How does alignment work with inheritance and virtual functions?

Question

Ryan McCombe · Accepted Answer

Inheritance and virtual functions add some complexity to memory alignment. Let's break this down by looking at different scenarios:

Basic Virtual Function Impact

Here's a code example that uses virtual functions:

1#include <iostream>
2
3// No virtual functions
4struct Basic {
5  int Value;
6  char Tag;
7};
8
9// With virtual functions
10struct Virtual {
11  virtual void Method() {}
12  int Value;
13  char Tag;
14};
15
16int main() {
17  std::cout << "Size without virtual: "
18    << sizeof(Basic) << " bytes\n"
19    << "Size with virtual: "
20    << sizeof(Virtual) << " bytes\n"
21    << "Alignment without virtual: "
22    << alignof(Basic) << " bytes\n"
23    << "Alignment with virtual: "
24    << alignof(Virtual) << " bytes\n";
25}

1Size without virtual: 8 bytes
2Size with virtual: 16 bytes
3Alignment without virtual: 4 bytes
4Alignment with virtual: 8 bytes

The size increase comes from the virtual function table pointer (vptr) added to objects with virtual functions. This pointer needs proper alignment too!

Inheritance Alignment

Here's an example that shows some inheritance scenarios:

1#include <iostream>
2
3class Base {
4  int BaseValue;
5  char BaseTag;
6};
7
8class Derived : public Base {
9  double DerivedValue;
10  char DerivedTag;
11};
12
13class VirtualBase {
14  int BaseValue;
15  char BaseTag;
16  virtual void Method() {}
17};
18
19class VirtualDerived : public VirtualBase {
20  double DerivedValue;
21  char DerivedTag;
22  virtual void Method() override {}
23};
24
25int main() {
26  std::cout << "Regular inheritance:\n"
27    << "  Base size: " << sizeof(Base)
28    << " bytes\n"
29    << "  Derived size: " << sizeof(Derived)
30    << " bytes\n"
31
32    << "\nVirtual inheritance:\n"
33    << "  Base size: " << sizeof(VirtualBase)
34    << " bytes\n"
35    << "  Derived size: " << sizeof(VirtualDerived)
36    << " bytes\n";
37}

1Regular inheritance:
2  Base size: 8 bytes
3  Derived size: 24 bytes
4
5Virtual inheritance:
6  Base size: 16 bytes
7  Derived size: 32 bytes

Key points about inheritance and alignment:

The vptr is typically stored at the start of the object
Base class alignment affects derived class alignment
Virtual inheritance may add additional overhead
Multiple inheritance can lead to complex alignment requirements

Padding and Alignment

Alignment with Virtual Functions

Basic Virtual Function Impact

Inheritance Alignment

Padding and Alignment

Game Development with SDL2

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