Classes vs Structs: A Developer’s Guide to Performance

Summary: Unlock the core differences between classes vs structs. Learn when to use each for high-performance, clean code in C#, Swift, C++, and more.

Introduction

The core difference between classes and structs is simple but decisive: classes are reference types and structs are value types. This distinction drives memory layout, copy behaviour, and runtime performance across languages like C#, C++, and Swift. Understanding when to choose one over the other is essential for predictable code and high-performing systems.

Understanding the Fundamental Difference

When you instantiate a class, the variable you hold is a reference that points to the object on the heap. Copying that variable copies the reference; multiple references can point to the same object, so changes via one reference are visible through others. This reference semantics is a key reason classes are used for entities with identity.¹

A struct represents the data itself. Creating a struct produces a concrete bundle of values—often stored on the stack or inline in arrays—so copying a struct yields an independent duplicate. Modifying the copy does not affect the original, which makes structs great for simple, immutable values.¹

For more on encapsulation and object design, see our guide on object-oriented encapsulation: https://cleancodeguy.com/blog/object-oriented-encapsulation.

Quick Comparison: Reference vs Value Types

Use a class when you need shared identity. Use a struct when you need a simple, self-contained value that can be copied without side effects.

This foundation informs deeper performance trade-offs—heap vs stack allocation, cache locality, and garbage collection pressure—which we explore below.

How Memory Allocation Dictates Speed

Memory layout affects CPU efficiency, throughput, and latency. A class access typically involves an indirection: a pointer on the stack references data on the heap. That extra lookup adds cost and can hurt cache behaviour. Structs, stored directly where the variable lives, often avoid that indirection and enable tighter memory layouts with better cache locality.¹

Garbage Collection Costs

Objects on the heap are subject to garbage collection. GC cycles can pause execution, increasing latency in real-time or high-throughput systems. Frequent allocation of short-lived class objects increases GC pressure and CPU overhead. Using value types for many small, short-lived objects reduces heap churn and GC work.³

Heap allocation adds potential GC overhead. Structs avoid that overhead when they stay value-based and unboxed.

This is especially important in systems designed for scale and responsiveness—reducing allocations directly reduces GC activity and can smooth runtime performance.

Cache Locality and Throughput

Modern CPUs rely on caches. Sequential layouts—such as arrays of structs—improve cache hits and throughput. Separate heap allocations for each class instance scatter data across memory, increasing cache misses and slowing processing. For tight loops and data-processing pipelines, contiguous value layouts are a major advantage.⁵

The Boxing Trap

Boxing occurs when a value type is converted to a reference type (for example, when placed into a collection that expects objects). Boxing allocates a heap object and copies the value into it, negating the struct’s performance advantages and increasing GC load. Avoiding boxing is a core principle of efficient value-type usage.⁴

How Languages Differ: C#, C++, and Swift

Different languages enforce different conventions and capabilities. Knowing the language-specific rules prevents applying one-language rules blindly to another.

C#: Clear Reference vs Value Model

In C#, class = reference type and struct = value type. Use classes for entities with identity (for example, Customer or DatabaseConnection) and structs for small, immutable values (for example, Point, Color). Keeping structs small and immutable avoids subtle bugs and copying overhead.¹

Common mistakes include making large or mutable structs; both lead to surprising bugs or performance regressions. Follow the immutable, small-struct guideline when optimizing in C#.

C++: Convention Over Language Restriction

In C++, the only syntactic difference between struct and class is default accessibility. Both can be allocated on stack or heap, have methods, and support inheritance. The convention is to use struct for plain data aggregates and class for encapsulated objects and RAII resource management.

This flexibility means C++ developers must rely on conventions and design choices rather than language-enforced value/reference distinctions. For guidance on polymorphism and inheritance patterns, see our C++ design notes: https://cleancodeguy.com/blog/polymorphism-vs-inheritance.

Swift: Value-Oriented by Default

Swift encourages preferring structs for most custom types. Structs in Swift support methods, extensions, and protocol conformance, making them powerful yet safe defaults. Choose classes only when reference semantics, identity, or Objective-C interoperability is required.²

This value-first design encourages immutability and easier reasoning about data flow, particularly in concurrent code.

When to Choose a Struct for Maximum Efficiency

Structs are ideal for small, immutable bundles of data whose identity is defined entirely by their values. Typical examples:

• Geometric data: Point2D or RGBColor
• Financial values: Money (amount + currency)
• Small DTOs used in high-throughput pipelines

A practical size guideline is the “16–32 byte” rule: if a struct’s fields fit in roughly that range, copying cost is modest and often cheaper than heap allocation. If a struct grows larger or must be mutable, a class is likely the better choice.⁵

Immutability and Size Rules

• Prefer immutable structs: values should be created once and replaced rather than mutated.
• Keep structs small: copying large structs frequently can become more expensive than passing references.

These rules help avoid silent bugs (from mutable copies) and performance traps (from excessive copying or boxing).

Common Pitfalls and Refactoring

Two common problems are mutable structs and excessive boxing.

Mutable structs lead to surprising behaviour because modifications affect only a copy. Refactor mutable structs into immutable ones that return new instances for state changes.

Boxing happens implicitly in many APIs and collections; identify and remove boxing hotspots to preserve struct performance benefits.

Example: Refactor a Mutable Point into an Immutable Struct (C#)

// PITFALL: Mutable struct public struct MutablePoint { public int X { get; set; } public int Y { get; set; }

public void Move(int dx, int dy)
{
    X += dx;
    Y += dy;
}

}

// REFACTOR: Immutable struct public readonly struct ImmutablePoint { public int X { get; } public int Y { get; }

public ImmutablePoint(int x, int y)
{
    X = x;
    Y = y;
}

public ImmutablePoint MovedBy(int dx, int dy)
{
    return new ImmutablePoint(X + dx, Y + dy);
}

}

This refactor makes intent explicit and eliminates accidental state corruption. For more clean-coding practices, see our principles guide: https://cleancodeguy.com/blog/clean-coding-principles.

Frequently Asked Questions (Concise Q&A)

Q1: When should I prefer a struct over a class?

A: Prefer a struct when the type is small, immutable, and represents a value rather than an identity. Structs shine for simple data like points, colors, or small DTOs.

Q2: What performance pitfalls should I watch for?

A: Avoid mutable structs, large structs (copying cost), and boxing into heap objects—these nullify the benefits of value types and can harm performance.

Q3: How do language differences affect my choice?

A: Follow language idioms: C# enforces value vs reference types; C++ uses convention; Swift prefers value types by default. Learn the platform rules before applying cross-language patterns.¹²

At Clean Code Guy, we help teams apply these principles to real codebases. Our Codebase Cleanups and AI-Ready Refactors make software faster, safer, and easier to maintain. Visit https://cleancode.com to learn more.