boost :: any的访问者模式 [英] visitor pattern for boost::any

问题描述

我发现这个 https://gist.github.com/2945472 ，但我需要一个不依赖于c的实施+ ++ 11。

I found this https://gist.github.com/2945472 but I need an implementation that does not depend on c++11. I tried my hand at converting it to use only boost, but I'm having some trouble.

这是我想出的：

#include <boost/any.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <boost/lambda/lambda.hpp>
#include <boost/unordered_map.hpp>

struct type_info_hash {
    std::size_t operator()(std::type_info const & t) const {
        return t.hash_code();
    }
};

struct equal_ref {
    template <typename T> bool operator()(boost::reference_wrapper<T> a,boost::reference_wrapper<T> b) const {
        return a.get() == b.get();
    }
};
struct any_visitor {
    boost::unordered_map<boost::reference_wrapper<std::type_info const>, boost::function<void(boost::any&)>, type_info_hash, equal_ref> fs;

    template <typename T> void insert_visitor(boost::function<void(T)> f) {
        try {
            fs.insert(std::make_pair(boost::ref(typeid(T)), boost::bind(f, boost::any_cast<T>(boost::lambda::_1))));
        } catch (boost::bad_any_cast& e) {
            std::cout << e.what() << std::endl;
        }
    }

    bool operator()(boost::any & x) {
        boost::unordered_map<boost::reference_wrapper<std::type_info const>, boost::function<void(boost::any&)>, type_info_hash, equal_ref>::iterator it = fs.find(boost::ref(x.type()));
        if (it != fs.end()) {
            it->second(x);
            return true;
        } else {
            return false;
        }
    }
};

struct abc {};

void fa(int i) { std::cout << "fa(" << i << ")" << std::endl; }
void fb(abc) { std::cout << "fb(abc())" << std::endl; }

int main() {
    any_visitor f;
    f.insert_visitor<int>(fa);
    f.insert_visitor<abc>(fb);

    std::vector<boost::any> xs;
    xs.push_back(1);
    xs.push_back(abc());
    xs.push_back(1.5);

    for (auto & x : xs) {
        if (!f(x)) std::cout << "no visitor registered" << std::endl;
    }
}

我在插入地图时收到一个bad_any_cast 。应该不是any_cast只被它调用 - > second（x）？

I'm getting a bad_any_cast when inserting into the map. Shouldn't any_cast only be called by it->second(x) ? What am I doing wrong?

推荐答案

您不能投射 _1 到 T （在绑定表达式时）。

You can't cast _1 to T (at the time of the bind expression).

你需要一个懒惰的演员。也许定义一个函数并使用一个嵌套的绑定表达式，或者使用Boost Phoenix和一个自定义的 any_cast actor。

You need a lazy cast. Perhaps define a function and use a nested bind expression, or use Boost Phoenix with a custom any_cast actor.

嵌套绑定方法：

#include <boost/any.hpp>
#include <boost/function.hpp>
#include <boost/bind.hpp>
#include <boost/lambda/lambda.hpp>
#include <boost/unordered_map.hpp>

struct type_info_hash {
    std::size_t operator()(std::type_info const & t) const {
        return 42; // t.hash_code();
    }
};

struct equal_ref {
    template <typename T> bool operator()(boost::reference_wrapper<T> a,boost::reference_wrapper<T> b) const {
        return a.get() == b.get();
    }
};
struct any_visitor {
    boost::unordered_map<boost::reference_wrapper<std::type_info const>, boost::function<void(boost::any&)>, type_info_hash, equal_ref> fs;

    template <typename T> static T any_cast_f(boost::any& any) { return boost::any_cast<T>(any); }

    template <typename T> void insert_visitor(boost::function<void(T)> f) {
        try {
            fs.insert(std::make_pair(boost::ref(typeid(T)), boost::bind(f, boost::bind(any_cast_f<T>, boost::lambda::_1))));
        } catch (boost::bad_any_cast& e) {
            std::cout << e.what() << std::endl;
        }
    }

    bool operator()(boost::any & x) {
        boost::unordered_map<boost::reference_wrapper<std::type_info const>, boost::function<void(boost::any&)>, type_info_hash, equal_ref>::iterator it = fs.find(boost::ref(x.type()));
        if (it != fs.end()) {
            it->second(x);
            return true;
        } else {
            return false;
        }
    }
};

struct abc {};

void fa(int i) { std::cout << "fa(" << i << ")" << std::endl; }
void fb(abc) { std::cout << "fb(abc())" << std::endl; }

int main() {
    any_visitor f;
    f.insert_visitor<int>(fa);
    f.insert_visitor<abc>(fb);

    std::vector<boost::any> xs;
    xs.push_back(1);
    xs.push_back(abc());
    xs.push_back(1.5);

    for (auto it=xs.begin(); it!=xs.end(); ++it)
        if (!f(*it)) std::cout << "no visitor registered" << std::endl;
}

打印输出：

fa(1)
fb(abc())
no visitor registered

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