研究员：
我正在研究一系列算法，这些算法作用于“Body”，“Spacecraft”，“Planet”等类。每个实例的构造可以通过不同的后端来完成。例如，我可以使用多个库（如NASA的SPICE系统）计算行星位置，并且我可以使用大量数据源和库“计算”天体的半径。
我的算法集合应该忽略数据源：例如，如果我想计算日食的时间，我只关心身体的相对位置及其半径（无论我在哪里获得这些数字）从）。
在下面的代码中，我使用Policy类来参数化两个不同的后端（简化，因为这是一个例子）。我有兴趣提出以下问题：
代码有点冗长，但我想“谈谈”我的理由。
谢谢。
我使用g ++ 4.7.2成功编译了下面的代码，如下所示：
g++ backends.cpp -std=c++11 -Wall -O2
（注意它使用了一些c ++ 11结构，比如auto）。
/**
Is it reasonable to parameterize different data back-ends using the
Policy Pattern?
The goal is to provide a unified interface to different classes
(e.g., `Body`, `Star`, `Spacecraft`). However, the construction of
specific instances requires data which can originate from different
sources.
For example, a "Body" has a radius and a gravitational parameter
(called "gm"). But these values can come from different sources
(different libraries which provide this kind of information).
Say that library 1 (called "Spice") is capable of providing the
radius given the body name:
double the_radius = compute_radius_with_spice("Mercury");
On the other hand, you could be using another library, which
computes the radius with a completely different interface, and with
completely different requirements:
double radii[3];
compute_the_radius_with_another_library("Mercury", radii)
double the_radius = (radii[0] + radii[1] + radii[2]) / 3.0;
Of course, the values computed with either library are similar, but
different enough to make a difference. What matters is CONSISTENCY
(stick to one back-end).
*/
#include<iostream>
#include<string>
#include<vector>
#include<memory>
/* Say that this is the uniform interface that I want to provide.*/
template<typename DataPolicy>
class Body:
private DataPolicy{
public:
Body(const std::string& name);
Body(const Body& body);
~Body();
std::string name() const;
double radius() const;
double gm() const;
private:
class BodyImpl * pimpl_;;
// std::unique_ptr<BodyImpl> pimpl_;
};
/* I use the pimpl_ idiom to hide the implementation */
struct BodyImpl{
std::string m_name;
double m_radius;
double m_gm;
BodyImpl(const std::string& name):
m_name(name){
}
};
/* The constructor has to build the pimpl step by step using the data
policy as a data source. */
template<typename DataPolicy>
Body<DataPolicy>::Body(const std::string& name):
pimpl_(new BodyImpl(name)){
pimpl_->m_radius = DataPolicy::get_radius(name);
pimpl_->m_gm = DataPolicy::get_gm(name);
}
template<typename DataPolicy>
Body<DataPolicy>::Body(const Body& body):
pimpl_(new BodyImpl(body.name())){
pimpl_->m_radius = body.radius();
pimpl_->m_gm = body.gm();
}
template<typename DataPolicy>
Body<DataPolicy>::~Body(){
delete pimpl_;
pimpl_ = 0;
}
/* The methods are simple forwarding calls to the implementation (in
reality it is not as simple as returning a primitive data type)*/
template<typename DataPolicy>
std::string Body<DataPolicy>::name() const{
return pimpl_->m_name;
}
template<typename DataPolicy>
double Body<DataPolicy>::radius() const{
return pimpl_->m_radius;
}
template<typename DataPolicy>
double Body<DataPolicy>::gm() const{
return pimpl_->m_gm;
}
/* Now I create a concrete data policy - in reality this would be more
extensive and complex, but the idea remains the same */
struct SPICEDataPolicy{
static double get_radius(const std::string& name){
std::cout<<"SPICEDataPolicy: calculating radius for "<<name<<std::endl;
return 0;
}
static double get_gm(const std::string& name){
std::cout<<"SPICEDataPolicy: calculating gm for "<<name<<std::endl;
return 0;
}
};
/* This is another data policy - it provides the same data but it may
call a completely different underlying library, and calculate the
values using completely different logic */
struct OtherDataPolicy{
static double get_radius(const std::string& name){
std::cout<<"OtherDataPolicy: calculating radius for "<<name<<std::endl;
return 0;
}
static double get_gm(const std::string& name){
std::cout<<"OtherDataPolicy: calculating gm for "<<name<<std::endl;
return 0;
}
};
/* My algorithms can now use the objects via the unified interface */
template<typename T>
void individual_complex_calculation(const Body<T>& body){
// Regardless of the body's data policy, I know I can call a uniform interface.
