String vs str in Rust: Main Differences with Examples

String vs str in Rust: Why Beginners Get Confused and How to Understand It Once and For All

If you're just starting to learn Rust, you've probably already encountered a situation: the compiler complains, you try to pass a String where &str is expected, or vice versa. Sound familiar? This is one of the most popular questions on Stack Overflow (thousands of views!), and for good reason — the topic is truly fundamental for understanding memory management in Rust.

In this article, we'll break down the differences between String and &str, when to use each type, and how to avoid typical mistakes. Let's go!

Short Answer for Those in a Hurry

If you need it right now — remember the main points:

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• String is an owning, mutable, dynamic data type that is stored on the heap.
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• &str is an immutable string slice that references data owned by someone else. It is most commonly encountered as &str.
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Now let's dig deeper.

What is String in Rust?

String is a type that owns its data. When you create a String, memory for it is allocated on the heap, and the string can grow, shrink, or be modified.

let mut s = String::from("Hello");\ns.push_str(", world!");\nprintln!("{}", s); // Output: Hello, world!

Here, s is the owner of the string. When s goes out of scope, the memory is automatically freed (thanks to RAII — Resource Acquisition Is Initialization).

Key Characteristics of String:

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• Owns the data (owning type)
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• Mutable
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• Stored on the heap
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• Can be created from a string literal: String::from("text")
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• Supports concatenation, insertion, and deletion of characters
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What is &str in Rust?

&str (pronounced "string slice") is a reference to a sequence of UTF-8 characters that belongs to someone else. By itself, &str is simply a pointer to the data and a length.

let greeting: &str = "Hello";\nprintln!("{}", greeting);

In this example, greeting is a reference to a string literal that is stored directly in the program's binary file (in the read-only memory segment). greeting does not own this data — it just looks at it.

Key Characteristics of &str:

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• Does not own the data (borrowed type)
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• Immutable
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• Stored either in the binary (string literals) or on the heap (as a slice from a String)
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• Lightweight — consists of two fields: pointer + length (16 bytes on a 64-bit system)
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• Ideal for passing to functions when you don't need to modify the string
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Key Differences Between String and &str (Table)

Let's summarize everything in a clear table:

Characteristic	String	&str
Ownership	Owns the data	Borrows the data
Mutability	Yes (mut)	No
Storage Location	Heap	Heap, stack, or binary
Size on Stack	24 bytes (ptr + len + cap)	16 bytes (ptr + len)
Creation	String::from("...")	"..." or &s[..]
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