In
part I I have written that templates should generally be preferred to virtual functions if dynamic polymorphism is not absolutely needed.
In this part I will deal with some common arguments in favor of using object-oriented style interfaces and critics of template.
So why C++ developers often prefer interfaces with virtual functions to solutions based on templates? Here is the list of motives I'm aware of:
- code that use OO interfaces can be hidden in .cpp/.CC files, whenever templates force to expose the whole code in the header file;
- templates will cause code bloat;
- OO interfaces are explicit, whenever requirements to template parameters are
implicit and exist only in developer's head;
- heavy usage of templates hurts compilation speed.
Let's scrutinize them in order.
Code that uses OO interfaces can be hidden in .cpp/.CC files, whenever templates force to expose the whole code in the header file
Here is how usually code which works with object-oriented hierarchies looks like:
// in header file
void f(Base const& t);
// in source file
void f(Base const& t)
{
// ...
// hairy implementation
// ...
}
So nobody sees implementation of f() guy (*).
Here is how templates usually implemented:
// in header file
template <class T>
void f(T const& t)
{
// ...
// hairy implementation
// not only hurts aesthetic feelings of every visitor
// but also couples the interface with dependencies of the implementation
// ...
}
Aesthetics can be amended very easy by extracting implementation to "implementation" header file.
Second problem -- exposing dependency on implementation -- is more difficult. This can be fixed, but only if you know a subset of types that your template will be instantiated with -- just provide explicit instantiation of template functions/classes with those parameters:
// in header file
template <class T>
void f(T const& t);
// explicit instantiation
template
void f<T1>(T1 const&);
// another one
template
void f<T2>(T2 const&);
// in source file
template <class T>
void f(T const& t)
{
// ...
// hairy implementation is now hidden
// ...
}
Templates will cause code bloat
Usually your code will actually
shrink comparing to non-template version -- because only those template functions, classes and member-functions instantiated, and with those type arguments only, that you actually
use in code! (**)
int f()
{
std::vector<int> v;
v.push_back(42);
// only constructor, destructor, push_back()
// and whatever they use internally will be instantiated,
// and only for type argument int
}
Bloating
can happen though if you are not careful. Classic example is containers that hold pointers. In C implementation there will be just one container that holds void*, and a couple of macros for convenient (but very unsafe) casting of those pointers to specific types. In naive C++ implementation there will be bunch of functions generated for each pointer type:
std::vector<int*> ints;
std::vector<float*> floats;
std::vector<Goblin*> goblins;
ints.push_back(0);
floats.push_back(0);
goblins.push_back(0);
// three pairs of push_back() will be generated... or not?
All decent implementations though provide a specialization for pointers:
// primary template
template <class T> class vector; // additional template omitted
// specialization for void*
template<> class vector<void*>
{
public:
// here comes the real implementation
void push_back(void* p) { ... }
};
// partial specialization for pointer types
template<T> class vector<T*> : private vector<void*>
{
private:
typedef vector<void*> base;
public:
// use version of vector<void*> with cast
// inlined, and doesn't cause any code bloating compared with
// version one would implement without templates
// (also safe: user cannot screw up)
void push_back(T* p) { return base::push_back(static_cast<void*>(v)); }
};
OO interfaces are explicit, whenever requirements to template parameters are implicit
This one is sad but absolutely true. Remember example from part I:
template <class T>
void f1(T const& t)
{
// no requirements on T except in comments if you are lucky
bool flag = t.do_something();
}
// serves as an explicit specification
class Base
{
public:
virtual bool do_something() const;
// ...
};
void f2(Base const& t)
{
// explicit requirement is Base's class interface
bool flag = t.do_something();
}
Concepts would have solved this problem, but unfortunately they
have been rejected from C++11. Let's hope that they appear in the next Standard.
Until then, you have two options.
- You can specify requirements in comments and documentation (like SGI STL documentation and C++ Standard do).
- Or you emulate concepts. Boost.Concept is the nice tool for that. Finally you can at least use constrains in templates implementation (***)
Heavy usage of templates hurts compilation speed
This one is also true. Compilation slows down due to following reasons:
- "real code" is generated from templates, and that takes time. Not much can be done with this that "issue". (Alternatively you can write all code by hand, but that would take even more time);
- templates usually implemented in header files (see the first point), and thus increase preprocessing time and introduce new dependencies that every template user depends on. Sometimes that could be mitigated with explicit instantiation requests, other times with accurate dependency management (not include what can be just forward declared etc.). Sometimes you can just live with it, and other times you can consider using other abstraction tool and not templates.
All in all, in my opinion in modern C++ templates and static polymorphism should be considered the basic design tool -- especially for libraries, and object-oriented techniques should be only considered after them, and not something you start with.
_____________________________________________________________________
(*) Unless developer wants to make it inline, which s/he usually doesn't -- if efficiency of this function was
so important s/he wouldn't use virtual functions here.
(**) Subject to some restrictions: for instance, virtual functions always instantiated, [provide second example]. For more details, read
C++ Templates -- The Complete Guide book.
(***) Constrains solve another (but closely related) problem with early diagnostics of violations on type requirements. Unfortunately they are poorly suitable for documenting interface as they are specified at implementation, not in interface.
