Memory Allocation in Rust: Box, Vec, and the Global Allocator

Thu, 18 Sep 2025 15:13:16 +0900

This post explores how Rust manages memory through Box, Vec, and the global allocator, uncovering what really happens under the hood.

Box and Vec Memory Allocation

Before diving into implementation details, let’s first understand how Box and Vec internally use allocators.

Box

Box is the simplest smart pointer in Rust. It allocates a value on the heap while making ownership explicit and safe.

Under the hood, though, Box is more complex than the Box<T> we usually write. Here’s the actual definition:

Rust: How the Compiler Manages Lifetimes

Thu, 11 Sep 2025 20:19:41 +0900

Most people just use lifetimes without understanding how they are implemented under the hood. So I decided to dig into the compiled binary to see how Rust actually manages lifetimes at the machine level.

The String Struct Layout

Before diving into the assembly, it helps to recall how Rust represents a String internally. A String is essentially a wrapper around a Vec<u8>, which in turn manages a heap-allocated buffer:

Field	Description
ptr	Pointer to the heap-allocated buffer
cap	Capacity (number of bytes the buffer can hold without reallocating)
len	Current length (number of bytes used)

Move value

In Rust, ownership ensures that when a value is passed to a function by move, the caller loses access to it. This guarantees that memory can be safely freed once the function finishes. Unlike in C, where the programmer decides when to free memory, Rust enforces this through the type system.

Mns03's Blog

Memory Allocation in Rust: Box, Vec, and the Global Allocator

Box and Vec Memory Allocation

Box

Rust: How the Compiler Manages Lifetimes

The String Struct Layout

Move value