vector internal array: always on heap; cleaned up via RAII

Across languages, vector is the #1 most common and useful container. Hashtable might be #2.

I used to feel that a vector (and hashtable) can grow its “backing array” without heap. There’s global memory and stack memory and perhaps more. Now I think that’s naive.

  • Global area is allocated at compile time. The array would be fixed in size.
  • For an array to be allocated on stack, the runtime must know its size. Once it’s allocated on that stack frame, it can’t grow beyond that stack frame.
  • In fact, any array can’t grow at all.

The vector grows its backing array by allocating a bigger array and copying the payload. That bigger array can’t be in global area because this type on-demand reallocation is not compile-time.

Anything on heap is accessed by pointer. Vector owns this hidden pointer. It has to delete the array on heap as in RAII. No choice.

Now the painful part — heap allocation is much slower than stack allocation. Static allocation has no runtime cost at all after program starts.

发表评论

Fill in your details below or click an icon to log in:

WordPress.com 徽标

您正在使用您的 WordPress.com 账号评论。 登出 /  更改 )

Google photo

您正在使用您的 Google 账号评论。 登出 /  更改 )

Twitter picture

您正在使用您的 Twitter 账号评论。 登出 /  更改 )

Facebook photo

您正在使用您的 Facebook 账号评论。 登出 /  更改 )

Connecting to %s