c++ - 为什么 auto 的推论不同？

Question

int main(){
    int x{};
    auto x2 = x;
    auto x3{x};

    static_assert(is_same<int, decltype(x)>::value, "decltype(x) is the same as int");
    static_assert(is_same<int, decltype(x2)>::value, "decltype(x2) is the same as int");
    static_assert(is_same<int, decltype(x3)>::value, "decltype(x3) is the same as int"); // Error here.
}

这段代码不能用 gcc 4.8.0 编译。 我什至不猜测 decltype(x3) 的类型。它是什么？为什么行为不同？

Answer 1

#include <initializer_list>
#include <type_traits>

using namespace std;

int main(){
    int x{};
    auto x2 = x;
    auto x3{x};

    static_assert(is_same<int, decltype(x)>::value, "decltype(x) is the same as int");
    static_assert(is_same<int, decltype(x2)>::value, "decltype(x2) is the same as int");
    static_assert(is_same<std::initializer_list<int>, decltype(x3)>::value, "decltype(x3) is the same as int");
}

这将编译。 x3被推断为 std::initializer_list<int>由于:

Let T be the type that has been determined for a variable identifier d. Obtain P from T [...] if the initializer is a braced-init-list (8.5.4), with std::initializer_list<U>.