c++ - 使用 std::ostream 在每一行之前插入文本

Question

我想知道是否可以从 std::ostream 继承，并以在每行的开头添加一些信息(比如行号)的方式覆盖 flush()。然后我想通过 rdbuf() 将它附加到 std::ofstream (或 cout)，以便我得到这样的东西:

ofstream fout("file.txt");
myostream os;
os.rdbuf(fout.rdbuf());

os << "this is the first line.\n";
os << "this is the second line.\n";

将其放入 file.txt

1 this is the first line.
2 this is the second line.

Answer 1

flush() 不会是在此上下文中要覆盖的函数，尽管您走在正确的轨道上。您应该在底层 std::streambuf 接口(interface)上重新定义 overflow()。例如:

class linebuf : public std::streambuf
{
public:
    linebuf() : m_sbuf() { m_sbuf.open("file.txt", std::ios_base::out); }

    int_type overflow(int_type c) override
    {
        char_type ch = traits_type::to_char_type(c);
        if (c != traits_type::eof() && new_line)
        {
            std::ostream os(&m_sbuf);
            os << line_number++ << " ";
        }

        new_line = (ch == '\n');
        return m_sbuf.sputc(ch);
    }

    int sync() override { return m_sbuf.pubsync() ? 0 : -1; }
private:
    std::filebuf m_sbuf;
    bool new_line = true;
    int line_number = 1;
};

现在您可以:

linebuf buf;
std::ostream os(&buf);

os << "this is the first line.\n";  // "1 this is the first line."
os << "this is the second line.\n"; // "2 this is the second line."

Live example