbuf());

    //写入out.txt文件中
    streamOut << "streamOut\n";
    std::cout << "std::cout\n";

    //注意恢复
    std::cout.rdbuf( OldBuf);
    return 0;
}

下面是如何使得上面的代码自动运行：

#include <iostream>

template< class char_type, class char_traits = std::char_traits< char_type> >
class basic_redirect_from_to
{
    typedef std::basic_ios< char_type, char_traits> ios_type;
    typedef std::basic_streambuf< char_type, char_traits> streambuf_type;
    typedef basic_redirect_from_to< char_type, char_traits> this_class;

    basic_redirect_from_to( const this_class & );
    this_class & operator=( this_class &);

public:
    basic_redirect_from_to( ios_type & streamFrom, ios_type & streamTo)
        : m_streamFrom( streamFrom)
    {
        m_pOriginalBuffer = m_streamFrom.rdbuf( streamTo.rdbuf());
    }

    ~basic_redirect_from_to()
    {
        m_streamFrom.rdbuf( m_pOriginalBuffer);
    }

private:
    ios_type & m_streamFrom;
    streambuf_type * m_pOriginalBuffer;
};

typedef basic_redirect_from_to< char> redirect_from_to;
typedef basic_redirect_from_to< wchar_t> wredirect_from_to;