Re: [boost] Metaprogramming: parameter pack expansion/evaluation trick

Subject: Re: [boost] Metaprogramming: parameter pack expansion/evaluation trick
From: Tim Song (t.canens.cpp_at_[hidden])
Date: 2015-07-03 03:35:22

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_at_[hidden]> 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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