std::cout<<"I am making a complex calculation involving "<<body.name()<<"."<<std::endl
<<"[This is my radius: "<<body.radius()<<", "
<<"and this is my gm: "<<body.gm()<<"]"<<std::endl;
}
template<typename T>
void complex_calculation(const std::vector<Body<T> > bodies){
for(auto it=bodies.begin(), finished=bodies.end(); it!=finished; it++)
individual_complex_calculation(*it);
}
int main(){
/* Now I can create a vector of bodies which are consistent with one
another */
std::cout<<"========== Using 'SPICEDataPolicy =========='"<<std::endl;
std::vector<Body<SPICEDataPolicy> > bodies;
bodies.push_back(Body<SPICEDataPolicy>("Mercury"));
bodies.push_back(Body<SPICEDataPolicy>("Venus"));
bodies.push_back(Body<SPICEDataPolicy>("Earth"));
bodies.push_back(Body<SPICEDataPolicy>("Mars"));
complex_calculation(bodies);
/* And even create other set of bodies consistent with one another,
but inconsistent with the previous ones.*/
std::cout<<"========== Using 'OtherDataPolicy' =========="<<std::endl;
std::vector<Body<OtherDataPolicy> > other_bodies;
other_bodies.push_back(Body<OtherDataPolicy>("Mercury"));
other_bodies.push_back(Body<OtherDataPolicy>("Venus"));
other_bodies.push_back(Body<OtherDataPolicy>("Earth"));
other_bodies.push_back(Body<OtherDataPolicy>("Mars"));
complex_calculation(other_bodies);
return 0;
}
./a.out的输出：
========== Using 'SPICEDataPolicy =========='
SPICEDataPolicy: calculating radius for Mercury
SPICEDataPolicy: calculating gm for Mercury
SPICEDataPolicy: calculating radius for Venus
SPICEDataPolicy: calculating gm for Venus
SPICEDataPolicy: calculating radius for Earth
SPICEDataPolicy: calculating gm for Earth
SPICEDataPolicy: calculating radius for Mars
SPICEDataPolicy: calculating gm for Mars
I am making a complex calculation involving Mercury.
[This is my radius: 0, and this is my gm: 0]
I am making a complex calculation involving Venus.
[This is my radius: 0, and this is my gm: 0]
I am making a complex calculation involving Earth.
[This is my radius: 0, and this is my gm: 0]
I am making a complex calculation involving Mars.
[This is my radius: 0, and this is my gm: 0]
========== Using 'OtherDataPolicy' ==========
OtherDataPolicy: calculating radius for Mercury
OtherDataPolicy: calculating gm for Mercury
OtherDataPolicy: calculating radius for Venus
OtherDataPolicy: calculating gm for Venus
OtherDataPolicy: calculating radius for Earth
OtherDataPolicy: calculating gm for Earth
OtherDataPolicy: calculating radius for Mars
OtherDataPolicy: calculating gm for Mars
I am making a complex calculation involving Mercury.
[This is my radius: 0, and this is my gm: 0]
I am making a complex calculation involving Venus.
[This is my radius: 0, and this is my gm: 0]
I am making a complex calculation involving Earth.
[This is my radius: 0, and this is my gm: 0]
I am making a complex calculation involving Mars.
[This is my radius: 0, and this is my gm: 0]
我尝试了另一种基于“特征”和“策略”的实现。它感觉更干净，但我仍然对你对它的看法感到好奇。
以下代码使用与上面相同的命令行参数进行编译。
/**
Multiple back-ends implemented as a mix of trait classes and policies.
This seems to be a better implementation because there is a clear
path to extend the different back-ends, and the class front-end is
completely independent from its back-end.
