C++ pimpl機制詳細講解

2022-08-10 22:00:33

什麼是PImpl機制

Pointer to implementation（PImpl ），通過將類的實現細節放在一個單獨的類中，從其物件表示中刪除它們，通過一個不透明的指標存取它們（cppreference 是這麼說的）

通過一個私有的成員指標，將指標所指向的類的內部實現資料進行隱藏

class Demo {
public:
	...
private:
	DemoImp* imp_;
}

為什麼用PImpl 機制

個人拙見

C++ 不像Java 後端型程式碼，能有行業定式的列目錄名形成規範（controller、Dao等）
隱藏實現，降低耦合性和分離介面（隱藏類的具體實現）
通過編譯期的封裝（隱藏實現類的細節）

業界實現

優秀開原始碼有實現

PImpl實現

方法一

cook_cuisine.h

#pragma once
#include <unordered_map>
#include <vector>
#include <memory>
//  Pointer to impl ementation
class CookImpl;
// 後廚
class Cook {
public:
    Cook(int, const std::vector<std::string>&);
    ~Cook();
    std::vector<std::string> getMenu();     /* 獲取選單 */
    uint32_t getChefNum();                  /* 獲取廚師數量 */
private:
    CookImpl* impl_;
};
typedef std::shared_ptr<Cook> CookPtr;		// 美妙的typedef 懶人工具

cook_cuisine.cc

#include "cook_cuisine.h"
class CookImpl {
public:
    CookImpl(uint32_t checf_num, const std::vector<std::string>& menu):checf_num_(checf_num), menu_(menu) {}
    std::vector<std::string> getMenu();
    uint32_t getChefNum();
private:
    uint32_t checf_num_;
    std::vector<std::string> menu_;
};
std::vector<std::string> CookImpl::getMenu() {
    return menu_;
}
uint32_t CookImpl::getChefNum() {
    return checf_num_;
}
Cook::Cook(int chef_num, const std::vector<std::string>& menu) {
    impl_ = new CookImpl(chef_num, menu);
}
Cook::~Cook() {
    delete impl_;
}
std::vector<std::string> Cook::getMenu() {
    return impl_->getMenu();
}
uint32_t Cook::getChefNum() {
    return impl_->getChefNum();
}

方法二

cook_cuisine.h

#pragma once
#include <unordered_map>
#include <vector>
#include <memory>
#include "cook_cuisine_imp.h"
// 後廚
class Cook {
public:
    Cook(int, const std::vector<std::string>&);
    ~Cook();
    std::vector<std::string> getMenu();     /* 獲取選單 */
    uint32_t getChefNum();                  /* 獲取廚師數量 */
private:
    CookImplPtr impl_;
};
typedef std::shared_ptr<Cook> CookPtr;

cook_cuisine.cc

#include "cook_cuisine.h"
Cook::Cook(int chef_num, const std::vector<std::string>& menu) {
    impl_.reset(new CookImpl(chef_num, menu));
}
Cook::~Cook() {
}
std::vector<std::string> Cook::getMenu() {
    return impl_->getMenu();
}
uint32_t Cook::getChefNum() {
    return impl_->getChefNum();
}

cook_cuisine_imp.h

#pragma once
#include <vector>
#include <unordered_map>
#include <memory>
class CookImpl {
public:
    CookImpl(uint32_t checf_num, const std::vector<std::string>& menu):checf_num_(checf_num), menu_(menu) {}
    std::vector<std::string> getMenu();
    uint32_t getChefNum();
private:
    uint32_t checf_num_;
    std::vector<std::string> menu_;
};
typedef std::shared_ptr<CookImpl> CookImplPtr;

cook_cusine_imp.cc

#include "cook_cuisine_imp.h"
std::vector<std::string> CookImpl::getMenu() {
    return menu_;
}
uint32_t CookImpl::getChefNum() {
    return checf_num_;
}

main.cc

#include "cook_cuisine.h"
#include <iostream>
using namespace std;    // Testing, 平時開發可千萬別用這句
int main() {
    int checf_num = 10;
    const std::vector<std::string> menus = { "Chicken", "Beef", "Noodle", "Milk" };
    CookPtr cook(new Cook(checf_num, menus));
    auto cook_menu = cook->getMenu();
    auto cook_checf_num = cook->getChefNum();
    cout << "======================Chinese Cook======================n";
    cout << "============Checf: " << cook_checf_num << " peoplen";
    cout << "==========Menun";
    for (size_t i = 0; i < cook_menu.size(); i++) {
        cout << "============" << i + 1 << " : " << cook_menu[i] << "n";
    }
    return 0;
}

CMakeLists.txt