Re: Referring to a base class with template template parameters

[Ian Lance Taylor <iant at google dot com>]

Deth <alexrepair@gmail.com> 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: âtemplate<template<class> class C> class Aâ used without template parameters


Ian