C++ FAQ / Section 35 / FAQ 35.21

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Revised Jul 4, 2012

[35.21] How can I create a container-template that allows my users to supply the type of the underlying container that actually stores the values?

First, let's clarify the question: the goal is to create a template Foo<>, but having the template parameter-list include some particular type of std::vector<T> or std::list<T> or some other (possibly non-standard) container to actually store the values.

Here's one way to do that:

template<typename Underlying>
class Foo {
public:
  // typename value_type is the type of the values within a Foo-container
  typedef typename Underlying::value_type  value_type;

  void insert(const typename value_type& x)
  {
    ← code to insert x into data_
  }

  ...
private:
  Underlying data_;
};

Foo<std::vector<int> > x;
Foo<std::list<double> > y;

If you want to allow your users to provide you with an underlying container that does not necessarily have a value_type typedef (such as some container from a third party), you could provide the value-type explicitly:

template<typename T, typename Underlying>
class Foo {
public:
  // typename value_type is the type of the values within a Foo-container
  typedef T  value_type;

  void insert(const typename value_type& x)
  {
    ← code to insert x into data_
  }

  ...

private:
  Underlying data_;
};

Foo<int, std::vector<int> > x;
Foo<double, std::list<double> > y;

However you cannot (yet) provide an unspecified template as a template parameter, such as this:

template<typename T, template<typename> class Underlying>  ← conceptual only; not C++
class Foo {
public:
  ...
private:
  Underlying<T> data_;     ← conceptual only; not C++
};

Foo<int, std::vector> x;   ← conceptual only; not C++
Foo<double, std::list> y;  ← conceptual only; not C++