std::apply 中的元组无法完美转发/移动构造

Question

我正在尝试使用嵌套的 lambda 捕获构建完美的转发（零复制）构造。我希望应该有零复制结构，但有些东西坏了。

我从可变参数转移。打包到元组（移动正常）然后我将元组（移动正常）传递给 std::apply 并在最终嵌套的 lambda 中我 assemble 另一个元组（预计移动正常但 Wrapper CTOR 是 COPY 而不是 MOVE：

#include <iostream>

// tuple printer (ignore it)
template<typename Type, unsigned N, unsigned Last>
struct tuple_printer {
    static void print(std::ostream& out, const Type& value) {
        out << std::get<N>(value) << ", ";
        tuple_printer<Type, N + 1, Last>::print(out, value);
    }
};

template<typename Type, unsigned N>
struct tuple_printer<Type, N, N> {
    static void print(std::ostream& out, const Type& value) {
        out << std::get<N>(value);
    }
};

template<typename... Types>
std::ostream& operator<<(std::ostream& out, const std::tuple<Types...>& value) {
    out << "(";
    tuple_printer<std::tuple<Types...>, 0, sizeof...(Types) - 1>::print(out, value);
    out << ")";
    return out;
}

// THE FUNCTION that returns lambda:
template <class ... Args>
auto f(Args && ... args)
{
    // v--- args is a tuple<arg1, arg2, arg3 ...>
    return [args_upper = std::make_tuple(std::forward<Args>(args)...)]()
    {
        // v-- lower "args" is a restored list of arguments - moved from the upper tuple "args"
        return std::apply([](auto && ... args) {
            // v--- here the Wrapper COPY-CTOR is called instead of moved from
            return std::make_tuple(std::forward<decltype(args)>(args)...);
        }, std::move(args_upper));
    };
}

struct Wrapper {
    Wrapper() {
        std::cout << "CTOR ";
    }
    Wrapper(const Wrapper & r) {
        std::cout << "COPY-CTOR ";
    }
    Wrapper(Wrapper&& r) {
        std::cout << "MOVE-CTOR ";
    }
    int w = 42;
    friend std::ostream& operator<<(std::ostream& out, const Wrapper& w);
};

std::ostream& operator<<(std::ostream& out, const Wrapper& w) {
    out << w.w;
    return out;
}

int main() {
    auto l = f(1,2.f,"st", Wrapper{});
    auto t = l(); // t is tuple
    std::cout << t; // tuple printer
//    std::cout << l();
}

给出输出 CTOR MOVE-CTOR COPY-CTOR (1, 2, st, 42)

Answer 1

return [args_upper = std::make_tuple(std::forward<Args>(args)...)]()

将此更改为

return [args_upper = std::make_tuple(std::forward<Args>(args)...)]() mutable

args_upper 隐含地 const 除非你使你的 lambda mutable。这将阻止移动语义。

std::apply 中的元组无法完美转发/移动构造

perfect forwarding / move construction isn't working from tuple in std::apply

c++

templates

lambda

variadic-templates

perfect-forwarding