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

here's part of eigen documentation:

matrix3f m; m << 1, 2, 3, 4, 5, 6, 7, 8, 9; std::cout << m;

output:

1 2 3 4 5 6 7 8 9

i couldn't understand how comma separated values captured operator<< above. did tiny experiment:

cout << "just commas: "; cout << 1, 2, 3, 4, 5; cout << endl; cout << "commas in parentheses: "; cout << ( 1, 2, 3, 4, 5 ); cout << endl;

predictably (according understanding of c++ syntax) 1 of values captured operator<< :

just commas: 1 commas in parentheses: 5

thus title question.

the basic thought overload both << , , operators.

m << 1 overloaded set 1 m , returns special proxy object - phone call p - holding reference m.

then p, 2 overloaded set 2 m , homecoming p, p, 2, 3 first set 2 m , 3.

a similar technique used boost.assign, though utilize += rather <<.

c++ initialization operator-overloading eigen

Five