Re: avoid inheritance from std::map

"" <>
Tue, 16 Dec 2008 06:54:38 -0800 (PST)
On Dec 16, 3:54 am, "Hicham Mouline" <> wrote:

"Juha Nieminen" <nos...@thanks.invalid> wrote in message


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++ runtime=


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 w=


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 difficul=


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.

If we just replicate part of map's public interface in MapDecorator<> abo=


and inherit from MapDecorator<>, then aren't we in the same situation?

MapDecorator also hasn't been designed to be a base class.

Should we change tjhe map's interface that we replicate to be virtual
functions insead,
and to add a virtual destructor,
then we'd get rid of 1 reason why we shouldn't inherit from STL container=


Our classes are curves of datapoints (x,y)- Hide quoted text -

- Show quoted text -

The correct way to do this IMHO (and picking up on Juha's

Make MapDecorator *contain* a map as part of it's (private) data and
expose what you need of it's functionality via accessor functions.
This gets you encapsulation.

If MapDecorator is going to function as a base class for derived
classes, then, and only then, make it have a virtual destructor. Make
other functions virtual (or not) only if needed. There are clear
guidelines as to whether you make a non-destructor function non-
virtual, virtual, or pure-virtual. I'm sure they're in the FAQ but I
learned the guidelines from Meyers.

Also, I would rename MapDecorator to something more domain specific so
it's clearer.


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