C++ 透過私有繼承復用實作類別實現工廠函式

架構

• 介面類別 IPerson：純虛擬介面。
• 實作類別 PersonInfo：具體資料與格式化邏輯。
• 具體類別 CPerson：public IPerson, private PersonInfo，覆寫介面並調用基類別實作。
• 工廠函式 makePerson：回傳 shared_ptr<IPerson>。

程式碼

/* test.cpp
 * Created on: 2014.04.14
 * Author: Spike
 */
#include <cstring>
#include <iostream>
#include <string>
#include <memory>
using namespace std;

class IPerson {
public:
    virtual ~IPerson() = default;
    virtual string name() const = 0;
    virtual string birthDate() const = 0;
};

struct DatabaseID { char name[256]; char birthDate[256]; };

class PersonInfo {
public:
    explicit PersonInfo(DatabaseID pid) {
        strcpy(m_name, pid.name);
        strcpy(m_birthDate, pid.birthDate);
    }
    virtual ~PersonInfo() = default;
    virtual const char* theName() const;
    virtual const char* theBirthDate() const;
protected:
    virtual const char* valueDelimOpen() const { return "["; }
    virtual const char* valueDelimClose() const { return "]"; }
private:
    char m_name[256], m_birthDate[256];
    static const int MaxLen = 512;
};

const char* PersonInfo::theName() const {
    static char v[MaxLen];
    strcpy(v, valueDelimOpen());
    strcat(v, m_name);
    strcat(v, valueDelimClose());
    return v;
}
const char* PersonInfo::theBirthDate() const {
    static char v[MaxLen];
    strcpy(v, valueDelimOpen());
    strcat(v, m_birthDate);
    strcat(v, valueDelimClose());
    return v;
}

class CPerson : public IPerson, private PersonInfo {
public:
    explicit CPerson(DatabaseID pid) : PersonInfo(pid) {}
    string name() const override { return PersonInfo::theName(); }
    string birthDate() const override { return PersonInfo::theBirthDate(); }
private:
    const char* valueDelimOpen() const override { return "---"; }
    const char* valueDelimClose() const override { return "---"; }
};

shared_ptr<IPerson> makePerson(DatabaseID id) {
    return make_shared<CPerson>(id);
}

DatabaseID askUserForDatabaseID() {
    DatabaseID di;
    strcpy(di.name, "Liu");
    strcpy(di.birthDate, "1998.9.25");
    return di;
}

int main() {
    DatabaseID id = askUserForDatabaseID();
    auto pp = makePerson(id);
    cout << pp->name() << endl;
    cout << pp->birthDate() << endl;
}

輸出

---Liu---
---1998.9.25---

設計重點

• 私有繼承 PersonInfo 復用實作，不暴露其介面。
• 覆寫 valueDelimOpen/Close 自訂格式化分隔符。
• 工廠函式隱藏具體型別，回傳介面智慧指標。