std::函数的复合模式

Composite pattern of std::functions

本文关键字：模式 复合 函数 std      更新时间：2023-10-16

我正在尝试使用模板类实现std::functions复合模式，其中每个复合类处理其子类的返回值。
因此，模式类可能如下所示：

class AbstractClass {
public:
virtual void process() = 0;
};
template<typename ReturnType>
class PrimitiveClass : public AbstractClass {
public: 
ReturnType process() {
// please note, that the result is not returned by the return statement
return this->func();  //this is just for simplicity
}
private:
std::function<ReturnType()> func;
}
template<typename ReturnType, typename ...Args>
class CompositeClass : public AbstractClass {
public:
ReturnType process() {
// --> This is where I want to process all children first and then pass their return values to this->func
// the following code is kind of a pseudo code:
for(auto it = vector.begin(); it != vector.end(); ++it {
results.add((**it).process())
}
return this->func(results)
}
private:
std::function<ReturnType(Args...)> func;
std::vector<std::shared_ptr<AbstractClass>> children;
};

例如，我有一个带有std::function<int(int, double, bool)的CompositeClass，该函数的参数类型也是其子函数的ReturnType。我想将孩子的返回值传递给上述std::function
谁能想到一种方法，我该如何实现这一目标？

如果我明白你想要什么(如果我没有错的话)......

(1)为了解决process()的无协变返回值的问题(参见Igor Tandetnik的评论)，你需要一个模板抽象类来表达正确的返回值;

template <typename T>
struct abstClass 
{ virtual T process() const = 0; };

(2)所以你的CompositeClass(在我的例子中更名为nodeClass)继承自abstClass<ReturnType>

(3)你的PrimitiveClass是无用的，因为你可以将案例(引用没有参数的函数)作为零CompositeClass进行管理Args

(4) 您需要一个leafClass来处理基本值

(5)在CompositeClass(nodeClass)，children，而不是shared_ptr<AbstractClass>的std::vector(不能做你想做的事)，可以是一个

std::tuple<std::shared_ptr<abstClass<Args>>...>  children;

鉴于这些要点，我提出了以下解决方案(不幸的是，它是 C++14，因为使用从 C++14 开始可用的std::index_sequence和std::make_index_sequence;但如果你需要一个 C++11 解决方案，写替代品并不难)

#include <tuple>
#include <memory>
#include <iostream>
#include <functional>
template <typename T>
struct abstClass 
{ virtual T process() const = 0; };
template <typename T>
class leafClass : public abstClass<T>
{
private:
T  value;
public:
leafClass (T && v0) : value { std::forward<T>(v0) }
{ }
T process () const
{ return value; };
};
template <typename RetT, typename ... ArgTs>
class nodeClass : public abstClass<RetT>
{
private:
using funcT = std::function<RetT(ArgTs...)>;
template <typename T>
using shrPAC = std::shared_ptr<abstClass<T>>;
funcT                         func;
std::tuple<shrPAC<ArgTs>...>  childrens;
template <std::size_t ... Is>
RetT processH (std::index_sequence<Is...> const &) const
{ return func(std::get<Is>(childrens)->process()...); }
public:
nodeClass (funcT && f0, shrPAC<ArgTs> && ... as)
: func { std::forward<funcT>(f0) },
childrens { std::forward<shrPAC<ArgTs>>(as)... }
{ }
RetT process () const
{ return processH(std::make_index_sequence<sizeof...(ArgTs)>{}); }
};
int main ()
{
auto func0 = [](int i, double d, bool b) { return int( b ? i+d : i-d ); };
auto shpLci = std::make_shared<leafClass<int>>(1);
auto shpLcd = std::make_shared<leafClass<double>>(2.2);
auto shpNb  = std::make_shared<nodeClass<bool>>([](){ return true; });
auto shpNc0 = std::make_shared<nodeClass<int, int, double, bool>>
(func0, shpLci, shpLcd, shpNb);
auto shpNc1 = std::make_shared<nodeClass<int, int, double, bool>>
(func0, shpNc0, shpLcd, shpNb);
auto shpNc2 = std::make_shared<nodeClass<int, int, double, bool>>
(func0, shpNc1, shpLcd, shpNb);
std::cout << shpNc0->process() << std::endl; // print 3
std::cout << shpNc1->process() << std::endl; // print 5
std::cout << shpNc2->process() << std::endl; // print 7
}
相关文章：