What is memory alignment and why does it matter?

Question

Accepted Answer

**Alignment** — data must start at addresses divisible by alignment.

```cpp
struct Bad {
    char a;   // 1 byte
    int b;    // 4 bytes (might need padding)
}; // sizeof might be 8, not 5

struct Good {
    int b;    // 4 bytes
    char a;   // 1 byte + 3 padding
}; // sizeof is 8 (better cache usage)
```

**alignof and alignas:**
```cpp
std::cout << alignof(int);   // 4
std::cout << alignof(double); // 8

alignas(64) char cache_line[64]; // aligned to 64 bytes
```

**Why it matters:**
- CPU reads aligned data faster
- Misaligned access causes faults on some architectures
- SIMD instructions require alignment
- Cache line optimization

C++: What is memory alignment and why does it matter?