從一個 結構 到一個 抽象資料類型（ADT） 代表設計理念的根本轉變。雖然一個 結構 通常只是變數的被動集合，而一個抽象資料類型則是主動的實體，透過 封裝來管理自身的狀態。
1. 設計意圖
在 C++ 中， class 關鍵字象徵著對 資料抽象的承諾。此策略將 介面 （使用者能執行的操作）與 實作 （資料如何儲存）分離。透過保護內部變數，程式設計師可確保物件維持自身內部的一致性。
2. 技術細節
技術上而言， 結構 和 class 之間唯一的差異是 預設存取層級。一個 結構 的成員預設為 公開 ，反映出其作為開放資料容器的角色。而一個 class 的成員預設為 私有 的成員則預設為私有，反映出其作為受控實體的角色。
$$\text{ADT} = \text{資料} + \text{操作}$$
main.py
TERMINALbash — 80x24
> Ready. Click "Run" to execute.
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QUESTION 1
What is the primary technical difference between a 'struct' and a 'class' in C++?
Structs cannot have member functions.
Classes are stored on the heap while structs are on the stack.
The default access level (public for struct, private for class).
Only classes support inheritance.
✅ Correct!
Correct! It is purely a matter of default access levels to facilitate different design intents.❌ Incorrect
Both can have functions and inheritance; the difference lies in default access permissions.QUESTION 2
What is the goal of 'Data Abstraction'?
To make the code run faster by removing data types.
To separate a type's interface from its implementation.
To allow variables to hold multiple types at once.
To prevent the use of pointers in a class.
✅ Correct!
Yes! Abstraction lets users interact with 'what it does' without worrying about 'how it works'.❌ Incorrect
Abstraction is about the logical separation of interface and implementation detail.QUESTION 3
If a class defines an operation 'combine()', who is responsible for maintaining internal consistency?
The user calling the function.
The operating system.
The class itself via its member functions.
The C++ compiler's garbage collector.
✅ Correct!
Exactly. An ADT manages its own state to ensure internal data remains logical.❌ Incorrect
In an ADT, the class encapsulates the logic to ensure data integrity.QUESTION 4
Why would the initialization 'Sales_data item = "9-999";' fail if it requires two conversions?
C++ allows zero implicit conversions.
C++ allows only one implicit user-defined conversion at a time.
Strings cannot be converted to objects.
Implicit conversions are only allowed for numeric types.
✅ Correct!
Correct. Converting char* to string and then string to Sales_data is one step too many for the compiler.❌ Incorrect
The compiler limits implicit user-defined chains to a single step to prevent ambiguity.QUESTION 5
In the provided code snippet, why is 'my_mem' accessible in main()?
Because it was declared in a struct and is public by default.
Because main() is a friend of struct A.
Because it is a static member.
Because all strings are globally accessible.
✅ Correct!
Indeed. Struct members default to public, allowing external access.❌ Incorrect
Check the default access level of a struct—it's public!Module Implementation Task: The Employee ADT
Applying Encapsulation and Static Members
You are tasked with creating a robust Employee management class. To ensure every employee has a unique identifier, you must use a shared counter that increments with every new instance. This demonstrates encapsulation and class-wide state management.
Q
[Writing Task] Exercise 13.18: Define an Employee class that contains an employee name and a unique employee identifier. Give the class a default constructor and a constructor that takes a string. Each should generate a unique ID. (Min 50 words code/explanation)
Solution:
To implement this, we define a static member `sn` to keep track of the next available ID. cpp class Employee { public: Employee() : id(sn++) { } Employee(const std::string& s) : name(s), id(sn++) { } private: std::string name; int id; static int sn; }; int Employee::sn = 0; // Initialize outside class This design ensures that the unique ID generation is handled internally by the class (encapsulation), and the shared state is maintained across all instances via the static member.
To implement this, we define a static member `sn` to keep track of the next available ID. cpp class Employee { public: Employee() : id(sn++) { } Employee(const std::string& s) : name(s), id(sn++) { } private: std::string name; int id; static int sn; }; int Employee::sn = 0; // Initialize outside class This design ensures that the unique ID generation is handled internally by the class (encapsulation), and the shared state is maintained across all instances via the static member.
Q
Explain why the initialization 'Sales_data item = {"978-0590353403", 25, 15.99};' would fail if any member of Sales_data were made private.
Solution:
This is aggregate initialization. It relies on the class being an aggregate, which requires all data members to be public and for there to be no user-defined constructors. Once you introduce the 'private' keyword for encapsulation, the class is no longer an aggregate, and you must instead provide a constructor that matches the provided argument list to initialize the object.
This is aggregate initialization. It relies on the class being an aggregate, which requires all data members to be public and for there to be no user-defined constructors. Once you introduce the 'private' keyword for encapsulation, the class is no longer an aggregate, and you must instead provide a constructor that matches the provided argument list to initialize the object.