Is there a way to get the values in a parameter pack without using recursion?

Question

I've seen many example codes that use recursion to extract the values from a parameter pack. Is there any way, other than recursion, to extract the values from a parameter pack?

The first element would be the degenerate case of recursing once, which is identical to calling one function. Why is that a problem? Is calling a non-recursive function OK? — Useless, Apr 26 '20 at 15:02
@Useless Because recursion is discouraged; it's compared to a chainsaw. — Sapphire_Brick, Apr 27 '20 at 16:39
@Useless see https://stackoverflow.com/questions/33511917/why-is-it-bad-to-call-functions-recursively — Sapphire_Brick, Apr 28 '20 at 00:36
`std::get` very likely _is_ recursive, which is why I asked for clarification. It's OK because the depth is fixed, and everything is inlined and optimized away, at compile time. — Useless, Apr 28 '20 at 21:51
Recursion is risky when you don't know in advance how deep it can go (with the possible exception of tail-call recursion with a good optimizer). If it's limited, with a reasonable upper limit, _especially_ one known at compile time - it can be fine, and is pretty frequently used in a compile-time-only way with type packs and old-style typelists (which are themselves recursive). — Useless, Apr 28 '20 at 22:07

Aykhan Hagverdili · Accepted Answer · 2020-05-17T16:49:12.287

1

Depending on what you want to do, you can use C++17 fold expressions:

template <int ... ints>
constexpr int product() {
    return (ints * ...);
}

edited May 17 '20 at 16:49

answered Apr 26 '20 at 15:05

Aykhan Hagverdili

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@Sapphire_Brick, no, it's not. Look up [recurion](https://en.wikipedia.org/wiki/Recursion) *"Recursion (adjective: recursive) occurs when a thing is defined in terms of **itself or of its type**"* – Aykhan Hagverdili Apr 28 '20 at 22:11
@Axyan I mean, it has the same disadvantages as recursion. (Stack limit, difficult to understand/debug, compiles to more assembly due to template parameters, etc) – Sapphire_Brick May 13 '20 at 01:42
No... It doesn't get simpler than a single function call. – Aykhan Hagverdili May 13 '20 at 08:05
@Axyan Are you sure that `std::foward` isn't implemented recursively? – Sapphire_Brick May 13 '20 at 16:48
@Sapphire_Brick `std::forward` is a conditional cast. It generates no executable code. Not a single byte. Thus, it's not recursive. You can look up how it's implemented in your standard library. – Aykhan Hagverdili May 13 '20 at 17:19
@Sapphire_Brick You can see implementation of `std::forward` for libstdc++ [here](https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/move.h#L74) – Aykhan Hagverdili May 13 '20 at 17:26
I'm sorry if my question wasn't clear. You're right about the getting the first value, it is as simple as calling another function, but I wanted to extract all values. I was unclear. – Sapphire_Brick May 15 '20 at 16:27
Your question wasn't clear previously, I've updated my answer. See if that's better – Aykhan Hagverdili May 15 '20 at 17:31
It is, but it's similar to mine. _Oh well_ – Sapphire_Brick May 15 '20 at 21:15
@Sapphire_Brick I thought you'd appreciate the simplification. – Aykhan Hagverdili May 16 '20 at 05:43
@Sapphire_Brick Seems like I don't quite understand your question. Please un-accept the answer so I can delete it. – Aykhan Hagverdili May 17 '20 at 16:05
I was wondering if there's a way to iterate over the items in a parameter pack without recursion. Your answer is perfectly fine. – Sapphire_Brick May 17 '20 at 16:09

score 1 · Answer 2 · answered Apr 26 '20 at 15:16

1

You can forward all pack parameters as a tuple, then call get<0>:

template<class ... Args>
void foo(Args&& ... args) {
    auto&& first = std::get<0>(std::forward_as_tuple(std::forward<Args>(args)...));
}

answered Apr 26 '20 at 15:16

rafix07

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Are you sure that std::get is not implemented recursively? – Useless Apr 27 '20 at 22:54

Sapphire_Brick · Answer 3 · 2021-01-14T03:02:27.837

It turns out that it there's a workaround. C++ parameter pack expansions can be used as for each loops:

#include <iostream>

template <int ... ints>
constexpr int product() {
    int ret = 1;
    auto _ = {
        ([&](int i) -> int {
            ret *= i;
            return i;
        })(ints)...
    };
    return ret;
}

int main() {
    std::cout << product<2, 6, 3>() << '\n';
    return 0;
}

The problems are:

the above code only works for non-type template parameter packs.
~~there is no break or continue~~

Edit

Apparently, it's legal to have a goto within a lambda within a parameter pack expansion. (I wasn't aware of this because Clang has a bug that restricts `goto` operands). Nevertheless, it's not a good idea, because it doesn't "clean up" before leaving the lambda.

Perhaps this helps if you want to pass non-type elements. You can use if as a continue. https://godbolt.org/z/CrCNiv — Aykhan Hagverdili, May 17 '20 at 16:49

Is there a way to get the values in a parameter pack without using recursion?

3 Answers3

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