How to you access a nested template class?

A

Adam Nielsen

Hi all,

I'm a bit confused about the syntax used to access a nested template
class. Essentially I have a bunch of class types to represent different
types of records in a database, and I want to store some cache data for
each datatype. The simplified code below demonstrates my issue - I
think I understand why it doesn't work as is (the structure only
declares the storage, I need to instantiate it for each template type)
but I'm not sure how to actually go about doing that.

Any pointers would be appreciated!

Thanks,
Adam.


class RecordTypeA { };
class RecordTypeB { };

class Main {

private:

template <typename T>
struct CacheData {
int i;
};

public:

template <typename T>
void set(T t, int x)
{
// Store some type-specific data for record type T
this->CacheData<T>::i = x;
}

};

int main(void)
{
RecordTypeA A;
RecordTypeB B;
Main m;
m.set(A, 1);
m.set(B, 2);

// At this point, I want this to be the case:
// m.CacheData<A>::i == 1
// m.CacheData<B>::i == 2

return 0;
}
 
K

Kai-Uwe Bux

Adam said:
Hi all,

I'm a bit confused about the syntax used to access a nested template
class. Essentially I have a bunch of class types to represent different
types of records in a database, and I want to store some cache data for
each datatype. The simplified code below demonstrates my issue - I
think I understand why it doesn't work as is (the structure only
declares the storage, I need to instantiate it for each template type)
but I'm not sure how to actually go about doing that.

Any pointers would be appreciated!

Thanks,
Adam.


class RecordTypeA { };
class RecordTypeB { };

class Main {

private:

template <typename T>
struct CacheData {
int i;
};
Click to expand...

This declares a type.
public:

template <typename T>
void set(T t, int x)
{
// Store some type-specific data for record type T
this->CacheData<T>::i = x;
Click to expand...

There is no member variable this->CacheData said:
}

};

int main(void)
{
RecordTypeA A;
RecordTypeB B;
Main m;
m.set(A, 1);
Click to expand...

You cannot call set that way. Templates do not make types into valid
arguments for functions.
m.set(B, 2);

// At this point, I want this to be the case:
// m.CacheData<A>::i == 1
// m.CacheData<B>::i == 2

return 0;
}
Click to expand...


You may want to ponder about the following:


#include <cassert>
#include <map>

template < typename T >
class typemap {

typedef void (*id) ( void );

template < typename A >
static
void type_identifier ( void ) {}

std::map< id, T > data;

public:

template < typename A >
T const & value ( void ) const {
return ( data[ &type_identifier<A> ] );
}

template < typename A >
T & value ( void ) {
return ( data[ &type_identifier<A> ] );
}

};

int main ( void ) {
typemap<int> table;
table.value<char>() = 2;
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}



Best

Kai-Uwe Bux
 
R

rajgkk

Joined
Oct 4, 2007
Messages
4
Reaction score
0
Clearly templates are not the solution to your problem.
Come out of generic programming mindset here. Use a map for each type.
I think you are running into a serious design error.
 
A

Alf P. Steinbach

* Kai-Uwe Bux:
* Adam Nielsen:

This declares a type.


There is no member variable this->CacheData<T> since CacheData<T> is a type.
Click to expand...

You mean, since class 'Main' does not derive from that type.
You cannot call set that way. Templates do not make types into valid
arguments for functions.
Click to expand...

Uh, there are no types as arguments here.

But you're right that it seems the OP is confusing types with data members.


Cheers,

- Alf
 
A

Adam Nielsen

You may want to ponder about the following:
#include <cassert>
#include <map>

template < typename T >
class typemap {

typedef void (*id) ( void );

template < typename A >
static
void type_identifier ( void ) {}

std::map< id, T > data;

public:

template < typename A >
T const & value ( void ) const {
return ( data[ &type_identifier<A> ] );
}

template < typename A >
T & value ( void ) {
return ( data[ &type_identifier<A> ] );
}

};

int main ( void ) {
typemap<int> table;
table.value<char>() = 2;
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}
Click to expand...

That makes sense, thanks for the example! Out of curiosity, is there
any way of doing this without using a runtime lookup? Something like a
'template-variable', if there were such a thing.

