Memory Layout in C++

Memory Layout in C++

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Prerequisites

- Struct
- Class

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1. Memory Layout

Memory layout defines how data is physically arranged in memory.

👉 Performance depends heavily on:

• Cache line usage
• Data locality
• Access pattern

2. Struct

struct A 
{
    char c;
    int i;
};

Memory (with padding):

[c][pad][pad][pad][i][i][i][i]

• CPU prefers aligned access
• Compiler inserts padding automatically

Why important?

• Affects size
• Affects cache efficiency

✔ Optimization by Reordering

// ❌ Inefficient
struct A 
{
    char c;
    int i;
    char d;
};

// ✔ Optimized
struct A 
{
    int i;
    char c;
    char d;
};

Less padding → better cache usage

3. Class

✔ Same as struct (mostly)

class A 
{
    int x;
    float y;
};

Same layout as struct

✔ With Virtual Function

class Base 
{
public:
    virtual void foo() {}
    int x;
};

Memory:

[vptr][x]

• vptr = pointer to virtual table
• Adds memory + runtime overhead

4. Optimization

AoS (Array of Structures)

struct Particle 
{
    float x, y, z;
    float vx, vy, vz;
};

Particle particles[N];

Memory:

[x y z vx vy vz][x y z vx vy vz][x y z vx vy vz]...

✔ Problem

for (int i = 0; i < N; i++) 
    particles[i].x += 1.0f;

• Loads entire struct
• Uses only x

❌ Wasted memory bandwidth
❌ Poor cache efficiency

5. SoA (Structure of Arrays)

✔ Definition

struct Particles 
{
    float x[N], y[N], z[N];
    float vx[N], vy[N], vz[N];
};

Memory:

[x x x x x ...]
[y y y y y ...]
[z z z z z ...]

✔ Advantage

for (int i = 0; i < N; i++) 
    particles.x[i] += 1.0f;

• Only loads required data
• Perfect sequential access

✔ Cache-friendly
✔ SIMD-friendly

Cache Perspective

CPU reads memory in **cache lines (~64 bytes)**

AoS

[x y z vx vy vz] → unnecessary data loaded

• AoS → wasted bandwidth

SoA

[x x x x x] → only needed data loaded

• SoA → efficient bandwidth