C++ method forwarding

Question

I need to implement a class Container which acts exactly as the contained template class:

template <typename T>
class Container {
public:
      //...
private:
      // T data_;
};

T can be either a predefined type (e.g., int) or a user-defined type.

The purpose is to intercept any read/write operations done on the contained type.

I've succesfully implemented most operators, and it works.

However, when I need to access methods specific of the contained class T, it doesn't work:

Container<myclass> a;
a.myclass_specific_method();

The reason is that Container obviously doesn't have such methods. Moreover, since T is a template, its methods cannot be known in advance.

I guess there is no solution to this problem, even with C++11, because operator . cannot be overloaded. Therefore, the only possible approach is to always rely on operator-> like smart pointers do.

Can you confirm ?

Answer 1

For a class type T, this will act a lot like a T:

template<class T, class=void>
struct Container : public T { // inheritance MUST be public
  using T::T;
  Container() = default; // or override
  Container( Container const& ) = default; // or override
  Container( Container && ) = default; // or override
  Container& operator=( Container const& ) = default; // or override
  Container& operator=( Container && ) = default; // or override
  // here, we override any method we want to intercept

  // these are used by operators:
  friend T& get_t(Container& self){return self;}
  friend T const& get_t(Container const& self){return self;}
  friend T&& get_t(Container&& self){return std::move(self);}
  friend T const&& get_t(Container const&& self){return std::move(self);}
};

for a non-class T, we detect it and use a different implementation:

template<class T>
struct Container<T, typename std::enable_if<!std::is_class<T>{}>::type > {
  T t;
  Container() = default; // or override
  Container( Container const& ) = default; // or override
  Container( Container && ) = default; // or override
  Container& operator=( Container const& ) = default; // or override
  Container& operator=( Container && ) = default; // or override

  // these are used by operators:
  friend T& get_t(Container& self){return self.t;}
  friend T const& get_t(Container const& self){return self.t;}
  friend T&& get_t(Container&& self){return std::move(self).t;}
  friend T const&& get_t(Container const&& self){return std::move(self).t;}
};

finally, we go off and override every operator we can find in a SFINAE friendly way, where the operator only participates in overload resolution if get_t(Container) would work in its place in the operator. This should all be done in a namespace, so the operators are found via ADL. An overload of get_t that returns its argument unchanged could be useful to massively reduce the number of overloads.

This could be another 100 or more lines of code.

Users of Container<T> can bypass the Container<T> and get the underlying T in the above system.

Answer 2

Are you opposed to having a getter for the inner data member? If no, then you can use something like this

#include <iostream>
#include <string>

template <typename T>
class Container
{
public:
    Container(T _data) : data{_data} {}
    T GetData() const { return data; }
private:
    T data;
};

int main()
{
    Container<std::string> c{"foo"};
    std::cout << c.GetData().size();
}

Otherwise you could internally access the method, and it will only compile if such a method exists for T

#include <iostream>
#include <string>

template <typename T>
class Container
{
public:
    Container(T _data) : data{_data} {}
    std::size_t size() const { return data.size(); }
private:
    T data;
};

int main()
{
    Container<std::string> c{"foo"};
    std::cout << c.size();
}

So this latter method would work if T was e.g. std::string, std::vector, std::list, etc.

Answer 3

The C++ committee is currently looking into "overloaded operator ." for future revisions of the language.

However, in your specific case, you can simply inherit from the type.

template <typename T>
class Container : private T {
public:
    using T::something_publicly_accessible;
};