Why is it not possible to overload class templates?

Question

Reading this question made me wonder: is there a technical reason for disallowing class templates overloads?

By overloading, I mean having several templates with the

Answer 1

You cannot "overload" type parameter, non-type argument and template template parameter, but you can specialize variadic template:

template <typename... T>
struct Foo;

template <typename T1>
struct Foo<T1> {};

template <typename T1, typename T2>
struct Foo<T1,T2> {};

Answer 2

Section 12.5 from Templates the Complete Guide (Amazon) contains this quote:

You may legitimately wonder why only class templates can be partially specialized. The reasons are mostly historical. It is probably possible to define the same mechanism for function templates (see Chapter 13).

In some ways the effect of overloading function templates is similar, but there are also some subtle differences. These differences are mostly related to the fact that the primary template needs to be looked up when a use is encountered. The specializations are considered only afterward, to determine which implementation should be used.

In contrast, all overloaded function templates must be brought into an overload set by looking them up, and they may come from different namespaces or classes. This increases the likelihood of unintentionally overloading a template name somewhat.

Conversely, it is also imaginable to allow a form of overloading of class templates. Here is an example:

// invalid overloading of class templates
template<typename T1, typename T2> class Pair; 
template<int N1, int N2> class Pair; 

However, there doesn't seem to be a pressing need for such a mechanism.

Furthermore, the Design and Evolution of C++ (Amazon) contains this quote in section 15.10.3

I therefore concluded that we needed a mechanism for "specializing" templates. This could be done either by accepting general overloading or by some more specific mechanism. I chose a specific mechanism because I thought I was primarily addressing irregularities caused by irregularities in C and because suggestions of overloading invariably creates a howl of protests. I was trying to be cautious and conservative; I now consider that a mistake. Specialization as originally defined was a restricted and anomalous form of overloading that fitted poorly with the rest of the language.

Bold emphasis mine. I interpret this as saying that function overload resolution is more difficult to implement (and get right by users) than class specialization. So probably no real technical obstacles (similary for function template partial specialization) but an historical accident.

Answer 3

This has been around for a while now, but I still found this post when searching. Thanks to @log0 for providing me with a good start. Here is a solution that avoids needing to provide a template specialisation for all possible enumerations. It does make one assumption: that you can define each template expansion in terms of itself and its base classes. (This would be done in FooImpl below):

template <typename... T>
struct Foo;

template<typename T>
struct Foo<T> { /* implementation of base class goes here*/};

template <typename C, typename Base>
struct FooImpl : public Base { /* implementation of derived class goes here */};

template<typename C, typename... Bases>
struct Foo<C, Bases...> : FooImpl<C, Foo<Bases...> > { /*NO IMPLEMENTATION HERE */};

The use of FooImpl breaks the ambiguous recursion that otherwise results. This then allows declarations such as the following:

Foo<int> foo_int;
Foo<int, double> foo_int_double;
Foo<int, float, double> foo_int_float_double;

Perhaps this is how the STL now does it?