c++ - SGI STL sub-allocation free-lists

Question

In sub-allocation of SGI STL allocator, there are 16 free-lists which separately manage small blocks of size 8,16,....,128. The structure of the free-lists' node is:

union obj{
    union obj *free_list_link;
    char client_data[1];
}

My question is : why it is designed like this? What is the member client_data used for?

Answer 1

这是一种常见的设计模式。client_data[] 实际上是 8、16、.. 128 字节。如果您查看分配 obj 的位置，它可能看起来像 malloc( sizeof(obj) + 128 ) 用于 128 字节块。