Tuesday, April 25, 2006

C++ templates should define traits

Exposing type information "out" from a class template appears to me as a good practice. For example, a class template Array<T> should define a trait which looks something like typedef T Array:TYPE in its public interface. Not only the parameter type, it should also expose any composite types used inside the template.

There are several good reasons behind it.

1. Member functions of the class can make use of the traits.
2. You can avoid typing really long names using these quick typedefs.
3. Consistency.
4. Other classes can make use of the traits. For example, derived classes. Sometimes it is simply not possible to write a non-template derived class of a parameterized class if the parameterized base-class does not expose necessary traits.

For example,
template <typename T> class Array;
template <typename T> class Access_Table {
Array<T> array_;
};

A new class AT_Adapter wants to adapt Access_Table<T> by adding get_array() function. It simply returns the Access_Table<T>::array_ by reference.

class AT_Adapter : Access_Table <char> {
const WHAT_IS_THIS_TYPE &get_array () const;
};

But what is the return type of the function? Therefore, class Access_Table<T> should be modified as

template <typename T> class Access_Table {
typedef Array<T> ARRAY_TYPE;
ARRAY_TYPE array_;
};

So that, the following works:

class AT_Adapter : Access_Table <char> {
const Access_Table<char>::ARRAY_TYPE &get_array () const;
};

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