rust - 在 Rust 中，向量如何在编译时既是动态的又是已知的？

Question

我对 documentation for vectors in Rust 中似乎相互矛盾的陈述感到困惑:

A ‘vector’ is a dynamic or ‘growable’ array, implemented as the standard library type Vec<T>.

和

Vectors store their contents as contiguous arrays of T on the heap. This means that they must be able to know the size of T at compile time (that is, how many bytes are needed to store a T?). The size of some things can't be known at compile time. For these you'll have to store a pointer to that thing: thankfully, the Box type works perfectly for this.

Rust 向量是动态增长的，但我看不出这与必须在编译时知道它们的大小的说法相符。

自从我使用低级语言以来我已经有一段时间了，在这种语言中我必须考虑内存分配，所以我可能遗漏了一些明显的东西。

Answer 1

注意措辞:

they must be able to know the size of T

这表示必须知道单个元素的大小。元素的总数以及分配的内存总量是未知的。

当向量分配内存时，它说“我想存储 12 个 FooBar 结构。一个 FooBar 是 24 字节，因此我需要总共分配 288 字节”。

12是向量的动态容量，24是一个元素(T)的静态大小。