它是与未知大小的数组的定义吗？ [英] Is it a definition of an array with unknow size?

问题描述

我们可以的 decalre 的一个不完整的对象类型的变量，但不能定义一个不完全类型的对象。
3.9 / 4说：

We can decalre a variable of an incomplete object type, but cannot define an object of an incomplete type. As 3.9/4 says:

这已宣告但尚未定义的类，在枚举类型
  某些情况下（7.2），或未知大小或不完整的阵列的
  的元素类型的，是不完全定义的对象类型。
  完全确定对象类型和无效类型是不完整
  类型（3.9.1）。 <青霉>物件不应被定义为具有一个不完整的的
  的类型。的

A class that has been declared but not defined, an enumeration type in certain contexts (7.2), or an array of unknown size or of incomplete element type, is an incompletely-defined object type. Incompletely defined object types and the void types are incomplete types (3.9.1). Objects shall not be defined to have an incomplete type.

所以下面的例子

int a[];//error: storage size of 'a' isn't known

int main()
{
}

是病态的。

演示

DEMO

但是，当我们初始化变量如下：

But, when we initialize that variable as follows:

int a[] = {1,2}; //OK

int main()
{
}

演示

DEMO

我不明白为什么在后者的例子 int类型的[] = {1,2}; 不会引发像的编译时错误一个不完全类型的对象的定义的。你能使用标准解释呢？

I don't understand why in the latter example int a[] = {1,2}; doesn't throw a compile-time error like Definition of an object of an incomplete type. Could you explain it using the Standard?

推荐答案

从N3936报价§8.3.4/ 1

Quoting from N3936 §8.3.4/1

...除以下，如果省略恒定前pression，的类型
   D 的标识符为导出声明符类型列表的约束 T的未知的阵列，一个不完整的对象类型。 ...

... Except as noted below, if the constant expression is omitted, the type of the identifier of D is "derived-declarator-type-list array of unknown bound of T", an incomplete object type. ...

除了如下面指出的是指§8.3.4/ 3

"Except as noted below" refers to §8.3.4/3

...也可在说明符后跟一个省略结合的阵列
  初始化（8.5）。在这种情况下，边界被从最初的元素（比如 N ）提供的数计算
  （8.5.1）和 D的标识符的类型是的 N个阵列 T 。......

... An array bound may also be omitted when the declarator is followed by an initializer (8.5). In this case the bound is calculated from the number of initial elements (say, N) supplied (8.5.1), and the type of the identifier of D is "array of N T." ...

因此​​，在

int a[] = {1, 2};

声明的类型 A 其实就是 INT [2] ，而不是为int [] 。

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