Declare another object output stream similar to cout

Question

I'm just curious whether it is possible to somehow declare something similar to cout but to define its function. For example:

SetCursorPosition(output_handle, some_COORD);
cout_coord << "Hi\nthere!";

to produce this output:

 .
               Hi
               there!

score 1 · Accepted Answer · answered Mar 01 '14 at 17:47

1

You could use operator overloading to make << do basically anything (just google "c++ operator overloading", you'll find tons of information), but such a thing as in your question is generally considered abuse of operator overloading. Overloading operators is intended to make code easier to read. In C++, everyone knows that << means "output something", just like += means "mathematically add something" and = means "assign something".

When I see << in some piece of code, then I certainly don't read it as "set cursor position and then print text". You could implement it like this, but it's bad programming style, akin to misnaming ordinary functions.

answered Mar 01 '14 at 17:47

Christian Hackl

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In C++, everyone knows that `<<` means "left shift". – Lightness Races in Orbit Mar 01 '14 at 18:13
A better way to implement this without mucking around with operators is to create an `ostream` wrapper that formats the output and exposes member functions that allow formatting options to be set, or perhaps uses custom manipulators. A simpler way, depending on requirements, is to just use existing manipulators, like `width()` and the alignment ones. – Jason C Mar 05 '14 at 02:26

score 0 · Answer 2 · edited Mar 01 '14 at 18:13

0

Well cout isn't a function, it's actually a std::ostream. You can create your own ostream that writes to stdout.

edited Mar 01 '14 at 18:13

Lightness Races in Orbit

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answered Mar 01 '14 at 17:38

wybourn

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score 0 · Answer 3 · answered Mar 01 '14 at 20:41

The way to implement this logic is to create a custom stream buffer which writes at the opportune location in its overflow() method and then to create a suitable std::ostream using this stream buffer. Here is a rough sketch on how that would look like:

class posbuf: public std::streambuf {
    int d_row;     // current row
    int d_column;  // column where to start output
    void move() { /* use curses, VT100 codes, whatever to move the cursor */ }
    int overflow(int c) {
        if (c == traits_type::eof()) { return traits_type::not_eof(c); }
        if (c == '\n') { ++this->d_row; this->move(); return '\n'; }
        return std::cout << traits_type::to_char_type(c)? c: traits_type::eof();
    }
    int sync() {
        return std::cout.flush()? 0: -1;
    }
public:
    posbuf(int row, int column): d_row(row), d_column(column) { this->move(); }
};
struct oposstream: private virtual posbuf, public std::ostream {
    oposstream(int row, int column): posbuf(row,c column), std::ostream(this) {}
};

int main() {
    oposstream out(4, 10);
    out << "hello\nworld";
}

Except for a couple of likely typos and the details on how to actually move the cursor, the above code should actually work and create an std::ostream which can be passed wherever an std::ostream& is expected. Any attempt to overload the output operator using some templates will not work as well.

This is interesting but looks a bit complex, I haven't finished the c++ book and I am not that familiar with some of the functions you use. I WILL look them up but actually I found what I was looking, see my answer on the question. — Bonnev, Mar 01 '14 at 21:00

Bonnev · Answer 4 · 2014-03-08T21:43:12.233

Thanks to Christian Hackl and another user that removed their answer I did a research and found the answer I was looking for.

If anyone is still curious my example in the question became:

std::ostream& operator<<(std::ostream& left, const char * right)
{
    string sRight = right;
    CONSOLE_SCREEN_BUFFER_INFO con;
    GetConsoleScreenBufferInfo(GetStdHandle(STD_OUTPUT_HANDLE), &con);
    COORD xy;
    xy.X = con.dwCursorPosition.X;
    xy.Y = con.dwCursorPosition.Y;
    for (int i = 0; i < sRight.length(); i++)
    {
        if (sRight[i] == '\n')
        {
            xy.Y++;
            SetConsoleCursorPosition(GetStdHandle(STD_OUTPUT_HANDLE), xy);                
            continue;
        }
        cout << sRight[i];
    }
    return left;
}

And then you just:

SetConsoleCursorPosition(output_handle, some_COORD);
cout_coord << "Hi\nthere!";

And "Hi" and "there!" are one under the other for every x given with some_COORD!

Declare another object output stream similar to cout

4 Answers4