inheritance - 如何从 std::ostream 继承？

Question

我一直在谷歌搜索，但我找不到一个简单的答案。它应该很简单，就像 STL 通常一样。

我想定义从 std::ostream 公开继承的 MyOStream。假设每次将某些内容写入我的流时，我都想调用 foo() 。

class MyOStream : public ostream {
public:
  ...
private:
   void foo() { ... }
}

我知道ostream的公共(public)接口(interface)是非虚拟的，那怎么办呢？
我希望客户能够在 MyOStream 上同时使用 operator<< 和 write() 和 put() 并使用我的类(class)的扩展能力。

Answer 1

我不知道这是否是正确的解决方案，但我以这种方式从 std::ostream 继承。它使用从 std::basic_streambuf 继承的缓冲区，一次获取 64 个字符(如果刷新则更少)，并将它们发送到一个通用的 putChars() 方法，在该方法中完成数据的实际处理。它还演示了如何提供用户数据。

Live Example

#include <streambuf>
#include <ostream>
#include <iostream>

//#define DEBUG

class MyData
{
    //example data class, not used
};

class MyBuffer : public std::basic_streambuf<char, std::char_traits<char> >
{

public:

    inline MyBuffer(MyData data) :
    data(data)
    {
        setp(buf, buf + BUF_SIZE);
    }

protected:

    // This is called when buffer becomes full. If
    // buffer is not used, then this is called every
    // time when characters are put to stream.
    inline virtual int overflow(int c = Traits::eof())
    {
#ifdef DEBUG
        std::cout << "(over)";
#endif
        // Handle output
        putChars(pbase(), pptr());
        if (c != Traits::eof()) {
            char c2 = c;
            // Handle the one character that didn't fit to buffer
            putChars(&c2, &c2 + 1);
        }
        // This tells that buffer is empty again
        setp(buf, buf + BUF_SIZE);

        return c;
    }

    // This function is called when stream is flushed,
    // for example when std::endl is put to stream.
    inline virtual int sync(void)
    {
        // Handle output
        putChars(pbase(), pptr());
        // This tells that buffer is empty again
        setp(buf, buf + BUF_SIZE);
        return 0;
    }

private:

    // For EOF detection
    typedef std::char_traits<char> Traits;

    // Work in buffer mode. It is also possible to work without buffer.
    static const size_t BUF_SIZE = 64;
    char buf[BUF_SIZE];

    // This is the example userdata
    MyData data;

    // In this function, the characters are parsed.
    inline void putChars(const char* begin, const char* end){
#ifdef DEBUG
        std::cout << "(putChars(" << static_cast<const void*>(begin) <<
            "," << static_cast<const void*>(end) << "))";
#endif
        //just print to stdout for now
        for (const char* c = begin; c < end; c++){
            std::cout << *c;
        }
    }

};

class MyOStream : public std::basic_ostream< char, std::char_traits< char > >
{

public:

    inline MyOStream(MyData data) :
    std::basic_ostream< char, std::char_traits< char > >(&buf),
    buf(data)
    {
    }

private:

    MyBuffer buf;

};

int main(void)
{
    MyData data;
    MyOStream o(data);

    for (int i = 0; i < 8; i++)
        o << "hello world! ";

    o << std::endl;

    return 0;
}