C++1z will have fold expressions for this: (std::cout << a << '\n', ...); Add optional (void) cast for the paranoid. Also, your implementation doesn't work if the type of the expression is `void`, as far as I can see. On Fri, Jul 3, 2015 at 2:16 AM, Antony Polukhin <antoshkka@gmail.com> wrote:

Hi,

Straight to the problem. Consider the situation, were a function accepts parameter pack and something need to be done with each argument (for example, we want to print each argument on a new line):

template <typename... Args> void print(Args&&... a) { // print each a on a separate line }

There's a known trick to do that:

template <typename... Args> void print(Args&&... a) { (void) (int []){ 0, (std::cout << a << std::endl, 0)... }; }

But the syntax is not very readable (and I always forget it and search for an example in Internet). I've tried to improve the syntax, and here's the result:

template<typename... Args> void print(Args&&... a) { expand_if<Args...> { std::cout << a << '\n'... }; }

Compete source could be found here: http://coliru.stacked-crooked.com/view?id=c2c59f22574671e5

Here's some more usage examples:

template<typename... Args> void printer_int(Args&&... args) { expand { std::cout << std::abs(args) << std::endl..., std::cout << "Printing once again all the numbers: " << std::endl, std::cout << " Once again " << std::abs(args) << std::endl..., std::cout << "EOF Printing once again all the numbers." << std::endl }; }

template<typename... Args> float sum(Args&&... args) { float res = 0; expand_if<Args...> { res += args... };

return res; }

I hope this would be helpful for someone or it could be used as an argument for a proposal to extend C++ and allow simpler syntax:

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

P.S.: There was a nicer solution that newer required "_if<Args..>" :

namespace detail { struct expand_impl final { template <class... Arg> constexpr inline expand_impl(Arg&&...) noexcept {} }; }

template <class... Arg> using expand = detail::expand_impl;

But unfortunately there's no way to force the compiler to evaluate expressions in constructor call from left to right. So it did not work on GCC (but work on Clang).

-- Best regards, Antony Polukhin

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namespace detail { struct expand_impl final { template <class... Arg> constexpr inline expand_impl(Arg&&...) noexcept {} }; }

template <class... Arg> using expand = detail::expand_impl;

But unfortunately there's no way to force the compiler to evaluate expressions in constructor call from left to right. So it did not work on GCC (but work on Clang).

There is. Just use {} and it's guaranteed to evaluate the args from left to right.