用.rubuf()函数改变STL流的方向

改变流的方向可以使用stream的.rdbuf()成员函数。.rdbuf()函数允许你得到以及设置底层流缓冲。如果你重新设置std::cout的底层流缓冲指向一个文件流缓冲，那么你就改变了它的方向。

#include <iostream>
#include <fstream>

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

    //下面这两行都写道out.txt文件中去
    streamOut << "using streamOut ";
    std::cout << "using std::cout ";

    //防止内存泄漏和其它问题
    std::cout.rdbuf( pOldBuf);
    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;

    //禁止拷贝
    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()
    {
        //恢复原来的缓冲
        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;

#include <fstream>

int main(int argc, char* argv[])
{
    std::ofstream streamOut( "out.txt");
    redirect_from_to r( std::cout, streamOut);
    redirect_from_to r2( std::cerr, streamOut);

    streamOut << "using streamOut. " << std::endl ;
    std::cout << "using std::cout "<< std::endl ;
    std::cerr << "using std::cerr "<< std::endl ;

    return 0;
}