template class to manage containers, getting type of contained data

Question

I have a templated c++ class which acts on containers. In the class implementation I need access to the type of the data the container contains. I've currently defined it like this:

template <typename ContainerType, typename DataType>
class MyClass {
    DataType a;
    DataType foo(DataVector& vec, DataType s);
};

And when I instantiate it I instantiate it like this

MyClass< vector<float>, float > c1;
MyClass< CustomVector<double>, double > c2;
MyClass< Lib::ContainerFloat, Lib::Float > c3; 

This works, but is there a way of doing this without duplicating the type info (i.e. float, double etc)? I.e. use some decltype-like magic to get the contained data type. So I'd like to be able to instantiate with:

MyClass< vector<float> > c1;
MyClass< CustomVector<double> > c2;
MyClass< Lib::ContainerFloat > c3; 

and declare the class with:

template <typename ContainerType>
class MyClass {
    // get DataType automatically from ContainerType
    DataType a;
    DataType foo(DataVector& vec, DataType s);
};

Answer 1

As a concept, all containers are expected to support value_type as a type. See http://en.cppreference.com/w/cpp/concept/Container.

Last I checked, value_type is supported by:

1. std::vector
2. std::list
3. std::set
4. std::multiset
5. std::unordered_set
6. std::unordered_multiset
7. std::queue
8. std::array
9. std::map
10. std::unordered_map
11. std::unordered_multimap
12. std::multimap
13. std::stack
14. std::priority_queue

I think it's safe for you to use:

template <typename ContainerType>
class MyClass {
   using DataType = ContainerType::value_type;
};

MyClass< vector<float>> c1;

If DataType can ever be different than ContainerType::value_type, it will be better to use (Thanks are due to @AlexeyAndronov for the suggestion):

template <typename ContainerType,
          typename DataType = ContainerType::value_type>
class MyClass {
};