Question

I would like to overload << operator for chaining like below

function1(param1, param2)<< obj2 << obj3 << string4

function1 would return an object.

What I want to do is after string4, I need to call a function using param1, param2.

My questions will be

1. How do I know string4 is the last parameters in the expression and hence I need to call another function2 using param1 and param2 or it isn’t possible to do so?

2. How do I pass param1 and param2 to the function to be called? I could not store the param1 and param2 in the object as it is a multithreaded.

Thanks.

Answer 1

You can return a helper object from function1 by value which calls function2 in it’s destructor.

Consider this example:

#include <iostream>

using namespace std;

void function2( int i, int j ) {
    cout << "function2 called with " << i << " and " << j << endl;
}

struct Function2Caller {
  Function2Caller( int param1, int param2 ) : m_param1( param1 ), m_param2( param2 ) {}
  ~Function2Caller() { function2( m_param1, m_param2 ); }

  int m_param1, m_param2;
};

Function2Caller &operator<<( Function2Caller &obj, int x ) {
    cout << "Streaming " << x << endl;
    return obj;
}

Function2Caller function1( int i, int j ) {
    cout << "function1 called with " << i << " and " << j << endl;
    return Function2Caller( i, j );
}

int main() {
    function1( 2, 3 ) << 4 << 6;
}

This program prints

function1 called with 2 and 3
Streaming 4
Streaming 6
function2 called with 2 and 3

The idea is that at the end of your line, the Function2Caller object goes out of scope and the destructor then does its work.

Please note that when implementing this you should probably forbid copying of Function2Caller objects and make function1 the only one who can call the Function2Caller constructor.