c++ - boost::fusion::push_back 的正确用法是什么？

Question

// ... snipped includes for iostream and fusion ...
namespace fusion = boost::fusion;

class Base
{
protected: int x;
public: Base() : x(0) {}
    void chug() { 
        x++;
        cout << "I'm a base.. x is now " << x << endl;
    }
};

class Alpha : public Base
{
public:
    void chug() { 
        x += 2;
        cout << "Hi, I'm an alpha, x is now " << x << endl;
    }
};

class Bravo : public Base
{
public:
    void chug() { 
        x += 3;
        cout << "Hello, I'm a bravo; x is now " << x << endl; 
    }
};

struct chug {
    template<typename T>
    void operator()(T& t) const
    {
        t->chug();
    }
};

int main()
{
    typedef fusion::vector<Base*, Alpha*, Bravo*, Base*> Stuff;
    Stuff stuff(new Base, new Alpha, new Bravo, new Base);

    fusion::for_each(stuff, chug());     // Mutates each element in stuff as expected

    /* Output:
       I'm a base.. x is now 1
       Hi, I'm an alpha, x is now 2
       Hello, I'm a bravo; x is now 3
       I'm a base.. x is now 1
    */

    cout << endl;

    // If I don't put 'const' in front of Stuff...
    typedef fusion::result_of::push_back<const Stuff, Alpha*>::type NewStuff;

    // ... then this complains because it wants stuff to be const:
    NewStuff newStuff = fusion::push_back(stuff, new Alpha);

    // ... But since stuff is now const, I can no longer mutate its elements :(
    fusion::for_each(newStuff, chug());

    return 0;
};

如何让 for_each(newStuff, chug()) 工作？

(注意:我只是根据 boost::fusion 上的 overly brief documentation 假设我应该在每次调用 push_back 时创建一个新 vector 。)

Answer 1

(Note: I'm only assuming from the overly brief documentation on boost::fusion that I am supposed to create a new vector every time I call push_back.)

您不是在创建新 vector 。 push_back 返回延迟计算的 view在扩展序列上。如果你想创建一个新的 vector ，那么例如typedef NewStuff 作为

typedef fusion::vector<Base*, Alpha*, Bravo*, Base*, Alpha*> NewStuff;

然后你的程序就可以运行了。

顺便说一句， fusion 是一种非常实用的设计。我认为如果您存储实际对象而不是指针并使用 transform，它会更像 fusion 。然后，chug 逻辑将从类中移出到 struct chug 中，它为每种类型提供了适当的 operator()。那时不必创建新的 vector ，您可以使用延迟评估的 View 。