Padding and Alignment

Understanding Natural Alignment

How do I know what the natural alignment of a type should be?

Abstract art representing computer programming

Natural alignment is typically based on the size of the data type. Here's a practical guide to common alignment requirements:

1#include <iostream>
2
3int main() {
4  std::cout << "Alignment requirements:\n"
5    << "char: " << alignof(char) << " bytes\n"
6    << "short: " << alignof(short) << " bytes\n"
7    << "int: " << alignof(int) << " bytes\n"
8    << "long: " << alignof(long) << " bytes\n"
9    << "float: " << alignof(float) << " bytes\n"
10    << "double: " << alignof(double) << " bytes\n"
11    << "pointer: " << alignof(int*) << " bytes\n";
12}

1Alignment requirements:
2char: 1 bytes
3short: 2 bytes
4int: 4 bytes
5long: 8 bytes
6float: 4 bytes
7double: 8 bytes
8pointer: 8 bytes

For compound types like structs and classes, the alignment is typically determined by the largest alignment requirement of any member:

1#include <iostream>
2
3struct Example {
4  char A;// 1 byte alignment
5  double B;// 8 byte alignment
6  int C;// 4 byte alignment
7};
8
9int main() {
10  std::cout << "Struct alignment: "
11    << alignof(Example) << " bytes\n";
12}

1Struct alignment: 8 bytes

You can use the alignas specifier to request specific alignment:

1#include <iostream>
2
3struct alignas(16) Custom {
4  int Value;
5};
6
7int main() {
8  std::cout << "Custom alignment: "
9    << alignof(Custom) << " bytes\n";
10
11  // This will always start at a 16-byte boundary
12  Custom Instance;
13}