*/
#include<iostream>
#include<string>
#include<vector>
#include<memory>
// forward declaration of the trait "data_traits"
template<typename T>
struct data_traits{
};
// each class would be defined as follows (with forward declaration of
// its implementation class)
template<typename T>
struct BodyImpl;
template<typename T>
class Body{
public:
Body(const std::string& name);
std::string name() const;
double radius() const;
double gm() const;
private:
std::unique_ptr<BodyImpl<T> > pimpl_;
};
// each class would be implemented in a cpp file with the following
// structure (notice full independence from any back-end)
template<typename T>
struct BodyImpl{
std::string m_name;
double m_radius;
double m_gm;
BodyImpl(const std::string& name):
m_name(name){
m_radius = data_traits<T>::get_radius(name);
m_gm = data_traits<T>::get_gm(name);
}
};
/* public interface simply forwards to pimpl */
template<typename T>
Body<T>::Body(const std::string& name):
pimpl_(new BodyImpl<T>(name)){
}
template<typename T>
std::string Body<T>::name() const{
return pimpl_->m_name;
}
template<typename T>
double Body<T>::radius() const{
return pimpl_->m_radius;
}
template<typename T>
double Body<T>::gm() const{
return pimpl_->m_gm;
}
/* the user or library writer can then write specific back-ends
according to the following interfaces */
struct SPICEBackEnd;
template<> struct data_traits<SPICEBackEnd>{
static double get_radius(const std::string& name){
std::cout<<"[SPICE] get radius for "<<name<<std::endl;
return 0;
}
static double get_gm(const std::string& name){
std::cout<<"[SPICE] get gm for "<<name<<std::endl;
return 0;
}
};
/*another back-end*/
struct OtherBackEnd;
template<> struct data_traits<OtherBackEnd>{
static double get_radius(const std::string& name){
std::cout<<"[OTHER] get radius for "<<name<<std::endl;
return 0;
}
static double get_gm(const std::string& name){
std::cout<<"[OTHER] get gm for "<<name<<std::endl;
return 0;
}
};
/* The algorithms can be obvlivious to the back-end used */
template<typename T>
void complex_calculation(const std::vector<Body<T> >& bodies){
for(auto it=bodies.begin(), finished=bodies.end(); it!=finished; it++){
std::cout<<"Body "<<it->name()<<" (r="<<it->radius()<<", mu="<<it->gm()<<")"<<std::endl;
}
}
int main(){
std::vector<Body<SPICEBackEnd> > spice_bodies;
spice_bodies.push_back(Body<SPICEBackEnd>("Mercury"));
spice_bodies.push_back(Body<SPICEBackEnd>("Venus"));
spice_bodies.push_back(Body<SPICEBackEnd>("Earth"));
spice_bodies.push_back(Body<SPICEBackEnd>("Mars"));
complex_calculation(spice_bodies);
std::vector<Body<OtherBackEnd> > other_bodies;
other_bodies.push_back(Body<OtherBackEnd>("Mercury"));
other_bodies.push_back(Body<OtherBackEnd>("Venus"));
other_bodies.push_back(Body<OtherBackEnd>("Earth"));
other_bodies.push_back(Body<OtherBackEnd>("Mars"));
complex_calculation(other_bodies);
}
答案 0 :(得分：1)
有趣的问题。
有许多不同的方法值得考虑，也许跳出最多的方法是Abstract Factory。为什么？因为您可以构造符合基本接口集的对象系列，然后使用它们而无需经常检查以查看应该执行的操作。另外，因为你提出了关于一致性的观点。
我在策略中看到的问题是它通常是一种封装不同方式来做同样事情的方法。例如，如果我们正在进行工资单，那么每个人都有一个系统同意应税工资总额 - 扣除，但这个价值的得出方式可能不同（坦率地说，在工资单中，抽象工厂也可能有意义，因为你不会怀疑需要不止一个变体，一旦你起草了一个变体，其中所有其他变体都来自同一个家族是必不可少的。）
这里设计方面有趣的另一个因素是你需要在一些非常不同的实体上计算一些常用的指标。这是Java中接口的巨大优势之一，和/或Objective-C或Scala等语言中的Traits（来自荒诞般的辉煌自我）。我很长一段时间没有写过很多C ++，但是我知道有很多方法可以做类似traits的事情，例如： Mixins（詹姆斯·科普利恩）。