> For the complete documentation index, see [llms.txt](https://bitcoin-dev-project.gitbook.io/rust-for-bitcoiners/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://bitcoin-dev-project.gitbook.io/rust-for-bitcoiners/curriculum/module_2/1_struct_type.md).

# Data Types

## Structs

In Rust structs are types that are composed of other types. You can think of structs as product types similar to a Cartesian product between sets. It is a collection of *n* types. At any point in time the value of a struct is the combination of all the *n* types. This is very similar to a simple class in Java or struct in C. In software design terms, `struct` enables Composition and record types.

**Example 1**

In rust the supported primitive unsigned integer types are `u8`, `u16`, ..., `u128`. In order to do elliptic curve cryptography we need to be able to represent unsigned integers using 256 bits,

```rust
// capital `U` is used to signify that this is a user defined type.

struct U256 {
    x0: u128, // least significant bits
    x1: u128, // most significant bits
}
```

The compiler knows that in order to store the value of type `U256` it would require 256 bits because of the field types. Hence every value of `U256` will be stored in the stack since its size is known at compile time. Moreover rust requires that the value of every field type should have a known size at compile time.

**Example 2**

If that is the case, then how do we deal with dynamically-sized data that cannot be known at compile time? Let's look at an example of a `vec` which is often modified at runtime. Here is a simplified version of how `Vec` is represented in rust,

```rust
struct Vec<T, A: Allocator> {
    buf: RawVec<T, A>,
    len: usize,
}

struct RawVec<T, A: Allocator> {
    ptr: Unique<T>, // Size of a pointer is either 32 or 64 bits based on the CPU architecture
    cap: Cap, // just a wrapper over `usize`
    alloc: A,
}
```

The `RawVec` type has a pointer field, `ptr`, which points to a block of memory (aka **buffer**) that is in the heap. The `alloc` field contains a type that has methods to manage heap memory allocations. The `cap` field indicates the buffer's capacity or allocated space in memory.

To better understand `cap` let's consider a scenario,

```rust
let xs: Vec<u32> = Vec::new();

for i in 0..10 {
    xs.push(i);
}
```

Initially `cap == 0`. When the first data push happens `cap` will be increased to `4`. Therefore, during the 2nd, 3rd and the 4th push the capacity will not be changed, and no new allocation will happen. During the 5th push, however, a new buffer that will hold 8 `u32`s will be created and the contents from the current buffer will be copied to the new buffer and `cap == 8`. After this whenever the `len == cap` a new buffer of `2 * cap` will be created and so on ..

This process is known as *reallocation* and is quite costly in terms of performance. If you know ahead of time that you will be pushing a lot of data to a `vec`, it's good practice to invoke a `vec` using the `with_capacity` method to indicate how much space to allocate ahead of time and avoid costly reallocations.

NOTE 1: The `RawVec` type uses unsafe code to handle allocations, and pointer management stuff. NOTE 2: The size of both `Vec` and `RawVec` to be stored in stack of the process is known at compile-time.

**Example 3**

Let's look at a widely used wrapper type,

```rust
struct String {
    vec: Vec<u8>,
}
```

These wrapper types are zero-cost abstractions, meaning these exists only at run-time. The number of bytes required to store a value of `String` is equal to the number of bytes required to store the underlying `Vec<u8>`.

The purpose is encapsulation, `String` can't be used like a `Vec<u8>`. Checkout the documentation for [String](https://doc.rust-lang.org/std/string/struct.String.html) and [Vec](https://doc.rust-lang.org/std/vec/struct.Vec.html), for more info.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://bitcoin-dev-project.gitbook.io/rust-for-bitcoiners/curriculum/module_2/1_struct_type.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.