研究员：
我正在研究一组算法，这些算法作用于“Body”、“Spacecraft”、“Planet”等类。每个实例的构建都可以通过不同的后端来完成。例如，我可以使用大量库（如 NASA 的 SPICE 系统）计算行星位置，并且我还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，我只关心物体的相对位置和它们的半径（不管我从哪里得到这些数字）。
在下面附加的代码中，我使用 Policy 类来参数化两个不同的后端（因为这是一个示例，所以进行了简化）。我有兴趣问以下问题：
- 为此使用策略模式是否合理？
- 如何删除类构造函数中包含的实现细节并将它们移动到 Pimpl 构造函数（参数化 BodyImpl？）
代码有点冗长，但我想“谈谈”我的基本原理。
谢谢你。
我使用 g++ 4.7.2 成功编译了以下代码，如下所示：
g++ backends.cpp -std=c++11 -Wall -O2
（请注意，它使用了一些 c++11 结构，例如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;
}
输出./a.out：
========== 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]
编辑：
我尝试了另一种基于“特征”和“策略”的实现。感觉干净多了，但我还是很好奇你对它的看法。
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);
}