Using fold expressions to print all variadic arguments with newlines inbetween

Question

The classic example for C++17 fold expressions is printing all arguments:

template
void print(Args ... args)
{
    (cout << ... &l         


        

Answer 1

Update: T.C.'s comment below provided a better solution:

template
void print(Args ... args)
{
    ((cout << args << '\n'), ...);
}

---

You can use a fold expression over the comma operator:

template
void print(Args ... args)
{
    ([](const auto& x){ cout << x << "\n"; }(args), ...);
}

Usage:

int main()
{
    print("a", 1, 1000);
}

a

1

1000

(Note: this prints a trailing newline as well.)

• live wandbox example

• assembly comparison on godbolt

---

Explanation:

• [](const auto& x){ cout << x << "\n"; } is a lambda that given x prints x and '\n'.

• [](const auto& x){ cout << x << "\n"; }(args) immediately invokes the lambda with args.

• ([](const auto& x){ cout << x << "\n"; }(args), ...) is a fold expression over the comma operator that expands in the following way:

  // (pseudocode)
  [](const auto& x){ cout << x << "\n"; }(args<0>),
  [](const auto& x){ cout << x << "\n"; }(args<1>),
  [](const auto& x){ cout << x << "\n"; }(args<2>),
  // ...
  [](const auto& x){ cout << x << "\n"; }(args)