Overload handling of std::endl?

Question

I want to define a class MyStream so that:

MyStream myStream;
myStream << 1 << 2 << 3 << std::endl << 5 << 6 << std::endl << 7 << 8 << std::endl;

gives output

[blah]123
[blah]56
[blah]78

Basically, I want a "[blah]" inserted at the front, then inserted after every non terminating std::endl?

The difficulty here is NOT the logic management, but detecting and overloading the handling of std::endl. Is there an elegant way to do this?

Thanks!

EDIT: I don't need advice on logic management. I need to know how to detect/overload printing of std::endl.

Answer 1

What you need to do is write your own stream buffer: When the stream buffer is flushed you output you prefix characters and the content of the stream.

The following works because std::endl causes the following.

1. Add '\n' to the stream.

2. Calls flush() on the stream

3. This calls pubsync() on the stream buffer.

   1. This calls the virtual method sync()
   2. Override this virtual method to do the work you want.

#include <iostream>
#include <sstream>

class MyStream: public std::ostream
{
    // Write a stream buffer that prefixes each line with Plop
    class MyStreamBuf: public std::stringbuf
    {
        std::ostream&   output;
        public:
            MyStreamBuf(std::ostream& str)
                :output(str)
            {}
            ~MyStreamBuf() {
                if (pbase() != pptr()) {
                    putOutput();
                }
            }
   
        // When we sync the stream with the output. 
        // 1) Output Plop then the buffer
        // 2) Reset the buffer
        // 3) flush the actual output stream we are using.
        virtual int sync() {
            putOutput();
            return 0;
        }
        void putOutput() {
            // Called by destructor.
            // destructor can not call virtual methods.
            output << "[blah]" << str();
            str("");
            output.flush();
        }
    };

    // My Stream just uses a version of my special buffer
    MyStreamBuf buffer;
    public:
        MyStream(std::ostream& str)
            :std::ostream(&buffer)
            ,buffer(str)
        {
        }
};


int main()
{
    MyStream myStream(std::cout);
    myStream << 1 << 2 << 3 << std::endl << 5 << 6 << std::endl << 7 << 8 << std::endl;
}

> ./a.out
[blah]123 
[blah]56 
[blah]78
>