C语言灵活数组成员

March 22, 2024

摘自 C99 §6.7.2.1, item 16：https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

As a special case, the last element of a structure with more than one named member may
have an incomplete array type; this is called a flexible array member. In most situations,
the flexible array member is ignored. In particular, the size of the structure is as if the
flexible array member were omitted except that it may have more trailing padding than
the omission would imply. However, when a . (or ->) operator has a left operand that is
(a pointer to) a structure with a flexible array member and the right operand names that
member, it behaves as if that member were replaced with the longest array (with the same
element type) that would not make the structure larger than the object being accessed; the
offset of the array shall remain that of the flexible array member, even if this would differ
from that of the replacement array. If this array would have no elements, it behaves as if
it had one element but the behavior is undefined if any attempt is made to access that
element or to generate a pointer one past it.

根据规范，一个拥有至少两个命名成员的结构体，允许其最后一个成员是一个数组，且没有写出长度（或者长度是 0，因此灵活数组成员也称零长度数组成员）

当然，还有可能有一些字节对齐的情况，此处省略，具体参见规范。

下面的示例实现了在堆上创建指定长度的数组（这有点类似与在栈上创建的变长数组）
此方法实现了数组的长度和数据都分配到一块连续内存，效果优于定义一个指针指向数据。

// preview: https://ideone.com/hWjI9C
#include <stdio.h>
#include <stdlib.h>

typedef struct
{
    int len;
    double array[]; // 注意这与定义一个指针的不同之处
} Vectord;

Vectord *newVectord(int len)
{
    Vectord *vec = (Vectord *)malloc(sizeof(int) + sizeof(double) * len);
    vec->len = len;
    for (int i = 0; i < len; i++)
        vec->array[i] = 0;
    return vec;
}

void printVectord(Vectord *vec)
{
    for (int i = 0; i < vec->len; i++)
        printf("%f ", vec->array[i]);
    puts("");
}

int main()
{
    Vectord *vec = newVectord(3);
    printVectord(vec); // -> 0.000000 0.000000 0.000000

    vec->array[0] = 2;
    vec->array[1] = 3;
    vec->array[2] = 3;

    printVectord(vec); // -> 2.000000 3.000000 3.000000
}