在C中动态创建和调用类方法的最简单方法是什么?

我想用类名和方法填充一个映射,一个唯一的标识符和指向该方法的指针.

typedef std::map<std::string, std::string, std::string, int> actions_type;
typedef actions_type::iterator actions_iterator;

actions_type actions;
actions.insert(make_pair(class_name, attribute_name, identifier, method_pointer));

//after which I want call the appropriate method in the loop

while (the_app_is_running)
{
    std::string requested_class = get_requested_class();
    std::string requested_method = get_requested_method();

    //determine class
    for(actions_iterator ita = actions.begin(); ita != actions.end(); ++ita)
    {
        if (ita->first == requested_class && ita->second == requested_method)
        {
            //class and method match
            //create a new class instance
            //call method
        }
    }
}

如果方法是静态的,那么一个简单的指针就足够了,问题很简单,
但我想动态创建对象,所以我需要存储一个指向类的方法和方法的偏移量,我不知道这是否有效(如果偏移量始终相同等).

问题是C缺乏反射,带有反射的解释语言中的等效代码应该如下所示(PHP中的示例):

$actions = array
(
     "first_identifier" => array("Class1","method1"),
     "second_identifier" => array("Class2","method2"),
     "third_identifier" => array("Class3","method3")
);

while ($the_app_is_running)
{
     $id = get_identifier();

     foreach($actions as $identifier => $action)
     {
         if ($id == $identifier)
         {
             $className = $action[0];
             $methodName = $action[1];

             $object = new $className() ;

             $method = new ReflectionMethod($className , $methodName);
             $method -> invoke($object);    
         }
     }
 }

PS:是的我正在尝试用C制作一个(web)MVC前端控制器.
我知道我知道为什么不使用PHP,Ruby,Python(在这里插入你最喜欢的网络语言)等等,我只想要C.

最佳答案
我在最后几个小时写了这些东西,并将它添加到我的有用的东西集合中.如果您要创建的类型不以任何方式相关,最困难的是应对工厂功能.我为此使用了boost :: variant.你必须给它一组你想要使用的类型.然后它将跟踪变体中当前“活动”类型的内容. (boost :: variant是一种所谓的歧视联盟).第二个问题是如何存储函数指针.问题是指向A成员的指针不能存储到指向B成员的指针.这些类型是不兼容的.为了解决这个问题,我将函数指针存储在一个对象中,该对象重载了其operator()并获取了boost :: variant:

return_type operator()(variant<possible types...>)

当然,所有类型的函数都必须具有相同的返回类型.否则整场比赛只会毫无意义.现在的代码:

#include <boost/variant.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <boost/tuple/tuple.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/function_types/parameter_types.hpp>
#include <boost/function_types/result_type.hpp>
#include <boost/function_types/function_arity.hpp>
#include <boost/preprocessor/repetition.hpp>
#include <map>
#include <string>
#include <iostream>

// three totally unrelated classes
// 
struct foo {
    std::string one() {
        return "I ";
    }
};

struct bar {
    std::string two() {
        return "am ";
    }
};

struct baz {
    std::string three() const {
        return "happy!";
    }
};

// The following are the parameters you have to set
//

// return type
typedef std::string return_type;
// variant storing an object. It contains the list of possible types you
// can store.
typedef boost::variant< foo, bar, baz > variant_type;
// type used to call a function on the object currently active in
// the given variant
typedef boost::function<return_type (variant_type&)> variant_call_type;

// returned variant will know what type is stored. C++ got no reflection, 
// so we have to have a function that returns the correct type based on
// compile time knowledge (here it's the template parameter)
template<typename Class>
variant_type factory() {
    return Class();
}

namespace detail {
namespace fn = boost::function_types;
namespace mpl = boost::mpl;

// transforms T to a boost::bind
template<typename T>
struct build_caller {
    // type of this pointer, pointer removed, possibly cv qualified. 
    typedef typename mpl::at_c<
        fn::parameter_types< T, mpl::identity<mpl::_> >,
        0>::type actual_type;

    // type of boost::get we use
    typedef actual_type& (*get_type)(variant_type&);

