Re: avoid inheritance from std::map

Juha Nieminen <nospam@thanks.invalid>
Mon, 15 Dec 2008 21:44:25 GMT
Hicham Mouline wrote:

we have bits of code where we inherit from std::map.

Why again shouldn't one inherit from STL containers in the C++ runtimes?

The proposal is to write a
template <typename Key, typename Value>
class MapDecorator {
  //replicate map's interface here and simply forward all calls to m_
  std::map<Key,Value> m_;
and then inherit from MapDecorator <....>

I agree one shouldn't inherit from std::map, though I don't remember why,
However the above seems to me redundant?

  It may be a question of abstraction.

  By inheriting directly from std::map you may be (rather ironically)
breaking good object-oriented design principles related to modularity
and abstraction. That's because you are fully exposing the data
container you are using in your class (as your class *is* the data

  Sometimes that doesn't matter. If your class really *is* a map, just
with added functionality, then it usually is ok.

  However, if your class is trying to represent some higher concept,
then by exposing to the outside that it really is a std::map, you are
lessening its abstraction. In some cases this can make it very difficult
to, for example, change the data container implementation in the future.

  Usually you don't really want to replicate the entire std::map
interface in your class. You want to implement what you class can do.
Whether it internally uses an std::map or something else (such as a
hash_map or whatever) should be a hidden implementation detail.

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