如何在模板类中返回成员变量的副本？

Question

Template newbie here. I'm playing with the following test class:

template<typename T>
class Container
{
public:

    Container(T t) : m_t(t) {}

    T clone()
    {
        return m_t;
    }

private:

    T m_t;
};

The clone() method returns a copy of the member variable. Obviously, this doesn't work as intended if T is a pointer, eg:

Container<SomeClass*> container(new SomeClass());

The clone() method would simply return a pointer and not a full clone. I'm aware of the beauty of if constexpr but unfortunately I'm stuck with a C++14 compiler. I would like to keep my class generic so that it would also work with pointers. Should I create two different methods? Can SFINAE be of any help here?

Answer 1

To specialize the entire class, as the other answer suggests, you might need to duplicate a lot of code (not a good thing).

Instead, I'd suggest good ol' tag dispatch :

template<typename T>
class Container
{
    T clone_low(std::false_type)
    {
        return m_t;
    }

    T clone_low(std::true_type)
    {
        return new std::remove_pointer_t<T>(m_t);
    }

  public:
    Container(T t) : m_t(t) {}

    T clone()
    {
        return clone_low(std::is_pointer<T>{});
    }

  private:
    T m_t;
};

Answer 2

You can use a partial specialization of the class. For example

#include <iostream>

template<typename T>
class Container
{
public:
    Container(T t) : m_t(t) {}
    T clone()
    {
        return m_t;
    }

private:

    T m_t;
};

template<typename T>
class Container<T*>
{
public:
    Container(T* t) : m_t(new T(*t)) {}
    T* clone()
    {
        return new T(*m_t);
    }
    ~Container() { delete m_t; }

private:
    T* m_t;
};

int main()
{
    std::cout << Container<int>(10).clone() << '\n';

    int x = 20;
    Container<int*> c(&x);

    int* p = c.clone();
    std::cout << *p << '\n';

    delete p;
    return 0;
}