Topic 37 of 91 7. C++ Structures and Unions Interactive Tutorial

Structures in C++: Building Custom Data Records With struct

15 mins read Last updated: 26 Sep 2026 21,600 views

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Direct Answer & Core Concept

A structure (struct) in C++ is a user-defined composite type that groups heterogeneous data members under a single identifier. In C++, the ONLY technical difference between a struct and a class is default member access: struct members and base classes are public by default, whereas class members and base classes are private by default. Both support constructors, member functions, inheritance, and templates.

Detailed Explanation & Mechanics

1. Struct vs Class Default Access:

Architecture Diagram

C++ STRUCT VS CLASS:
┌─────────────────┬──────────────────────────┬────────────────────────────┐
│ Feature         │ struct                   │ class                      │
├─────────────────┼──────────────────────────┼────────────────────────────┤
│ Default Members │ PUBLIC                   │ PRIVATE                    │
│ Default Inherit │ PUBLIC                   │ PRIVATE                    │
│ Idiomatic Use   │ Passive Data Structures  │ Encapsulated Objects / OOP │
└─────────────────┴──────────────────────────┴────────────────────────────┘

2. Struct Memory Alignment & Padding:

Compilers insert padding bytes between struct members to align them to CPU word boundaries for faster memory bus transfers (alignof(Type)).

Runnable Code Example: Structures in C++

C++20 / C++23

main.cpp

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#include <iostream>
#include <string>

// Plain Old Data (POD) / Aggregate struct
struct DatabaseRecord {
    int recordId;
    std::string tableName;
    double queryLatencyMs;

    void display() const {
        std::cout << "[Record #" << recordId << "] Table: " << tableName 
                  << " | Latency: " << queryLatencyMs << " ms
";
    }
};

int main() {
    // Designated initializers (C++20)
    DatabaseRecord record{1001, "users_auth", 1.42};
    record.display();

    std::cout << "DatabaseRecord Size: " << sizeof(record) << " bytes
";
    return 0;
}

Common C++ Technical Interview Questions & Answers

Interview Prep

Q: What is struct memory padding and alignment in C++?

A: CPU hardware reads memory efficiently in aligned chunks (e.g. 4 or 8 bytes). If a 1-byte char is followed by an 8-byte double, the compiler inserts 7 padding bytes to ensure the double starts at a multiple of 8.

Q: What is an aggregate struct in C++?

A: An aggregate is a class/struct with no user-declared constructors, no private/protected non-static data members, and no virtual functions, allowing direct brace initialization `StructName{a, b, c}`.

Key Takeaways, Modern Best Practices & Idioms

C++ Best Practices & Production Rules

1. Use struct for passive Plain Old Data (POD) / Data Transfer Objects (DTOs).
2. Use class when enforcing invariants, private encapsulation, and complex behavior.
3. Order struct data members from largest to smallest to minimize memory padding waste.

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