// prints _2 if n is 0
#define PLACEHOLDER_print(z, n, unused) BOOST_PP_CAT(_, BOOST_PP_ADD(n, 2))
#define GET_print(z, n, unused)                                         \
    template<typename U>                                                \
    static variant_call_type get(                                       \
        typename boost::enable_if_c<fn::function_arity<U>::value ==     \
            BOOST_PP_INC(n), U>::type t                                 \
        ) {                                                             \
        /* (boost::get<actual_type>(some_variant).*t)(n1,...,nN) */     \
        return boost::bind(                                             \
            t, boost::bind(                                             \
                (get_type)&boost::get<actual_type>,                     \
                _1) BOOST_PP_ENUM_TRAILING(n, PLACEHOLDER_print, ~)     \
            );                                                          \
    }

// generate functions for up to 8 parameters
BOOST_PP_REPEAT(9, GET_print, ~)

#undef GET_print
#undef PLACEHOLDER_print

};

}

// incoming type T is a member function type. we return a boost::bind object that
// will call boost::get on the variant passed and calls the member function
template<typename T>
variant_call_type make_caller(T t) {
    return detail::build_caller<T>::template get<T>(t);
}

// actions stuff. maps an id to a class and method.
typedef std::map<std::string, 
                 std::pair< std::string, std::string >
                 > actions_type;

// this map maps (class, method) => (factory, function pointer)
typedef variant_type (*factory_function)();
typedef std::map< std::pair<std::string,      std::string>, 
                  std::pair<factory_function, variant_call_type> 
                  > class_method_map_type;

// this will be our test function. it's supplied with the actions map, 
// and the factory map
std::string test(std::string const& id,
                 actions_type& actions, class_method_map_type& factory) {
    // pair containing the class and method name to call
    std::pair<std::string, std::string> const& class_method =
        actions[id];

    // real code should take the maps by const parameter and use
    // the find function of std::map to lookup the values, and store
    // results of factory lookups. we try to be as short as possible. 
    variant_type v(factory[class_method].first());

    // execute the function associated, giving it the object created
    return factory[class_method].second(v);
}

int main() {
    // possible actions
    actions_type actions;
    actions["first"] = std::make_pair("foo", "one");
    actions["second"] = std::make_pair("bar", "two");
    actions["third"] = std::make_pair("baz", "three");

    // connect the strings to the actual entities. This is the actual
    // heart of everything.
    class_method_map_type factory_map;
    factory_map[actions["first"]] = 
        std::make_pair(&factory<foo>, make_caller(&foo::one));
    factory_map[actions["second"]] = 
        std::make_pair(&factory<bar>, make_caller(&bar::two));
    factory_map[actions["third"]] = 
        std::make_pair(&factory<baz>, make_caller(&baz::three));

    // outputs "I am happy!"
    std::cout << test("first", actions, factory_map)
              << test("second", actions, factory_map)
              << test("third", actions, factory_map) << std::endl;
}

它使用来自boost预处理器,函数类型和绑定库的非常有趣的技术.可能循环复杂,但如果你得到该代码中的键,那就不再需要掌握了.如果要更改参数计数,只需调整variant_call_type即可:

typedef boost::function<return_type (variant_type&, int)> variant_call_type;

现在,您可以调用带有int的成员函数.以下是呼叫方面的外观:

return factory[class_method].second(v, 42);

玩得开心!

如果你现在说上面的内容过于复杂,我必须同意你的观点.它很复杂,因为C并不是真正用于这种动态使用.如果您可以在要创建的每个对象中对方法进行分组和实现,则可以使用纯虚函数.或者,您可以在默认实现中抛出一些异常(如std :: runtime_error),因此派生类不需要实现所有内容:

struct my_object {
    typedef std::string return_type;

    virtual ~my_object() { }
    virtual std::string one() { not_implemented(); }
    virtual std::string two() { not_implemented(); }
private:
   void not_implemented() { throw std::runtime_error("not implemented"); }
};

对于创建对象,通常的工厂会这样做

struct object_factory {
    boost::shared_ptr<my_object> create_instance(std::string const& name) {
        // ...
    }
};

地图可以通过映射将ID映射到一对类和函数名称(与上面相同),以及映射到boost :: function的映射:

typedef boost::function<my_object::return_type(my_object&)> function_type;
typedef std::map< std::pair<std::string, std::string>, function_type> 
                  class_method_map_type;
class_method_map[actions["first"]] = &my_object::one;
class_method_map[actions["second"]] = &my_object::two;

调用该函数将如下工作:

boost::shared_ptr<my_object> p(get_factory().
    create_instance(actions["first"].first));
std::cout << class_method_map[actions["first"]](*p);

当然,通过这种方法,您可以放松灵活性(可能没有分析)效率,但是您可以大大简化设计.

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