获取 C++17 / C++20 中参数包的最后一个元素

Question

I would like to get the last element of a parameter pack. I did it with the following code GodBolt :

template<typename... Args>
auto last(Args&&... args){
   const auto& last = (args, ...);
   return last;
}

But now I get the warning

left operand of comma operator has no effect

But this is exactly what I wanted to achieve... Is there a way to make it clear. In general I like to get warnings for unused values so I don't want to disable all of them (-Wno-unused-value).

In addition I could do it with a recursive template, but a fold expression seems to be better here.

Answer 1

Wrapping args in any function call would remove the warning.

You could make an identity function specifically for this purpose, but you might as well use std::forward and get proper forwarding as a bonus.

template<typename... Args>
decltype(auto) last(Args&&... args){
   return (std::forward<Args>(args), ...);
}

Answer 2

Make a tuple and get the last element:

template<typename... Args>
auto last(Args&&... args)
{
    return std::get<sizeof...(Args)-1>(std::forward_as_tuple(args...));
}

Test:

int main()
{
    auto&& last_element = last(1, 1.0, true);
    static_assert(std::is_same_v<bool, std::remove_cvref_t<decltype(last_element)>>);
}

Demo

C++11 Friendly Demo

Answer 3

As suggested by HolyBlackCat the easiest way to remove this warning is to use an "identity / dummy-function" like this:

template<typename T>
constexpr inline decltype(auto) identityFkt(T&& t)
{
    return std::forward<T>(t);
}

It can be used this way:

const auto& last = (identityFkt(std::forward<AParamPack>(aParamPack)), ...);

It results in the same assembly (checked @ godbolt) - no overhead (with -O3).

Thank you: @HolyBlackCat

Answer 4

You can use another implementation (it's less efficient, but compiler should be able remove inefficiency here)

template <class T>
decltype(auto) last(T && ... t) { return t; }

template <class ... TT,class T>
decltype(auto) last(T &&,TT && ... tt) { return last( std::forward<TT>(tt)... ); }