从另一个注释：所以默认情况下我不应该依赖它（因为它可能取决于编译器）

From another comment: "So by default I should not rely on it (as it may depend on the compiler)"

不，它不依赖于编译器，实际上无论如何。任何编译器都不会浪费时间构造一个A，然后复制它。

No, it does not depend on the compiler, practically anyway. Any compiler worth a grain of sand won't waste time constructing an A, then copying it over.

在标准中，它明确表示它完全可以接受 T = x; 等价于说 T（x）; 。 （§12.8.15，pg.211）这样做与 T（T（x））显然是多余的，所以它删除内部 / code>。

In the standard it explicitly says that it is completely acceptable for T = x; to be equivalent to saying T(x);. (§12.8.15, pg. 211) Doing this with T(T(x)) is obviously redundant, so it removes the inner T.

为了获得所需的行为，你必须强制编译器默认构造第一个A：

To get the desired behavior, you'd force the compiler to default construct the first A:

A a;
// A is now a fully constructed object,
// so it can't call constructors again:
a = A(5);