std :: map中的C ++模板值

Question

我想实现一个包含字典等的类。有一个简化的示例，其中我使用地图来实现它：

#include <map>
#include <iostream>

class Position {
public:
    typedef std::string AttrKey;
    typedef std::string AttrValue;
    typedef std::map<AttrKey, AttrValue> AttrMap;

    Position() = default;

    void set(const AttrKey& key, const AttrValue& value)
    {
        attrs[key] = value;
    }

    AttrValue get(const AttrKey& key) const
    {
        const auto found = attrs.find(key);
        if (found != attrs.end())
            return found->second;
        return "";
    }

    const AttrMap& getAttrs() const
    {
        return attrs;
    }

private:
    AttrMap attrs;

};

int main()
{
    Position p;
    p.set("some_key", "some_value");
    std::cout << "This is the value: "<< p.get("some_key");
    return 0;
}

现在，我想动态地决定值的类型，以便字典不仅包含字符串值。我是pythonist，所以对自己的工作了解很少。但是，我尝试制作一个Foo模板类，以便将任何值放入其中并将其作为AttrValue的别名（，而且我也不想使用Boost ）：

#include <map>
#include <iostream>

template <class T>
class Foo
{
public:
    typedef T Bar;
};


class Position {
public:
    template <typename T>
    using Bar = typename Foo<T>::Bar;

    typedef std::string AttrKey;
    typedef Bar AttrValue;
    typedef std::map<AttrKey, AttrValue> AttrMap;

    Position() = default;

    void set(const AttrKey& key, const AttrValue& value)
    {
        attrs[key] = value;
    }

    AttrValue get(const AttrKey& key) const
    {
        const auto found = attrs.find(key);
        if (found != attrs.end())
            return found->second;
        return "";
    }

    const AttrMap& getAttrs() const
    {
        return attrs;
    }

private:
    AttrMap attrs;

};

int main()
{
    Position p;
    p.set("some_key", 42);
    std::cout << "This is the Answer to the Ultimate Question of Life, The Universe, and Everything: "<< p.get("some_key");
    return 0;
}

我在这里遇到错误invalid use of template-name ‘Position::Bar’ without an argument list。