C++ EBCO and [[no_unique_address]]

Introduction: “Why does empty class occupy 1 byte?"

"Why is std::unique_ptr<T, Deleter> 16 bytes instead of 8 bytes?”

In C++, empty class (Empty Class—class without member variables) also occupies minimum 1 byte. This is because each object must have unique address. But this rule causes wasted memory when having stateless functors or custom deleters as members.

EBCO (Empty Base Class Optimization) is a compiler optimization where inheriting empty class as base makes size 0. C++20’s [[no_unique_address]] attribute allows applying this optimization to member variables too.

What This Guide Covers:

• Empty class rule: Why it occupies 1 byte
• EBCO principle: Size 0 when inherited as base class
• [[no_unique_address]]: C++20 member variable optimization
• Practical usage: std::tuple, std::unique_ptr, compressed pair
• Problem scenarios: When memory layout optimization is needed
• Complete examples: Custom smart pointer, compressed pair implementation
• Common errors: EBCO failure cases, ABI compatibility
• Production patterns: Standard library implementation techniques

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1. Empty Class Rule and EBCO

Why Empty Class Occupies 1 Byte

C++ standard requires all objects have unique address. Even empty class needs minimum 1 byte for each array element to have different address.

struct Empty {};

int main() {
    Empty e1, e2;
    std::cout << sizeof(Empty) << '\n';  // 1
    std::cout << &e1 << " vs " << &e2 << '\n';  // Different addresses
    
    Empty arr[10];
    std::cout << &arr[0] << " vs " << &arr[1] << '\n';  // 1 byte apart
}

Output:

1
0x7ffc1234 vs 0x7ffc1235
0x7ffc1240 vs 0x7ffc1241

Problem When Having as Member

struct Empty {};

struct Container {
    int value;      // 4 bytes
    Empty empty;    // 1 byte + 3 bytes padding (due to alignment)
    // Total 8 bytes
};

static_assert(sizeof(Container) == 8);

Problem: Empty has no state but wastes 4 bytes (including padding).

EBCO: Size 0 When Inheriting as Base Class

EBCO (Empty Base Class Optimization) is compiler optimization making empty base class size 0.

// Type definition
struct Empty {};

struct Optimized : Empty {
    int value;  // 4 bytes
    // Empty is size 0 → Total 4 bytes
};

static_assert(sizeof(Optimized) == 4);

Key: Base class is not array element so unique address rule is relaxed. Compiler places empty base at same address as derived class start to save space.

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2. C++20 [[no_unique_address]]

Basic Usage

C++20 [[no_unique_address]] attribute allows applying EBCO to member variables.

struct Empty {};

struct Optimized {
    int value;                          // 4 bytes
    [[no_unique_address]] Empty empty;  // 0 bytes
    // Total 4 bytes
};

static_assert(sizeof(Optimized) == 4);

Comparison with EBCO

Method	Syntax	Limitation
EBCO	Inherit as base	Inheritance required
[[no_unique_address]]	Member attribute	C++20 required

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Summary

Key Points

1. Empty class: Occupies minimum 1 byte
2. EBCO: Size 0 when inherited as base class
3. [[no_unique_address]]: C++20 member optimization
4. Use cases: Smart pointers, tuple, compressed pair
5. Conditions: Empty type, no virtual functions

When to Use

✅ Use EBCO/[[no_unique_address]] when:

• Member is empty type
• Want to minimize memory
• Implementing smart pointers/containers
• Optimizing memory layout

❌ Don’t use when:

• Type has state
• Has virtual functions
• Adds unnecessary complexity

Best Practices

• ✅ Use for stateless functors
• ✅ Apply in custom smart pointers
• ✅ Use [[no_unique_address]] in C++20
• ❌ Don’t assume EBCO always applies
• ❌ Don’t forget ABI compatibility

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Related Articles

• C++ Alignment and Padding
• C++ Cache and Data-Oriented Design
• C++ Pimpl

Master EBCO for memory-efficient C++ code! 🚀

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자주 묻는 질문 (FAQ)

Q. 이 내용을 실무에서 언제 쓰나요?

A. EBCO and C++20 [[no_unique_address]] solving the problem of empty classes occupying memory. From std::tuple, std::unique… 실무에서는 위 본문의 예제와 선택 가이드를 참고해 적용하면 됩니다.

Q. 선행으로 읽으면 좋은 글은?

A. 각 글 하단의 이전 글 또는 관련 글 링크를 따라가면 순서대로 배울 수 있습니다. C++ 시리즈 목차에서 전체 흐름을 확인할 수 있습니다.

Q. 더 깊이 공부하려면?

A. cppreference와 해당 라이브러리 공식 문서를 참고하세요. 글 말미의 참고 자료 링크도 활용하면 좋습니다.

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같이 보면 좋은 글 (내부 링크)

이 주제와 연결되는 다른 글입니다.

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• C++ 캐시 효율적인 코드: 데이터 지향 설계 가이드
• C++ Pimpl Idiom 완벽 가이드 | 구현 은닉과 컴파일 시간 단축

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