Memory Impact of Padding

The impact of padding on memory usage can be significant, but it varies depending on your data structures and how they're used. Let's look at some practical examples:

Small Scale Impact

Here’s an example where we create an inefficient struct that is 8 bytes larger than it needs to be:

1#include <iostream>
2
3struct Inefficient {
4  char A;// 1 byte
5  int B;// 4 bytes
6  char C;// 1 byte
7  double D;// 8 bytes
8};
9
10struct Optimized {
11  double D;// 8 bytes
12  int B;// 4 bytes
13  char A;// 1 byte
14  char C;// 1 byte
15};
16
17int main() {
18  std::cout << "Inefficient size: "
19    << sizeof(Inefficient) << " bytes\n"
20    << "Optimized size: "
21    << sizeof(Optimized) << " bytes\n";
22
23  // Calculate waste in large array
24  constexpr int Count{1000000};
25  size_t Waste = (sizeof(Inefficient)
26    - sizeof(Optimized)) * Count;
27
28  std::cout << "Memory wasted in array of "
29    << Count << " elements: "
30    << Waste << " bytes ("
31    << (Waste / 1024.0 / 1024.0) << " MB)\n";
32}

1Inefficient size: 24 bytes
2Optimized size: 16 bytes
3Memory wasted in array of 1000000 elements: 8000000 bytes (7.62939 MB)

The waste can add up in large collections, but modern systems have strategies to help:

• Memory pools group similarly-sized allocations
• Cache lines often need padding anyway
• Virtual memory can make padding "free" in some cases

Tips to Minimize Waste

Order members by size (largest to smallest):

1// Bad layout
2struct PlayerBad {
3  char Level;// 1 byte + 7 padding
4  double Health;// 8 bytes
5  char Class;// 1 byte + 7 padding
6  double Mana;// 8 bytes
7};// Total: 32 bytes
8
9// Better layout
10struct PlayerGood {
11  double Health;// 8 bytes
12  double Mana;// 8 bytes
13  char Level;// 1 byte
14  char Class;// 1 byte
15  // 6 padding bytes
16};// Total: 24 bytes

Group small members together:

1// Bad - each bool needs 3 padding bytes
2struct FlagsBad {
3  bool IsActive;
4  // 3 padding bytes
5  int x;
6  
7  bool IsVisible;
8  // 3 padding bytes
9  int y;
10
11  bool IsEnabled;
12  // 3 padding bytes
13  int z;
14};
15
16// Good - bools packed together
17struct FlagsGood {
18  int x;
19  int y;
20  int z;
21  bool IsActive;
22  bool IsVisible;
23  bool IsEnabled;
24  // 1 padding byte
25};

In practice, the memory impact of padding rarely matters for small programs. However, for large-scale applications or memory-constrained systems, careful structure layout can make a significant difference in memory usage.