Gabriel de Grimouard Gabriel de Grimouard - 3 months ago 23
C++ Question

C++17 fold expression in cout

I am learning the new c++17 fold expression and I saw this code from c++17 fold expression. I would like to know why this code work :

template<typename ...Args>
void printer(Args&&... args) {
(std::cout << ... << args) << '\n';
}


but not this one :

template<typename ...Args>
void printer(Args&&... args) {
(std::cout << args << ...) << '\n';
}


which could seems logic too and would reverse the print order in my opinion.

Answer

As seen on cppreference, binary folds can have the following two forms:

Screenshot from cppreference/fold

Where E is the pack expression and I is the initialization expression.


There is no binary fold that matches your (std::cout << args << ...), which has the form of (I op E op ...).