c++ - 数组新表达式中的直接初始化与列表初始化

Question

基本上，为什么这是有效的:

auto p1 = new int[10]{5};

但这不是:

auto p1 = new int[10](5);

更一般地说，新表达式初始化器的规则是什么？

我发现了以下内容:

— If the new-initializer is omitted, the object is default-initialized (8.5). [ Note: If no initialization is performed, the object has an indeterminate value. — end note ] — Otherwise, the new-initializer is interpreted according to the initialization rules of 8.5 for direct- initialization.

那么第二种情况是否无效，因为像 T((5)) 这样的事情是无效的(直接从表达式 (5) 初始化)？或者是什么原因？

编辑:好吧，我对 (()) 的建议似乎很愚蠢，因为我看不出为什么它只适用于数组 new 表达式。

Answer 1

第一个是有效的，因为 list initialization (自 C++11 起)，

new T { arg1, arg2, ... }

和

If T is an aggregate type, aggregate initialization is performed.

int[10]是数组类型，属于聚合类型，则aggregate initialization执行，

If the number of initializer clauses is less than the number of members and bases (since C++17) or initializer list is completely empty, the remaining members and bases (since C++17) are initialized by their default initializers, if provided in the class definition, and otherwise (since C++14) by empty lists, in accordance with the usual list-initialization rules (which performs value-initialization for non-class types and non-aggregate classes with default constructors, and aggregate initialization for aggregates). If a member of a reference type is one of these remaining members, the program is ill-formed.

所以 auto p1 = new int[10]{5}; 将创建一个指向数组的指针，其第一个元素被初始化为 5，其余元素被初始化作为 0。

第二个是direct initialization ，所以 new int[10](5); 表示从单个 int 5int[10]直接；这是无效的。

其实对于array new expression你不能指定一个非空的带括号的初始化器。

If type is an array type, an array of objects is initialized.

• If initializer is absent, each element is default-initialized
• If initializer is an empty pair of parentheses, each element is value-initialized.
• If initializer is a brace-enclosed list of arguments, the array is aggregate-initialized. (since C++11)

所以

auto p1 = new int[10];    // valid
auto p2 = new int[10]();  // valid
auto p3 = new int[10]{5}; // valid

auto p4 = new int[10](5); // invalid

编辑

从标准的角度来看，正如您引用的那样，[expr.new]/18 :

A new-expression that creates an object of type T initializes that object as follows:

• If the new-initializer is omitted, the object is default-initialized ([dcl.init]). [ Note: If no initialization is performed, the object has an indeterminate value. — end note ]

=> 适用于 auto p1 = new int[10];，导致默认初始化。

• Otherwise, the new-initializer is interpreted according to the initialization rules of [dcl.init] for direct-initialization.

和[dcl.init]/17 :

• (17.1) If the initializer is a (non-parenthesized) braced-init-list or is = braced-init-list, the object or reference is list-initialized.

=> 申请auto p3 = new int[10]{5};，导致列表初始化，细节在上面解释。

• (17.4) If the initializer is (), the object is value-initialized.

=> 适用于 auto p2 = new int[10]();，导致值初始化。

• (17.5) Otherwise, if the destination type is an array, the program is ill-formed.

=> 适用于 auto p4 = new int[10](5);。