Operator<<, in nested templated classes.

[Eddie Parker]

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!

[Victor Bazarov]

Eddie Parker wrote:

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. :)
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.
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) {};
Drop the traling semicolon from the line above.
};
TemplatedOuter() : m_NestedInner() {};
Drop the traling semicolon from the line above.

NestedInner m_NestedInner;
};

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

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

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!

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