How to get the value of a template class? - c++

Question

I'm trying to get the value of a template class. To get the value of a class, I can easily do like:
int get_value()
{
return *this;
}
But I want to create a class, and extend it, and don't make get_value() in all classes again. So, I did that:
template<typename T>
class _extend : public T
{
public:
auto _get_value()
{
return *this;
}
};
template<typename T>
class extend : public _extend<T>
{
public:
T get_value()
{
auto value = this->_get_value(); /* It was `_get_value()`, changed to `this->_get_value()` due to the comments */
T output = value;
return output;
}
};
But it's not working: output is nothing.
Edit
Sample program:
#include <iostream>
namespace kc
{
template<typename T>
class _extend : public T
{
public:
auto _get_value()
{
return *this;
}
};
template<typename T>
class extend : public _extend<T>
{
public:
T get_value()
{
auto value = this->_get_value();
T output = value;
return output;
}
};
class A : public std::string, public extend<std::string>
{
public:
using std::string::string;
};
}
int main()
{
kc::A a("a");
std::cout << a.get_value() << std::endl;
}

Answer 1

The problem with the code in the question is the multiple inheritance. The *this that _extend<string>::get_value() will be called with is not the string that has a value.
Although I agree with #NicolBolas in comments in that I don't understand why you want this, but you can make a class template that will return the value of what it derives from with only one level of inheritance. You just need to give it a perfect forwarding constructor and have get_value() cast to the base type, i.e.
#include <iostream>
#include <string>
namespace kc
{
template<typename T>
class extend : public T
{
public:
template<typename... Args>
extend(Args&&... args) : T(std::forward<Args>(args)...)
{}
T get_value()
{
return *static_cast<T*>(this);
}
};
}
int main()
{
kc::extend<std::string> a("foo");
std::cout << a.get_value() << std::endl;
}

Answer 2

Basically:
template<class C>
class extend : public C
{
public:
using C::C;
auto get()
{
return *this;
}
};
And a full example:
#include <iostream>
template<class C>
class extend : public C
{
public:
using C::C;
auto get()
{
return *this;
}
};
class e_string : public extend<std::string>
{
public:
using extend<std::string>::extend;
};
int main()
{
e_string s = "Test";
std::cout << s.get() << std::endl;
}