带有 std::index_sequence 的三元运算符

Question

在以下代码中：

#include <iostream>
#include <utility>
#include <set>

template <typename... Args>
void f(Args... args) {
    std::cout << sizeof...(Args) << " elements.\n";
}

template <std::size_t... Is>
void g (std::index_sequence<Is...>, const std::set<int>& set) {
    f((set.find(Is) == set.end() ? Is : 2*Is)...);
}

int main() {
    g (std::make_index_sequence<10>{}, {1,3,7,8});
}

如果set.find(Is) == set.end()，我希望f((set.find(Is) == set.end() ? Is : 2*Is)...); 使用Is，否则什么都不使用（而不是2*Is）。因此传递的参数数量不是固定的。如何实现？

编辑： 抱歉，我把问题简化得太多了。下面的代码更能反映真正的问题：

#include <iostream>
#include <utility>
#include <tuple>

template <typename... Args>
void f(Args... args) {
    std::cout << sizeof...(Args) << " elements.\n";
}

struct NullObject {};

template <typename Tuple, std::size_t... Is>
void g (std::index_sequence<Is...>, const Tuple& tuple) {
    f ((std::get<Is>(tuple) != std::get<Is+1>(tuple) ? std::get<Is>(tuple) : NullObject{})...);
}

int main() {
    g (std::make_index_sequence<8>{}, std::make_tuple(2,1.5,'a','a',true,5,5,false));
}

由于三元运算符传递的混合类型，上面没有编译，但我想你可以在这里看到我的想法。如果条件为std::get<Is>(tuple) != std::get<Is+1>(tuple)，我不想传递任何内容，所以我传递NullObject{}，然后以某种方式尝试从f 的参数中删除所有NullObjects，以获得传递给f 的真实参数。

Answer 1

您不能为此使用三元运算符 - 这需要两个具有共同类型的表达式。这里没有骰子。我不知道根据 runtime 比较有条件地返回不同的 type 方法。

您必须有条件地将类型转发给不同的函数，只需引入另一个构建 Args... 的辅助函数和一个简单的 if 语句：

template <size_t... Js>
void fhelper (std::index_sequence<>, const std::set<int>& , std::index_sequence<Js...>) {
    f(Js...);
}

template <std::size_t I, size_t... Is, size_t... Js>
void fhelper (std::index_sequence<I, Is...>, const std::set<int>& set, std::index_sequence<Js...>) {
    if (set.find(I) == set.end()) {
        fhelper(std::index_sequence<Is...>{}, set, std::index_sequence<Js..., I>{});
    }
    else {
        fhelper(std::index_sequence<Is...>{}, set, std::index_sequence<Js...>{});
    }
}

int main() {
    fhelper (std::make_index_sequence<10>{}, {1,3,7,8}, std::index_sequence<>{});
}

Answer 2

我只是想分享我所寻求的一般问题的解决方案。巴里对上述特定问题的解决方案使之成为可能：

#include <iostream>
#include <utility>
#include <tuple>

template <template <std::size_t, typename> class Check, typename F, size_t... Js, typename Tuple>
void screenArguments (std::index_sequence<>, std::index_sequence<Js...>, const Tuple& tuple) {
    F()(std::get<Js>(tuple)...);
}

template <template <std::size_t, typename> class Check, typename F, std::size_t I, size_t... Is, size_t... Js, typename Tuple>
void screenArguments (std::index_sequence<I, Is...>, std::index_sequence<Js...>, const Tuple& tuple) {
    if (Check<I, Tuple>::execute(tuple))
        screenArguments<Check, F>(std::index_sequence<Is...>{}, std::index_sequence<Js..., I>{}, tuple);
    else
        screenArguments<Check, F>(std::index_sequence<Is...>{}, std::index_sequence<Js...>{}, tuple);
}

template <template <std::size_t, typename> class Check, typename F, typename Tuple, std::size_t N = std::tuple_size<Tuple>::value>
void passCertainArguments (const Tuple& tuple) {
    screenArguments<Check, F> (std::make_index_sequence<N>{}, std::index_sequence<>{}, tuple);
}

// Testing
#include <typeinfo>

template <typename... Args>
void foo (Args&&...) {
    std::cout << sizeof...(Args) << " elements passed into foo.\n";
}

struct Foo {
    template <typename... Args>
    void operator()(Args&&... args) {
        foo(std::forward<Args>(args)...);
    }
};

template <typename... Args>
void bar (Args&&...) {
    std::cout << sizeof...(Args) << " elements passed into bar.\n";
}

struct Bar {
    template <typename... Args>
    void operator()(Args&&... args) {
        bar(std::forward<Args>(args)...);
    }
};

template <std::size_t N, typename Tuple>
struct CheckArguments {
    static bool execute (const Tuple& tuple) {
        return std::get<N>(tuple) != std::get<N+1>(tuple);
    }
};

struct NullObject {};

template <std::size_t N, typename Tuple>
struct CheckNotNullObject {
    static bool execute (const Tuple& tuple) {
        return typeid(std::get<N>(tuple)) != typeid(NullObject);
    }
};

template <typename F, typename... Args>
void executeWithoutNullObject (Args&&... args) {
    passCertainArguments<CheckNotNullObject, F> (std::forward_as_tuple(args...));
}

template <typename F, typename... Args>
void executeRemoveConsecutiveRepeatElements (Args&&... args) {
    const auto tuple = std::forward_as_tuple(args...);
    passCertainArguments<CheckArguments, F, decltype(tuple), sizeof...(Args) - 1> (tuple);
}

int main() {
    executeWithoutNullObject<Foo> (3, 5, 'a', true, NullObject{}, 'b', 5.8, NullObject{}, NullObject{}, '!', 2);
    // 8 elements passed into foo.  (the 3 NullObjects are removed)
    executeRemoveConsecutiveRepeatElements<Bar> (2, 1.5, 'a', 'a', true, 5, 5, false);
    // 5 elements passed into bar.  (the first 'a', the first 5, and 'false' removed)
}