Learning boost SP 1 — 什麼是complete type

在學習boost的過程中，我們發現在boost的文檔中，多處提及了complete type和incomplete type。到底什麼是complete type呢？經過我尋找msdn，終於發現了，原來complete type是編譯器能夠確定類型大小的類型，incomplete type是編譯器不能確定類型大小的類型。incomplete type有：

• 沒有確定成員的struct
• 沒有確定成員的union
• 沒有確定大小的數組

看看msdn吧：

C Language Reference

Incomplete Types

An incomplete type is a type that describes an identifier but lacks information needed to determine the size of the identifier. An "incomplete type" can be:

• A structure type whose members you have not yet specified.
• A union type whose members you have not yet specified.
• An array type whose dimension you have not yet specified.

The void type is an incomplete type that cannot be completed. To complete an incomplete type, specify the missing information. The following examples show how to create and complete the incomplete types.

• To create an incomplete structure type, declare a structure type without specifying its members. In this example, the ps pointer points to an incomplete structure type called student.

  struct student *ps;

• To complete an incomplete structure type, declare the same structure type later in the same scope with its members specified, as in

  struct student{    int num;}                   /* student structure now completed */

• To create an incomplete array type, declare an array type without specifying its repetition count. For example:

  char a[];  /* a has incomplete type */

• To complete an incomplete array type, declare the same name later in the same scope with its repetition count specified, as in

  char a[25]; /* a now has complete type */

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