How could comma separated initialization such as in Eigen be possibly implemented in C++?

Question

Here\'s a part of Eigen documentation:

Matrix3f m;
m << 1, 2, 3,
     4, 5, 6,
     7, 8, 9;
std::cout << m;

Output:

Answer 1

The comma itself is an operator in c++ which can be overloaded (and apparently is by eigen). I don't know the exact way eigen implements the overloading but I am sure that you can search for the sources of eigen to look it up.

To your little experient you have to understand how the unoverloaded comma operator works in c++.

The comma operator has the form , and is evaluated to whatever the second statement is evaluated to. The operator << has a higher precedence than the operator ,. Because of that the cout is evaluated before the rest of the comma operations are evaluated.

Because , is Left-to-right associative the code (1,2,3,4,5) is equal to ((((1,2),3),4),5) which evaluates to the most right value, which is 5.