c - 无维数组字段，它的用途？

Question

struct test{
    unsigned long int asd[][3][6];
};

sizeof(struct test) 返回 0。因此，如果这是

的确切别名

struct test{
    unsigned long int asd[0][3][6];
};

这样的字段声明有实际用途吗？您可能还会考虑模板元编程的东西，这总是令人惊讶。

Answer 1

第一个示例演示了灵活数组成员 的使用，这是 C99 的一个特性。然而，为了让该片段能够编译，您需要在您的 struct 中有另一个成员，即:

struct test{
    int a;
    unsigned long int asd[][3][6];
};

This documentation在 gcc 上告诉你为什么 sizeof 求值为零，以及普通数组的语法差异:

In ISO C90, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:

• Flexible array members are written as contents[] without the 0.
• Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero.
• Flexible array members may only appear as the last member of a struct that is otherwise non-empty.
• A structure containing a flexible array member, or a union containing such a structure (possibly recursively), may not be a member of a structure or an element of an array. (However, these uses are permitted by GCC as extensions.)