Deth <alexrepair@xxxxxxxxx> writes:
> Thanks Ian, but my code is definitely correct - lookup "template template"
> parameters if you're not familiar with the syntax in my post.
You're right, I misunderstood what you wanted to do, but I'm not
convinced your code is correct according to the standard. Your
original example said
template <typename>
class B : public A {
so it didn't provide any type for A's template parameter. Shouldn't
it be
template <typename>
class B : public A<B> {
? Otherwise, how does the compiler know what to use for A's template
parameter?
The problem then boils down to: how do you write the constructor
B(int i)
: A<...>(i) {}
The problem is that when you write
B(int i)
: A<B>(i) {}
B refers to the instantiated class B<>, not the template B. So you
need to qualify it. Is this code what you are after?
template <template <typename> class T>
class A {
public:
A(int) {}
};
template <typename>
class B : public A<B> {
public:
B(int i)
: A< ::B>(i) {}
};
(note the space required between "<" and "::" to avoid a digraph).
> This code without inheritance illustrates the problem a little clearer:
>
> //template template syntax is required to allow A::makeAnIntC() to
> //return a new template instantiation of C
> template <template <typename> class C>
> class A {
> C<int> makeAnIntC() {return C<int>(5);}
> };
>
> template <typename T>
> class B {
> public:
> B<int> makeAnIntB() {return A::makeAnIntC();} //Compile error here
> };
You're using A with no template parameter, and that is precisely what
the compiler complains about.
> Note that the following will compile and work:
> class Foo {
> B<int> makeAnIntB() {return A::makeAnIntC();} //Not self-referential
> };
I see an error with this version as well.
foo.cc:15: error: ‘template<template<class> class C> class A’ used without template parameters
Ian
