当参数是参数包时，右值引用不起作用

Question

当我从模板参数包中生成右值时，它无法编译，但如果它是 'simple' 模板参数，则可以正常编译。

在此代码中，for_each_in_tup1 可以正常编译，但 for_each_in_tup3 不能。我不明白为什么这不会编译，但 GCC 9.2 和 VC v142 都认为这是错误的。

为什么这个语法无效？

这个例子：

#include <tuple>
using namespace std;

template< typename Tuple, size_t N = 0>
void for_each_in_tup1( Tuple&& tup ) {
    if constexpr(N < tuple_size<decay_t<Tuple>>::value)
        for_each_in_tup1<Tuple, N + 1>(forward<Tuple>(tup));
}

template< template<typename...> typename Tuple, typename... Ts>
void for_each_in_tup2( const Tuple<Ts...>& tup) {
}

template< template<typename...> typename Tuple, typename... Ts>
void for_each_in_tup3( Tuple<Ts...>&& tup) {
}

void test_lazy() {
    tuple<uint32_t, uint32_t> tup;
    for_each_in_tup1(tup);
    for_each_in_tup2(tup);
    for_each_in_tup3(tup);
}

失败：

<source>:20:22: error: cannot bind rvalue reference of type 'std::tuple<unsigned int, unsigned int>&&' to lvalue of type 'std::tuple<unsigned int, unsigned int>'
   20 |     for_each_in_tup3(tup);
      |                      ^~~

<source>:15:39: note:   initializing argument 1 of 'void for_each_in_tup3(Tuple<Ts ...>&&) [with Tuple = std::tuple; Ts = {unsigned int, unsigned int}]'
   15 | void for_each_in_tup3( Tuple<Ts...>&& tup) {
      |                        ~~~~~~~~~~~~~~~^~~

godbolt

Answer 1

在 for_each_in_tup1 中，您正在使用 转发参考:

template< typename Tuple, size_t N = 0>
void for_each_in_tup1( Tuple&& tup )

当你传递左值时，Tuple被推导为Tuple&，在引用合并后你将得到for_each_in_tup1(Tuple&)。左值可以绑定到左值引用，这就是这段代码有效的原因。如果您将右值传递给 for_each_in_tup1，参数将是 Tuple&&，并且它只能接受右值。

在for_each_in_tup3中有普通的右值引用，它只能绑定右值。要调用 for_each_in_tup3，您必须将 tup 转换为右值，例如 std::move:

for_each_in_tup3(std::move(tup));

当参数是参数包时，右值引用不起作用

rvalue reference not working when parameter is a parameter pack

c++

variadic-templates

c++17