How to specialize a template sub-class?

Question

I'm trying to specialize a template class inside another class but the compiler won't let me. The code works outside of class Foo but not inside and I want struct Bla to be private to class Foo.

class Foo {
   template<typename ... Ts> struct Bla; 
   template<> struct Bla<> { static constexpr int x = 1; };
};

error: explicit specialization in non-namespace scope 'class Foo'

score 4 · Answer 1 · answered Apr 09 '15 at 23:29

4

You simply cannot do that. The error sums it up nicely. Class templates can only be specialized in namespace scope. class Foo is not a namespace.

You can do it external to the class, as per this example from the standard [temp.class.spec]:

A class template partial specialization may be declared or redeclared in any namespace scope in which its definition may be defined (14.5.1 and 14.5.2). [ Example:
 template<class T> struct A {  
     struct C {
         template<class T2> struct B { };
     };
 };

 // partial specialization of A<T>::C::B<T2>
 template<class T> template<class T2>
   struct A<T>::C::B<T2*> { };

A<short>::C::B<int*> absip; // uses partial specialization
—end example ]

answered Apr 09 '15 at 23:29

Barry

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upvoted for the standard quotes, you are really fast :) – vsoftco Apr 09 '15 at 23:35
@vsoftco Just start at chapter Templates and keep searching for "scope" :) – Barry Apr 09 '15 at 23:37
1

yeah I know how it works, but still, I was too lazy :) – vsoftco Apr 09 '15 at 23:37

vsoftco · Accepted Answer · 2015-04-09T23:47:26.903

3

You cannot specialize inside the class, use:

class Foo {
public: // so we can test it easily
   template<typename ... Ts> struct Bla; 
};

// specialize it outside the class
template<> class Foo::Bla<> { static constexpr int x = 1; }; 

int main()
{
    std::cout << Foo::Bla<>::x;
}

edited Apr 09 '15 at 23:47

answered Apr 09 '15 at 23:30

vsoftco

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Thanks, that's exactly the solution. – Goswin von Brederlow Apr 09 '15 at 23:36
@GoswinvonBrederlow note that I used `public` here, so I can display `Bla::x`. However I guess you are using a `friend` or displaying via a member function, in which case you should be ok. – vsoftco Apr 09 '15 at 23:39
This makes me think though. Doesn't that allow violating the privacy of a class? `class Foo { static constexpr int y = 1; public: template struct Bla; }; template<> struct Foo::Bla<> { static constexpr int x = y; };` Foo::y is private but by specializing Bla any user of Foo can gain access to it through Bla::x. – Goswin von Brederlow Apr 09 '15 at 23:40
@GoswinvonBrederlow my answer is too long to post here. check this out http://stackoverflow.com/q/424104/3093378 and search for "backdoor" – vsoftco Apr 09 '15 at 23:41
@GoswinvonBrederlow and that's probably exactly what you meant: http://ideone.com/dBV0eU – vsoftco Apr 10 '15 at 00:00

How to specialize a template sub-class?

2 Answers2