Question

Suppose I want to rot13 every string passed to cout (or another ostream), so that, say cout<<"Foo Bar Baz.;" (or even cout<<rot13<<"Foo Bar Baz.";) outputs Sbb One Onm.

How would I go about doing that?

(My first idea was to replace cout’s streambuf with a streambuf-derived class that would do all the work. But seeing as the original streambuf is responsible for directing stuff to the console… that didn’t work out at all.)

Answer 1

You can write your own stream that overload operator<< for char*, std::string and others and prints a transformed text.

#include <iostream>
#include <string>
#include <algorithm>
#include <iterator>

using namespace std;

class ostream_rot13 : public basic_ostream <char, char_traits<char> >
{
public:
    ostream_rot13(std::basic_streambuf<char, char_traits<char> >* sb) 
    : basic_ostream<char, char_traits<char> >(sb) {}

    ostream_rot13& operator<<(const char* text)
    {
        std::string s(text);

        int rot=13;
        std::transform(std::begin(s), std::end(s), ostream_iterator<char>(*this), [rot] (char c) { 
            if(c >= 'a' && c <= 'z')
                return 'a' + (c + rot - 'a') % 26;
            else if(c >= 'A' && c <= 'Z')
                return 'A' + (c + rot - 'A') % 26;

            return (int)c;
        });

        return *this;
    }
};

The next step is to declare a global variable of this type and then a macro that replaces cout with the new variable.

ostream_rot13 cout_rot13(std::cout.rdbuf());

#define cout cout_rot13

And then all instances of cout will become cout_rot13.

int main() 
{
    cout << "Foo Bar Baz";

    return 0;
}