Cheers,
Adam.
 
K

Kai-Uwe Bux

Adam said:
You may want to ponder about the following:


#include <cassert>
#include <map>

template < typename T >
class typemap {

typedef void (*id) ( void );

template < typename A >
static
void type_identifier ( void ) {}

std::map< id, T > data;

public:

template < typename A >
T const & value ( void ) const {
return ( data[ &type_identifier<A> ] );
}

template < typename A >
T & value ( void ) {
return ( data[ &type_identifier<A> ] );
}

};

int main ( void ) {
typemap<int> table;
table.value<char>() = 2;
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}
Click to expand...

That makes sense, thanks for the example! Out of curiosity, is there
any way of doing this without using a runtime lookup? Something like a
'template-variable', if there were such a thing.
Click to expand...

You can do this for static data (e.g., you can have such data on a per class
basis):

#include <cassert>

template < typename T >
struct static_typemap {

template < typename A >
static
T & value ( void ) {
static T data;
return ( data );
}

};


int main ( void ) {

static_typemap<int>::value<char>() = 1;
static_typemap<int>::value<int>() = 2;

assert( static_typemap<int>::value<char>() == 1 );
assert( static_typemap<int>::value<int>() == 2 );

}


_If_ we had templated virtual member functions, we could do such tricks on a
per object basis (and all sorts of other cool stuff).


Best

Kai-Uwe Bux
 
A

Adam Nielsen

You may want to ponder about the following:
#include <cassert>
#include <map>

template < typename T >
class typemap {

typedef void (*id) ( void );

template < typename A >
static
void type_identifier ( void ) {}

std::map< id, T > data;

public:

template < typename A >
T const & value ( void ) const {
return ( data[ &type_identifier<A> ] );
}

template < typename A >
T & value ( void ) {
return ( data[ &type_identifier<A> ] );
}

};

int main ( void ) {
typemap<int> table;
table.value<char>() = 2;
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}
Click to expand...

Hi Kai-Uwe,

I've just started implementing this design in my code, but I can't
figure out why it won't work inside a template.

For example, if I delete your main() function above and replace the code
with this:

template <typename X>
int m ( void ) {
typemap<int> table;
table.value<char>() = 2; // line 59 in the error below
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}

int main ( void ) {
m<int>();
}

When I compile it I end up with this:

t4.cpp: In function `int m()':
t4.cpp:59: error: parse error before `>' token
t4.cpp:60: error: parse error before `>' token
t4.cpp:61: error: parse error before `>' token
t4.cpp:62: error: parse error before `>' token

I don't understand why this happens, as I didn't think it mattered
*where* you used the code. Is there something special you must do in
this situation?

Thanks again,
Adam.
 
K

Kai-Uwe Bux

Adam said:
You may want to ponder about the following:


#include <cassert>
#include <map>

template < typename T >
class typemap {

typedef void (*id) ( void );

template < typename A >
static
void type_identifier ( void ) {}

std::map< id, T > data;

public:

template < typename A >
T const & value ( void ) const {
return ( data[ &type_identifier<A> ] );
}

template < typename A >
T & value ( void ) {
return ( data[ &type_identifier<A> ] );
}

};

int main ( void ) {
typemap<int> table;
table.value<char>() = 2;
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}
Click to expand...

Hi Kai-Uwe,

I've just started implementing this design in my code, but I can't
figure out why it won't work inside a template.

For example, if I delete your main() function above and replace the code
with this:

template <typename X>
int m ( void ) {
typemap<int> table;
table.value<char>() = 2; // line 59 in the error below
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}

int main ( void ) {
m<int>();
}

When I compile it I end up with this:

t4.cpp: In function `int m()':
t4.cpp:59: error: parse error before `>' token
t4.cpp:60: error: parse error before `>' token
t4.cpp:61: error: parse error before `>' token
t4.cpp:62: error: parse error before `>' token

I don't understand why this happens, as I didn't think it mattered
*where* you used the code. Is there something special you must do in
this situation?
Click to expand...

I cannot reproduce the error. The code you posted compiles for me. This
maybe due to you giving instructions on how to create the code from the
present pieces instead of posting a complete piece.


