Can template arguments have nested template classes?

[Alex]

Is there something wrong with requiring that a template argument has a nested class template? Why can I not do the following?

template<typename A, typename T> struct B
{
typedef typename A::AA<T> my_type;
};

VS2013 gives a compile error, even if B is not used at all.

What could I do instead?

Thanks, Alex



p.s. what I really want to do is the following:

template<typename A>
struct B
{
template<typename T>
struct BB : A::AA<T>
{};
};

 

[Marcel Müller]

Is there something wrong with requiring that a template argument has a nested class template? Why can I not do the following?

Maybe the same problem than with forward declarations of nested classes...

template<typename A, typename T> struct B
{
typedef typename A::AA<T> my_type;
};

VS2013 gives a compile error, even if B is not used at all.

Works as expected here, even back to gcc 3.2.2.

What could I do instead?

Pass the nested type as template argument to B?


Marcel

 

[Victor Bazarov]

Is there something wrong with requiring that a template argument has
anested class template? Why can I not do the following?

template<typename A, typename T> struct B
{
typedef typename A::AA<T> my_type;

Perhaps it ought to be

typedef typename A::template AA<T> my_type;

? A wild guess...

};

VS2013 gives a compile error, even if B is not used at all.

WHAT compiler error do you get?

What could I do instead?

Thanks, Alex



p.s. what I really want to do is the following:

template<typename A>
struct B
{
template<typename T>
struct BB : A::AA<T>
{};
};

Is there a guarantee that a particular argument you provide to
instantiate 'B' actually does have a nested template? And how is
compiler to know that A::AA is a template (after which the compiler
should expect a template argument clause with angle brackets and not an
expression that can contain a less-than operator)?

V

 

[Alex]

Perhaps it ought to be



typedef typename A::template AA<T> my_type;



? A wild guess...

Perfect! Solves my problem! Thanks.

WHAT compiler error do you get?

error C2059: syntax error : '<'
error C2238: unexpected token(s) preceding ';'

Is there a guarantee that a particular argument you provide to

instantiate 'B' actually does have a nested template? And how is

compiler to know that A::AA is a template (after which the compiler

should expect a template argument clause with angle brackets and not an

expression that can contain a less-than operator)?

I thought using the typename keyword made that clear. And it did work on VS2012. Anyway, your solution is very logical and works.