c++ - C++ 20从带括号的值列表初始化聚合，不支持内部数组

Question

C++ 20采用了p0960-允许从带括号的值列表中初始化聚合。

确切的措辞([dcl.init] 17.6.2.2)说:

[...] if no constructor is viable, the destination type is an aggregate class, and the initializer is a parenthesized expression-list, the object is initialized as follows.

Let e₁ , …, e_n be the elements of the aggregate ([dcl.init.aggr]).

Let x₁, …, x_k be the elements of the expression-list.

If k is greater than n, the program is ill-formed.

The element e_i is copy-initialized with x_i for 1 ≤ i ≤ k . The remaining elements are initialized with their default member initializers [...]

这不允许使用带括号的值列表初始化内部数组:

struct Foo {
    int i, j;
};

struct Moo {
    int arr[2];
};

int main() {
    // before C++20:
    Foo foo1{1, 2};
    // with C++20:
    Foo foo2(1, 2); // p0960! we are good

    // before C++20:
    Moo moo1{1, 2};
    // C++20 - oops p0960 doesn't help here:
    Moo moo2(1, 2); // error: too many initializers

    // before C++20:
    std::array<int, 2> arr1{1, 2};   // OK
    std::array<int, 2> arr2({1, 2}); // OK
    std::array<int, 2> arr3{{1, 2}}; // OK
    // C++20 - oops p0960 doesn't help here:
    std::array<int, 2> arr4(1, 2); // error: too many initializers
}

不能使用圆括号将 std::array初始化这一事实使它无法参与从值列表中创建未知类型 T的对象的通用代码中(例如，使用 make_shared， make_unique和 make_from_tuple等的算法)。

为什么p0960没有采取更简单的方法使 ()初始化更像 {} ？

例如，类似:

if no constructor is viable, the destination type is an aggregate class, and the initializer is a parenthesized expression-list, the object would be initialized as if the values were sent with brace-initialization.

Answer 1

p0960在r1和r2之间改变:

r2: This revision changes the mental model from the original “literal rewrite to a braced list” to “as if a synthesized, explicit constructor with appropriate mem-initializers was called” ¹. This has the effect of allowing narrowing conversions in the parenthesized list, even when narrowing conversions would be forbidden in the corresponding braced list syntax. It also clarifies the non-extension of temporary lifetimes of temporaries bound to references, the absence of brace elision, and the absence of a well-defined order of evaluation of the arguments.

可以在更改的p0960设计原理中找到进行此更改的原因:

r1: Parenthesized initialization and braced-initialization should be as similar as possible.

r2:Parenthesized initialization and braced-initialization should be as similar as possible, but as distinct as necessary to conform with the existing mental models of braced lists and parenthesized lists.(emphasis mine)

“为什么p0960没有采用更简单的方法使()初始化更像{}？”:
当决定采用现有的心理模型 1 时，不允许括号删除似乎是唯一的方法。