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Is it okay to inherit implementation from STL containers, rather than delegate?

Posted by: admin November 29, 2017 Leave a comment

Questions:

I have a class that adapts std::vector to model a container of domain-specific objects. I want to expose most of the std::vector API to the user, so that he/she may use familiar methods (size, clear, at, etc…) and standard algorithms on the container. This seems to be a reoccurring pattern for me in my designs:

class MyContainer : public std::vector<MyObject>
{
public:
   // Redeclare all container traits: value_type, iterator, etc...

   // Domain-specific constructors
   // (more useful to the user than std::vector ones...)

   // Add a few domain-specific helper methods...

   // Perhaps modify or hide a few methods (domain-related)
};

I’m aware of the practice of preferring composition to inheritance when reusing a class for implementation — but there’s gotta be a limit! If I were to delegate everything to std::vector, there would be (by my count) 32 forwarding functions!

So my questions are… Is it really so bad to inherit implementation in such cases? What are the risks? Is there a safer way I can implement this without so much typing? Am I a heretic for using implementation inheritance? 🙂

Edit:

What about making it clear that the user should not use MyContainer via a std::vector<> pointer:

// non_api_header_file.h
namespace detail
{
   typedef std::vector<MyObject> MyObjectBase;
}

// api_header_file.h
class MyContainer : public detail::MyObjectBase
{
   // ...
};

The boost libraries seem to do this stuff all the time.

Edit 2:

One of the suggestions was to use free functions. I’ll show it here as pseudo-code:

typedef std::vector<MyObject> MyCollection;
void specialCollectionInitializer(MyCollection& c, arguments...);
result specialCollectionFunction(const MyCollection& c);
etc...

A more OO way of doing it:

typedef std::vector<MyObject> MyCollection;
class MyCollectionWrapper
{
public:
   // Constructor
   MyCollectionWrapper(arguments...) {construct coll_}

   // Access collection directly
   MyCollection& collection() {return coll_;} 
   const MyCollection& collection() const {return coll_;}

   // Special domain-related methods
   result mySpecialMethod(arguments...);

private:
   MyCollection coll_;
   // Other domain-specific member variables used
   // in conjunction with the collection.
}
Answers:

The risk is deallocating through a pointer to the base class (delete, delete[], and potentially other deallocation methods). Since these classes (deque, map, string, etc.) don’t have virtual dtors, it’s impossible to clean them up properly with only a pointer to those classes:

struct BadExample : vector<int> {};
int main() {
  vector<int>* p = new BadExample();
  delete p; // this is Undefined Behavior
  return 0;
}

That said, if you’re willing to make sure you never accidentally do this, there’s little major drawback to inheriting them—but in some cases that’s a big if. Other drawbacks include clashing with implementation specifics and extensions (some of which may not use reserved identifiers) and dealing with bloated interfaces (string in particular). However, inheritance is intended in some cases, as container adapters like stack have a protected member c (the underlying container they adapt), and it’s almost only accessible from a derived class instance.

Instead of either inheritance or composition, consider writing free functions which take either an iterator pair or a container reference, and operate on that. Practically all of <algorithm> is an example of this; and make_heap, pop_heap, and push_heap, in particular, are an example of using free functions instead of a domain-specific container.

So, use the container classes for your data types, and still call the free functions for your domain-specific logic. But you can still achieve some modularity using a typedef, which allows you to both simplify declaring them and provides a single point if part of them needs to change:

typedef std::deque<int, MyAllocator> Example;
// ...
Example c (42);
example_algorithm(c);
example_algorithm2(c.begin() + 5, c.end() - 5);
Example::iterator i; // nested types are especially easier

Notice the value_type and allocator can change without affecting later code using the typedef, and even the container can change from a deque to a vector.

Questions:
Answers:

You can combine private inheritance and the ‘using’ keyword to work around most of the problems mentioned above: Private inheritance is ‘is-implemented-in-terms-of’ and as it is private you cannot hold a pointer to the base class

#include <string>
#include <iostream>

class MyString : private std::string
{
public:
    MyString(std::string s) : std::string(s) {}
    using std::string::size;
    std::string fooMe(){ return std::string("Foo: ") + *this; }
};

int main()
{
    MyString s("Hi");
    std::cout << "MyString.size(): " << s.size() << std::endl;
    std::cout << "MyString.fooMe(): " << s.fooMe() << std::endl;
}

Questions:
Answers:

As everyone has already stated, STL containers do not have virtual destructors so inheriting from them is unsafe at best. I’ve always considered generic programming with templates as a different style of OO – one without inheritance. The algorithms define the interface that they require. It is as close to Duck Typing as you can get in a static language.

Anyway, I do have something to add to the discussion. The way that I have created my own template specializations previously is to define classes like the following to use as base classes.

template <typename Container>
class readonly_container_facade {
public:
    typedef typename Container::size_type size_type;
    typedef typename Container::const_iterator const_iterator;

    virtual ~readonly_container_facade() {}
    inline bool empty() const { return container.empty(); }
    inline const_iterator begin() const { return container.begin(); }
    inline const_iterator end() const { return container.end(); }
    inline size_type size() const { return container.size(); }
protected: // hide to force inherited usage only
    readonly_container_facade() {}
protected: // hide assignment by default
    readonly_container_facade(readonly_container_facade const& other):
        : container(other.container) {}
    readonly_container_facade& operator=(readonly_container_facade& other) {
        container = other.container;
        return *this;
    }
protected:
    Container container;
};

template <typename Container>
class writable_container_facade: public readable_container_facade<Container> {
public:
    typedef typename Container::iterator iterator;
    writable_container_facade(writable_container_facade& other)
        readonly_container_facade(other) {}
    virtual ~writable_container_facade() {}
    inline iterator begin() { return container.begin(); }
    inline iterator end() { return container.end(); }
    writable_container_facade& operator=(writable_container_facade& other) {
        readable_container_facade<Container>::operator=(other);
        return *this;
    }
};

These classes expose the same interface as an STL container. I did like the effect of separating the modifying and non-modifying operations into distinct base classes. This has a really nice effect on const-correctness. The one downside is that you have to extend the interface if you want to use these with associative containers. I haven’t run into the need though.

Questions:
Answers:

In this case, inheriting is a bad idea: the STL containers do not have virtual destructors so you might run into memory leaks (plus, it’s an indication that STL containers are not meant to be inherited in the first place).

If you just need to add some functionality, you can declare it in global methods, or a lightweight class with a container member pointer/reference. This off course doesn’t allow you to hide methods: if that is really what you are after, then there’s no other option then redeclaring the entire implementation.

Questions:
Answers:

Virtual dtors aside, the decision to inherit versus contain should be a design decision based the class you are creating. You should never inherit container functionality just because its easier than containing a container and adding a few add and remove functions that seem like simplistic wrappers unless you can definitively say that the class you are creating is a kind-of the container. For instance, a classroom class will often contain student objects, but a classroom isn’t a kind of list of students for most purposes, so you shouldn’t be inheriting from list.

Questions:
Answers:

It is easier to do:

typedef std::vector<MyObject> MyContainer;

Questions:
Answers:

The forwarding methods will be inlined away, anyhow. You will not get better performance this way. In fact, you will likely get worse performance.