Operator<<, in nested templated classes.

Hi!

I'm having an interesting problem that I can't seem to get to work, and
I'm curious if someone could either a) tell me how to make it work, or
b) tell me why it *can't* work. :)

Anyhow, here's a small test case that should compile on most compilers:

#include <iostream>

using namespace std;

template<typename T>
class TemplatedOuter
{
public:
class NestedInner
{
public:
int a;

NestedInner() : a(0) {};
};


TemplatedOuter() : m_NestedInner() {};

NestedInner m_NestedInner;
};

/**
* This does not.
*/
template<typename T>
ostream operator<<(ostream &os, typename TemplatedOuter<T>::NestedInner
const &nestedInner)
{
}

/**
* This works...
*/
template<typename T>
ostream operator<<(ostream &os, TemplatedOuter<T> const
&templatedOuter)
{
}

int main(void)
{
TemplatedOuter<int> a;

// This works.
cout << a;

// This does not.
cout << a.m_NestedInner;
}


/**

Output:

main.cpp: In function `int main()':
main.cpp:39: error: no match for 'operator<<' in 'std::cout <<
a.TemplatedOuter<int>::m_NestedInner'

*/

As you can see, I'm trying to basically have a nested public class, and
print out the contents of it... But it can't seem to find it... Any
suggestions on what I'm doing wrong, would be appreciated!

Thanks!

re: Operator<<, in nested templated classes.

Eddie Parker wrote:[color=blue]
> I'm having an interesting problem that I can't seem to get to work, and
> I'm curious if someone could either a) tell me how to make it work, or
> b) tell me why it *can't* work. :)[/color]

You're running into "non-deducible context". It's not possible to deduce
the template argument from a template nested class. What's happening is
this: you give 'a.m_NestedInner' from which 'T' is supposed to be deduced.
The list of deducible contexts does *not* include template-name<T>::type.
[color=blue]
> Anyhow, here's a small test case that should compile on most compilers:
>
> #include <iostream>
>
> using namespace std;
>
> template<typename T>
> class TemplatedOuter
> {
> public:
> class NestedInner
> {
> public:
> int a;
>
> NestedInner() : a(0) {};[/color]

Drop the traling semicolon from the line above.
[color=blue]
> };
>
>
> TemplatedOuter() : m_NestedInner() {};[/color]

Drop the traling semicolon from the line above.
[color=blue]
>
> NestedInner m_NestedInner;
> };
>
> /**
> * This does not.
> */
> template<typename T>
> ostream operator<<(ostream &os, typename TemplatedOuter<T>::NestedInner
> const &nestedInner)[/color]

template<typename T>
ostream& ...
[color=blue]
> {[/color]

return os;
[color=blue]
> }
>
> /**
> * This works...
> */
> template<typename T>
> ostream operator<<(ostream &os, TemplatedOuter<T> const[/color]

template<typename T>
ostream& ...
[color=blue]
> &templatedOuter)
> {[/color]

return os;
[color=blue]
> }
>
> int main(void)
> {
> TemplatedOuter<int> a;
>
> // This works.
> cout << a;
>
> // This does not.
> cout << a.m_NestedInner;
> }
>
>
> /**
>
> Output:
>
> main.cpp: In function `int main()':
> main.cpp:39: error: no match for 'operator<<' in 'std::cout <<
> a.TemplatedOuter<int>::m_NestedInner'
>
> */
>
> As you can see, I'm trying to basically have a nested public class, and
> print out the contents of it... But it can't seem to find it... Any
> suggestions on what I'm doing wrong, would be appreciated![/color]

The only work-around I know is to specify the template argument explicitly
in a call:

::operator << <int> (cout, a.m_NestedInner);

Another work-around, of course, is to add a member function 'print' to the
'NestedInner' class and use it as

a.m_NestedInner.print(cout);

V