# C++ Empty base optimization

**[Empty base optimization](https://en.cppreference.com/w/cpp/language/ebo)** (EBO) is a compiler technique where an empty base class takes no space in the derived object. Since an empty class has size 1 (to give it a unique address), deriving from it would normally add 1 byte to the derived object; EBO allows the derived object to have the same size as if the base didn't exist.

Prefer using `[[no_unique_address]]` (C++20) or composition over inheritance for empty types to clarify intent and enable optimization.

## Example

This example shows EBO reducing object size when inheriting from empty classes.

```cpp
// compile: g++ -std=c++20 -o ebo ebo.cpp
// run: ./ebo
// description: empty base optimization reduces object size

#include <iostream>

class Empty {};

class WithEBO : Empty {
public:
    int value;
};

class NoEBO : public Empty {
public:
    int value;
};

class Modern {
public:
    [[no_unique_address]] Empty e;
    int value;
};

int main() {
    std::cout << "sizeof(Empty): " << sizeof(Empty) << "\n";
    std::cout << "sizeof(WithEBO): " << sizeof(WithEBO) << "\n";
    std::cout << "sizeof(NoEBO): " << sizeof(NoEBO) << "\n";
    std::cout << "sizeof(Modern): " << sizeof(Modern) << "\n";
    
    return 0;
}
```