問題描述

我有一個帶有 varargs 模板參數的模板函數，就像這樣

I have a template function with varargs template arguments, like this

template<typename Args...>
void ascendingPrint(Args... args) { /* ... */ }

我想寫

template<typename Args...>
void descendingPrint(Args... args) {
  /* implementation using ascendingPrint()? */
}

我如何反轉參數包 args 的順序，然后再傳遞它，即在偽代碼中:

How do I reverse the order of the parameter-pack args before passing it along, i.e. in pseudo-code:

template<typename Args...>
void descendingPrint(Args... args) {
  ascendingPrint( reverse(args) );
}

推薦答案

這里是一個遞歸的特殊revert<>實現:

Here is a recursive implementation of a specialized revert<>:

// forward decl
template<class ...Tn>
struct revert;

// recursion anchor
template<>
struct revert<>
{
    template<class ...Un>
    static void apply(Un const&... un)
    {
        ascendingPrint(un...);
    }
};

// recursion
template<class T, class ...Tn>
struct revert<T, Tn...> 
{
    template<class ...Un>
    static void apply(T const& t, Tn const&... tn, Un const&... un)
    {
        // bubble 1st parameter backwards
        revert<Tn...>::apply(tn..., t, un...);
    }
};

// using recursive function
template<class A, class ...An>
void descendingPrint(A const& a, An const&... an)
{
    revert<An...>::apply(an..., a);
}

它適用于 gcc-4.6/7/8 和 clang 并且可能符合標準——唯一困難的部分是調用revert::apply(tn..., t, un...).

It works with gcc-4.6/7/8 and clang and is probably standard compliant -- the only difficult part being the call of revert<Tn...>::apply(tn..., t, un...).

雖然它有缺點(就像遞歸經常有的那樣)，它會生成目標函數的大量模板實例(代碼膨脹)并且不使用完美轉發，這可能是一個問題(但也許可以改進使用它).

It has drawbacks though (as recursion often has), that it generates a lot of template-instantiations of the target function (code bloat) and does not use perfect forwarding, which may be an issue (but maybe could be improved to use it).

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