Best

Kai-Uwe Bux
 
A

Adam Nielsen

I cannot reproduce the error. The code you posted compiles for me. This
maybe due to you giving instructions on how to create the code from the
present pieces instead of posting a complete piece.
Click to expand...

That's strange then - well here is the exact code I'm compiling.
Perhaps if someone else using GCC can test it as well to see whether
it's a compiler issue?

#include <cassert>
#include <map>

template < typename T >
class typemap {

typedef void (*id) ( void );

template < typename A >
static
void type_identifier ( void ) {}

std::map< id, T > data;

public:

template < typename A >
T const & value ( void ) const {
return ( data[ &type_identifier<A> ] );
}

template < typename A >
T & value ( void ) {
return ( data[ &type_identifier<A> ] );
}

};

template <typename X>
int m ( void ) {
typemap<int> table;
table.value<char>() = 2;
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}

int main ( void ) {
m<int>();
}

$ g++ -o t t.cpp
t.cpp: In function `int m()':
t.cpp:32: syntax error before `>' token
t.cpp:33: syntax error before `>' token
t.cpp:34: syntax error before `>' token
t.cpp:35: syntax error before `>' token
$ g++ --version
g++-gcc-3.2.3 (GCC) 3.2.3

I've also tried it with GCC 3.3.4 with the same result.

Cheers,
Adam.
 
K

Kai-Uwe Bux

Adam said:
I cannot reproduce the error. The code you posted compiles for me. This
maybe due to you giving instructions on how to create the code from the
present pieces instead of posting a complete piece.
Click to expand...

That's strange then - well here is the exact code I'm compiling.
Perhaps if someone else using GCC can test it as well to see whether
it's a compiler issue? [code snipped]
I've also tried it with GCC 3.3.4 with the same result.
Click to expand...

Thanks for the complete code. It compiles for me with g++ (gcc 3.4.6 and
4.1.1).


Best

Kai-Uwe Bux
 
J

James Kanze

That's strange then - well here is the exact code I'm
compiling. Perhaps if someone else using GCC can test it as
well to see whether it's a compiler issue?
Click to expand...
#include <cassert>
#include <map>
Click to expand...
template < typename T >
class typemap {
Click to expand...
typedef void (*id) ( void );
template < typename A >
static
void type_identifier ( void ) {}
Click to expand...
std::map< id, T > data;
Click to expand...
public:
template < typename A >
T const & value ( void ) const {
return ( data[ &type_identifier<A> ] );
}
Click to expand...
template < typename A >
T & value ( void ) {
return ( data[ &type_identifier<A> ] );
}
};
Click to expand...
template <typename X>
int m ( void ) {
typemap<int> table;
table.value<char>() = 2;
table.value<int>() = 1;
assert( table.value<char>() == 2 );
assert( table.value<int>() == 1 );
}
Click to expand...
int main ( void ) {
m<int>();
}
Click to expand...
$ g++ -o t t.cpp
t.cpp: In function `int m()':
t.cpp:32: syntax error before `>' token
t.cpp:33: syntax error before `>' token
t.cpp:34: syntax error before `>' token
t.cpp:35: syntax error before `>' token
$ g++ --version
g++-gcc-3.2.3 (GCC) 3.2.3
Click to expand...
I've also tried it with GCC 3.3.4 with the same result.
Click to expand...

I get the same errors with 3.2.3, but it compiles with g++
4.1.0. If you replace typemap<int> with typemap<X> in the first
line of m, you get errors with both compilers, of course.
Changing each of the function calls to:

table.template value<...>...

works in all cases however.

It's an interesting case, since it's the only case I know of off
hand in which a (non-instantiated) template definition can
trigger the instantiation of another template. Apparently, g++
pre-4.0 is not instantiating typemap<int> here, and so is
treating the < as if it were the less than operator. (Note that
in the case of typemap<X>, the context is dependent, so you are
required to tell the compiler when the name in question -- here,
value -- is a type or a template.)
 
A

Adam Nielsen

Changing each of the function calls to:
table.template value<...>...

works in all cases however.
Click to expand...

Ah, that's excellent - that's saved me a compiler upgrade :)

Cheers,
Adam.
 

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