C++ Aggregates & Aggregate Initialization

Why aggregates matter

Aggregates let you skip constructors for simple “bags of data” and use brace initialization instead.

struct Point {
    int x;
    int y;
};
int main() {
    Point p = {10, 20};
}

Table of contents

1. Aggregate conditions
2. Basic syntax
3. C++17/20 changes
4. Arrays
5. Common errors
6. Production patterns
7. Full example

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1. Aggregate conditions (C++17-style summary)

• No user-provided constructors (per standard rules—verify with is_aggregate)
• No private/protected non-static data members
• No virtual functions
• No problematic base classes (see standard; public base allowed since C++17)

struct Point { int x, y; };  // aggregate
struct Derived : Point { int z; };  // often aggregate (C++17+)
struct NotAgg {
    virtual void f() {}
    int x;
};

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2. Basic syntax

struct Person {
    std::string name;
    int age;
    double height;
};
Person p1 = {"Alice", 30, 165.5};
Person p2 = {"Bob", 25};       // height value-initialized
Person p3 = {};                // all value-initialized
Person p4{"Charlie", 35, 180.0};

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3. C++17/20

• C++17: aggregates may have public base classes.
• C++20: designated initializers .member = value.

struct Point { int x, y, z; };
Point p = { .x = 10, .y = 20, .z = 30 };

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4. Arrays

int arr1[5] = {1, 2, 3, 4, 5};
int arr2[5] = {1, 2};
int arr3[] = {1, 2, 3};
int matrix[2][3] = { {1,2,3}, {4,5,6} };

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5. Common errors

• User-defined ctor → no longer aggregate brace list in the same way.
• Private members break aggregate rules.
• Too many initializers for members.

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6. Production patterns

Configuration

struct DatabaseConfig {
    std::string host = "localhost";
    int port = 5432;
    std::string database = "mydb";
};
DatabaseConfig prod = {
    .host = "prod.example.com",
    .port = 5432,
    .database = "production"
};

Test tables

struct TestData {
    int input;
    int expected;
    std::string description;
};
std::vector<TestData> cases = {
    {0, 0, "zero"},
    {5, 25, "five squared"}
};

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Summary

Concept	Meaning
Aggregate	Simple type per standard rules
Init	Usually { ... }
C++17	Bases allowed
C++20	Designated initializers
Related: Initialization order.

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FAQ

Q: Aggregate vs POD?
A: POD is stricter (trivial + aggregate-related rules). Aggregate is broader. Q: Partial init?
A: Remaining members value-initialized or use member defaults. Resources: cppreference — Aggregate initialization.

Related posts

• C++ Uniform initialization
• C++ Value initialization
• C++ Designated initializer

Keywords

C++, aggregate, aggregate initialization, struct, POD, C++20.

More links

• C++ Aggregate initialization
• C++ struct vs class

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같이 보면 좋은 글 (내부 링크)

이 주제와 연결되는 다른 글입니다.

• C++ Aggregate Initialization | ‘집합체 초기화’ 가이드
• C++20 Designated Initializers 완벽 가이드 | 명확한 구조체 초기화
• C++ Default Initialization | ‘기본 초기화’ 가이드

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