Question

I need some way to achieve a “reverse template alias”. So I’d use a template typedef to select the correct class to use at compile time. I’d like to do the following:

typedef ClassA Temp<int>;
typedef ClassB Temp<char>;

ClassA and ClassB are not template classes, but I’d like to select the right class through the use of a template. So if a Temp< int > was needed it would actually use ClassA. Is anything like this even possible in C++? I tried the following but it didn’t work.

template<>
typedef ClassA Temp<int>;

template<>
typedef ClassB Temp<char>;

I got the following error in GCC

error: template declaration of ‘typedef’

Answer 1

No, typedef can’t define type templates, only types. The two closest things you can do are:

template <typename T>
struct Temp;

template <>
struct Temp<int> : ClassA {}

template <>
struct Temp<char> : ClassB {}

so you write just Temp<int>, but it’s a derived class, not the class itself, or

template <typename T>
struct Temp;

template <>
struct Temp<int> { typedef ClassA Type; }

template <>
struct Temp<char> { typedef ClassB Type; }

so you can get the ClassA and ClassB themselves, but you have to write Temp<int>::Type.