Why C++ doesn't allow template overloading? [duplicate]

Question

Sometime I want to write two templates like:

template <typename Type1>
class A{
    ...
};

template <typename Type1, typename Type2>
class A{
    ...
};

But it seems that it is illegal to have two templates shared the same name but have different parameters. I have to name it like A_1, A_2. I think it might be useful if I can do this especially when implementing Functors.

Why C++ doesn't allow this? Does it difficult to implement or have ambiguity in some circumstance? Will this be supported on later version of C++?

Answer 1

It is extremely useful, but like you say, C++ doesn't allow you to do this directly. However, you can do almost the same thing with partial specialisation.

This is particularly easy if you use variadic templates in C++11, as you can do the following:

template <typename... T>
struct A; // Declared but not defined

template <typename T, typename U>
struct A<T, U>
{
    // definition of the two-parameter case
};

template <typename T>
struct A<T>
{
   // definition of the one-parameter case
};

Effectively, this allows you to have A<T, U> and A<T> as completely separate types. Attempting to instantiate an A with more template parameters will lead to a compile error as the general case is undefined (you could use a static_assert to give a nice error message if you wanted).

Something similar can be achieved in C++03 using default template parameters (set to an empty dummy struct, or void), but the C++11 version is much nicer IMO.