How does alignment work with unions?

Question

Ryan McCombe · Accepted Answer

Unions are special because they overlay their members in memory - the size and alignment of a union is determined by its largest member. Let's explore this with examples:

Basic Union Alignment

Here's a code example using a basic union:

1#include <iostream>
2
3union SimpleUnion {
4  char C;// 1 byte
5  int I;// 4 bytes
6  double D;// 8 bytes
7};
8
9int main() {
10  std::cout << "Union size: "
11    << sizeof(SimpleUnion)
12    << " bytes\n"
13    << "Union alignment: "
14    << alignof(SimpleUnion)
15    << " bytes\n"
16    << "Individual alignments:\n"
17    << "  char: " << alignof(char) << " bytes\n"
18    << "  int: " << alignof(int) << " bytes\n"
19    << "  double: " << alignof(double)
20    << " bytes\n";
21
22  SimpleUnion U;
23  U.D = 3.14;
24  std::cout << "Address of U: "
25    << static_cast<void*>(&U) << "\n"
26    << "Address of U.C: "
27    << static_cast<void*>(&U.C) << "\n"
28    << "Address of U.I: "
29    << static_cast<void*>(&U.I) << "\n"
30    << "Address of U.D: "
31    << static_cast<void*>(&U.D) << "\n";
32}

1Union size: 8 bytes
2Union alignment: 8 bytes
3Individual alignments:
4  char: 1 bytes
5  int: 4 bytes
6  double: 8 bytes
7Address of U: 000000CF4974F8B8
8Address of U.C: 000000CF4974F8B8
9Address of U.I: 000000CF4974F8B8
10Address of U.D: 000000CF4974F8B8

Union with Structures

In this example, we use a union that includes a user-defined type:

1#include <iostream>
2
3struct AlignedStruct {
4  double Value;// 8 bytes
5  char Tag;// 1 byte + padding
6};
7
8union ComplexUnion {
9  char Simple;// 1 byte
10  AlignedStruct Complex;// Determines union size
11};
12
13int main() {
14  std::cout << "Struct size: "
15    << sizeof(AlignedStruct) << " bytes\n"
16    << "Union size: "
17    << sizeof(ComplexUnion) << " bytes\n"
18    << "Union alignment: "
19    << alignof(ComplexUnion) << " bytes\n";
20}

1Struct size: 16 bytes
2Union size: 16 bytes
3Union alignment: 8 bytes

Important points about union alignment:

All members share the same starting address
Size matches the largest member
Alignment matches the strictest member requirement
Accessing inactive members can cause undefined behavior
Unions are useful for type punning but require careful handling

Padding and Alignment

Understanding Union Alignment

Basic Union Alignment

Union with Structures

Padding and Alignment

Game Development with SDL2

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