c++ - 大括号初始化列表中的对象创建顺序

Question

#include <iostream>

struct A
{
    A() { std::cout << "(A::A)"; }
};

struct B
{
    B() { std::cout << "(B::B)"; }
};

struct C
{
    template<typename ...Args>
    C(Args && ...) {}
};

int main(int agrc, char *argv[])
{
    C {A(), B()}; // <-- prints (B::B)(A::A)
    std::cout << std::endl;
    C {(A(), B())}; // <-- prints (A::A)(B::B)
    std::cout << std::endl;

    return 0;
}

我有 2 个问题:

• 为什么在第一个大括号初始化列表中对象是按从右到左的顺序创建的？
• 为什么第二种情况下的括号会恢复此顺序？

编辑:我已经用 msvs 2013 编译了它

Answer 1

在第二种情况下，您实际上只是使用 B(); 进行初始化；通过逗号操作符的使用，A()首先被构造和丢弃。

C {(A(), B())};
//|^^^^^^^^^^|
//      \--------> (A(), B())
//                  ^^^  ^^^
//                   |    |
//                   /    \
//            evaluated,   THEN evaluated,
//            discarded      used

---

另一方面，在第一个实例中，您通过初始化列表从 both 临时变量中初始化 C，其元素也应该从左到右进行评估- 对，碰巧，但你的编译器在这方面有问题:

[C++11: 8.5.4/4]: Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions (14.5.3), are evaluated in the order in which they appear. That is, every value computation and side effect associated with a given initializer-clause is sequenced before every value computation and side effect associated with any initializer-clause that follows it in the comma-separated list of the initializer-list. [ Note: This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of the initializer-list are interpreted as arguments of a constructor call, even though ordinarily there are no sequencing constraints on the arguments of a call. —end note ]

我可以使用 GCC 4.8^* 重现该问题，但 Clang 3.5 的行为正常^†。 std-discussion 列表中已经讨论过该错误^‡，但我还没有找到 GCC Bugzilla ID^§。 p>

C {A(), B()};
// ^^^  ^^^
//  |    \
// eval-  THEN
// uated   evaluated
//  \       /
//   \     /
//  both used

---

^{* http://coliru.stacked-crooked.com/a/1f18e0d1f8973f3c
† http://coliru.stacked-crooked.com/a/5a6e7506e9be97c3
‡ https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/TQUnBFkUBDg
§ #51253可能是相关的。}