why parameter pack expansion is so limited?

Question

I was wondering, why parameter pack expansion is so limited in C++11 - is it just oversight in C++11 standard? Why is it not possible to do just bar(args)...;? It is possible to workaround that better than I did?

#include <iostream>

int bar(int i)
{
    std::cout << i << ' ';
    return 0;
}

template <typename T>
void boo(T t)
{
    bar(t);
}

template <typename T, typename... Args>
void boo(T t, Args&&... args)
{
    bar(t);
    boo(args...);
}

template <typename... Args>
void dummy(Args&&... args)
{
}

template <typename... Args>
void foo(Args&&... args)
{
    dummy(bar(args)...);

    std::cout << std::endl;

    int dummy1[sizeof...(Args)] = { (bar(args), 0)... };

    std::cout << std::endl;

    boo(args...);
}

int main()
{
    foo(1, 3, 5);
}

Probably because of potential ambiguity. Also, C++ has a habit of implementing the least possible amount of functionality required to make something not entirely impossible. — user541686, Jun 19 '14 at 21:41
Rename "dummy" to "for_each_arg", and you've got a winner. :-) — Howard Hinnant, Jun 19 '14 at 21:46
@HowardHinnant note that bar() has to return something, so there would need to be more code to wrap void functions — kpx1894, Jun 19 '14 at 21:56

score 3 · Answer 1 · answered Jun 19 '14 at 22:35

3

The reference mentions the different contexts in which parameter pack expansions are allowed. So, a slightly more succinct way of doing this is

auto a = {bar(args)...};

This won't work if the return type of bar is void.

answered Jun 19 '14 at 22:35

Pradhan

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3

To get it to handle `void` return type, and a potential comma operator overload, use `auto a = {(bar(args)..., void(), 0)};` – Praetorian Jun 19 '14 at 22:49
1

@Praetorian: That is very cool, but you have a slight syntax error: `auto a = {(bar(args), void(), 0)...};` – Howard Hinnant Jun 20 '14 at 02:16
@HowardHinnant and you should end it with a `,0`, in case `args...` is empty. – Yakk - Adam Nevraumont Oct 31 '14 at 19:55

why parameter pack expansion is so limited?

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