C++ pimpl机制详细讲解

Pointer to implementation(PImpl ),通过将类的实现细节放在一个单独的类中,从其对象表示中删除它们,通过一个不透明的指针访问它们(cppreference 是这么说的)

C++ pimpl机制详细讲解

通过一个私有的成员指针,将指针所指向的类的内部实现数据进行隐藏

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

为什么用PImpl 机制

个人拙见

C++ 不像Java 后端型代码,能有行业定式的列目录名形成规范(controller、Dao等)隐藏实现,降低耦合性和分离接口(隐藏类的具体实现)通过编译期的封装(隐藏实现类的细节)

业界实现

优秀开源代码有实现

PImpl实现

方法一

cook_cuisine.h

#pragma once
#include <unordered_map>
#include <vector>
#include <memory>
//  Pointer to impl ementation
class CookImpl;
// 后厨
class Cook {
public:
Cook(int, const std::vector<std::string>&);
~Cook();
std::vector<std::string> getMenu(); /* 获取菜单 */
uint32_t getChefNum();  /* 获取厨师数量 */
private:
CookImpl* impl_;
};
typedef std::shared_ptr<Cook> CookPtr;		// 美妙的typedef 懒人工具

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"
// 后厨
class Cook {
public:
Cook(int, const std::vector<std::string>&);
~Cook();
std::vector<std::string> getMenu(); /* 获取菜单 */
uint32_t getChefNum();  /* 获取厨师数量 */
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, 平时开发可千万别用这句
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 << " people\n";
cout << "==========Menu\n";
for (size_t i = 0; i < cook_menu.size(); i++) {
cout << "============" << i + 1 << " : " << cook_menu[i] << "\n";
}
return 0;
}

CMakeLists.txt

mkdir buildcd buildcmake ..

PImpl 缺点

空间开销:每个类都需要额外的指针内存指向实现类

时间开销:每个类间接访问实现的时候多一个间接指针操作的开销

阅读开销:使用、阅读和调试上带来一些不便(不是啥问题)

总结

每种设计方法都有它的优点和缺点

PImpl 用一些内存空间和额外类的实现换取耦合性的下降,是可以接受的

但重点在:在性能/内存要求不敏感处,PImpl 技术才更优不错的发挥舞台

极端例子:

你不可能在斐波那契的实现中还加个PImpl 机制,多此一举