Re: Syntax for specialization of nested class
Am 13.02.2012 18:46, schrieb asif.lse:
I have something like the following:
template<typename T>
struct Outer
{
template<typename U> class TraitsClass {} //the skeleton
You are missing a semicolon here after the definition of TraitsClass.
template<typename V> class Nested;
template<> class Nested
{
};
You cannot define the specialization of Nested in class scope. The last definition has to be moved to namespace scope like so:
template <>
template <typename T>
class Outer<T>::Nested
{
};
};
typedef Outer::Nested nested_t; //COMPILER IS COMPLAINING HERE - HOW
TO GET THIS RIGHT?
In such cases I always recommend to quote the compiler complaint, because it usually gives a good hint what's going wrong here.
I want to get the typedef right because later I want to derive sub-
classes from the nested class Outer::Nested and I am having a hard-
time getting that syntax right as well. Can you help me?
No kidding intended, but you either must have had a *very* long night or you need to get some more basic understanding of C++ templates ;-) [I hope you forgive my mild form of sarcasm]
So let me reply with another question:
What do you expect what the meaning of Outer is without any template arguments? Outer is a template and you need to provide some template argument for the type "placeholder" (in all but some very rare situations). In addition, Nested within Outer is another template, so what do you expect what the meaning of Outer::Nested is without providing another set of template arguments for the inner template?
Honestly, I have no idea (in this context), so we both must either agree, that you missed to provide some arguments here, e.g. Outer<int>::Nested<double>, or that you did not explain sufficiently precise what you *actually* want to realize.
For example you might want to realize something like a "template-typedef". If you have a so-called C++11-capable compiler (E.g. gcc4.7 or clang 3.x), you can use a alias template for this, like
template<typename T, typename U>
using nested_t = typename Outer<T>::template Nested<U>;
or - if you just have a good C++03 compiler you need to simulate this via a wrapper template, e.g. like so
template<typename T, typename U>
struct nested_t {
typedef typename Outer<T>::template Nested<U> type;
};
HTH & Greetings from Bremen,
Daniel Kr?gler
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