Question

I am trying to write very simple Any object, which can hold object of any type. I want it use inside container, to achieve heterogeneous container.

#include <iostream>
#include <vector>
#include <string>

struct Any
{
    template < typename T >
    Any(const T & t) 
        :p(new storageImpl<T>(t)) { }

    ~Any() 
    {
        delete p;
    }

    struct storage
    {
        virtual ~storage() {}
    };

    template <typename T>
    struct storageImpl : storage
    {
        storageImpl(const T & t) : data(t) {}
        T data;
    };

    template <typename T>
    T & get()
    {
        storageImpl<T> * i = static_cast<storageImpl<T>*>(p);
        return i->data;
    }

    storage * p;
};

usage

int main ()
{
    //block1
    Any copy(Any(std::string("foo")));      
    std::cout << copy.get<std::string>();   

    //block2
    std::vector<Any> cont;
    cont.push_back(Any(5));
    cont.push_back(Any(37.9f));
    std::cout << cont[0].get<int>();    
    std::cout << cont[1].get<float>();  
}

I have problem with copy-construction.

When I push Any into vector (//block2), the unnamed Any gets destroyed, so pointer gets deleted, and pushed object is no longer valid.

So I have 2 questions:

1, How to write copy constructor for class Any?

2, Why isn’t unnamed Any in block1 destroyed, so its pointer isn’t deleted?\

EDIT
I have tried

template <typename T>
Any(const Any & rhs)
    :p(new storageImpl<T>(rhs.get()))
{
}

but it doesn’t get triggered.

Answer 1

Any is not a template class. Trying to template the copy-constructor as template <typename T> Any(const Any & rhs) is meaningless.

What you could do is to use the virtual constructor idiom, to let the storageImpl copy itself. This is also the method used in Boost.Any.

struct Any {
    template < typename T >
    Any(const T& t) : p(new storageImpl<T>(t)) {}

    Any(const Any& other) : p(other.p->clone()) {}
 // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    ~Any() { delete p; }

    struct storage {
        virtual ~storage() {}
        virtual storage* clone() = 0;
     // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    };

    template <typename T>
    struct storageImpl : storage {
        storageImpl(const T & t) : data(t) {}

        virtual storage* clone() { return new storageImpl(data); }
     // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

        T data;
    };

    template <typename T>
    T& get() {
        storageImpl<T>* i = static_cast<storageImpl<T>*>(p);
        return i->data;
    }

    storage * p;
};

Notice that this implementation have a lot of problems, e.g. the get() method won’t check whether the Any is really holding a T. It is still better to use a well-tested library e.g. Boost.Any.

---

Why isn’t unnamed Any in block1 destroyed, so its pointer isn’t deleted?

Copy elision.