C++ 多态工厂如何从字符串名称和参数映射构建函数对象

Question

#include <cassert>
#include <iostream>
#include <map>
#include <string>
#include <vector>
#include <any>
#include <functional>
#include <type_traits>
using namespace std;

using MapAny = map<string, any>;

class FunctionBase {
public:
    FunctionBase() {}
    FunctionBase(const MapAny &param_maps) {}
    virtual any operator()(const any &data) const = 0;
};

class Func1 : public FunctionBase {
private:
    int a,b;

public:
    Func1(const MapAny &param_map) {
        a = any_cast<int>(param_map.at("a"));
        b = any_cast<int>(param_map.at("b"));
    }

    virtual any operator()(const any &data) const override {        
        // calculate some F1(data, a, b)
        return a + b;
    }
};

class Func2 : public FunctionBase {
private:
    float c,d;    
public:
    Func2(const MapAny &param_map) {        
        c = any_cast<float>(param_map.at("c"));
        d = any_cast<float>(param_map.at("d"));
    }

    virtual any operator()(const any &data) const override {
        // calculate some F2(data, a, b)
        return c * d;
    }
};

FunctionBase* functionFactory(string name, const MapAny &param_map)
{
    if (name=="func1") return new Func1(param_map);
    if (name=="func2") return new Func2(param_map);
    return nullptr;
}

int main()
{
    // map<string, ???> functionMapping;
    // functionMapping["func1"] = ...
    // functionMapping["func2"] = ...   
    vector<float> data;

    MapAny param_map1;
    param_map1["a"] = 3;
    param_map1["b"] = 5;    
    FunctionBase* func1 = functionFactory("func1", param_map1); //functionMapping["func1"](param_map1);
    assert(any_cast<int>(func1->operator()(data))==8);

    MapAny param_map2;
    param_map2["c"] = 2.0f;
    param_map2["d"] = 4.0f;
    FunctionBase* func2 = functionFactory("func2", param_map2); //functionMapping["func2"](param_map2);
    assert(any_cast<float>(func2->operator()(data))==8.0);

    cout << "Correct\n";
}

我正在尝试创建一个类似于 Python 的 C++ 库，用户将在其中提供 map 字符串和参数。代码会将映射解析为函数，然后执行函数。最重要的是，我需要能够写：

std::any param_map; // = something, doesn't matter here
FunctionBase *myfunc = new functionMapping["myfunc"](param_map);

给定示例中的代码，

的值类型应该是什么

map<string, ???> functionMapping;

编辑： 添加了一个使用丑陋的 if 语句的等效解决方案

Answer 1

答案是使用 lambda 和 std::function

map<string, std::function<FunctionBase*(const MapAny&)>> functionMapping;
functionMapping["func1"] = [](const MapAny& prmap) {return new Func1(prmap);};
functionMapping["func2"] = [](const MapAny& prmap) {return new Func2(prmap);};
FunctionBase *func1 = functionMapping["func1"](param_map1);
FunctionBase *func2 = functionMapping["func2"](param_map2);

Answer 2

您可以使用基于模板的抽象工厂。这是非常通用的，可用于不同类型的键和值。

也许下面的通用代码可以让您了解如何实现您的工厂。

#include <iostream>
#include <map>
#include <utility>
#include <any>


// Some demo classes ----------------------------------------------------------------------------------
struct Base {
    Base(int d) : data(d) {};
    virtual ~Base() { std::cout << "Destructor Base\n"; }
    virtual void print() { std::cout << "Print Base\n"; }
    int data{};
};
struct Child1 : public Base {
    Child1(int d, std::string s) : Base(d) { std::cout << "Constructor Child1 " << d << " " << s << "\n"; }
    virtual ~Child1() { std::cout << "Destructor Child1\n"; }
    virtual void print() { std::cout << "Print Child1: " << data << "\n"; }
};
struct Child2 : public Base {
    Child2(int d, char c, long l) : Base(d) { std::cout << "Constructor Child2 " << d << " " << c << " " << l << "\n"; }
    virtual ~Child2() { std::cout << "Destructor Child2\n"; }
    virtual void print() { std::cout << "Print Child2: " << data << "\n"; }
};
struct Child3 : public Base {
    Child3(int d, long l, char c, std::string s) : Base(d) { std::cout << "Constructor Child3 " << d << " " << l << " " << c << " " << s << "\n"; }
    virtual ~Child3() { std::cout << "Destructor Child3\n"; }
    virtual void print() { std::cout << "Print Child3: " << data << "\n"; }
};



using UPTRB = std::unique_ptr<Base>;


template <class Child, typename ...Args>
UPTRB createClass(Args...args) { return std::make_unique<Child>(args...); }

// The Factory ----------------------------------------------------------------------------------------
template <class Key, class Object>
class Factory
{
    std::map<Key, std::any> selector;
public:
    Factory() : selector() {}
    Factory(std::initializer_list<std::pair<const Key, std::any>> il) : selector(il) {}

    template<typename Function>
    void add(Key key, Function&& someFunction) { selector[key] = std::any(someFunction); };

    template <typename ... Args>
    Object create(Key key, Args ... args) {
        if (selector.find(key) != selector.end()) {
            return std::any_cast<std::add_pointer_t<Object(Args ...)>>(selector[key])(args...);
        }
        else return nullptr;
    }
};

int main()
{
    // Define the factory with an initializer list
    Factory<int, UPTRB> factory{
        {1, createClass<Child1, int, std::string>},
        {2, createClass<Child2, int, char, long>}
    };

    // Add a new entry for the factory
    factory.add(3, createClass<Child3, int, long, char, std::string>);


    // Some test values
    std::string s1(" Hello1 "); std::string s3(" Hello3 ");
    int i = 1;  const int ci = 1;   int& ri = i;    const int& cri = i;   int&& rri = 1;

    UPTRB b1 = factory.create(1, 1, s1);
    UPTRB b2 = factory.create(2, 2, '2', 2L);
    UPTRB b3 = factory.create(3, 3, 3L, '3', s3);

    b1->print();
    b2->print();
    b3->print();
    b1 = factory.create(2, 4, '4', 4L);
    b1->print();
    return 0;
}

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