Name for a common idiom

[Gareth Stockwell]

I've been using an technique whereby a tag used as a parameter to a
template class C determines which of several potential base classes
(A,B) C inherits from. What I want to know is whether there is an
accepted name for this idiom.

To illustrate the point, here is some code which demonstrates it in
action...

// Selector classes - these are typically just empty structs
struct A_Tag { };
struct B_Tag { };

// Potential base classes
class A { /* ... */ };
class B { /* ... */ };

// Definition of the class which selects the appropriate base for C
template<class Tag> struct Selector { };

// Specialisations of the selector map tags onto base classes
template<> struct Selector<A_Tag> { typedef A base_t; };
template<> struct Selector<B_Tag> { typedef B base_t; };

// OK, here is the class which we actually instantiate
template<class Tag>
class C : public Selector<Tag>::base_t
{ /* ... */ };

// And now the instantiations...
C<A_Tag> c_inheriting_from_A;
C<B_Tag> c_inheriting_from_B;


Gareth

 

[Gianni Mariani]

I've been using an technique whereby a tag used as a parameter to a
template class C determines which of several potential base classes
(A,B) C inherits from. What I want to know is whether there is an
accepted name for this idiom.

I don't think so, but then I would probably not know if there was one.

....

// OK, here is the class which we actually instantiate
template<class Tag>
class C : public Selector<Tag>::base_t

This is a syntax error. Should be:

class C : public typename Selector<Tag>::base_t

 

[Peter Koch Larsen]

I've been using an technique whereby a tag used as a parameter to a
template class C determines which of several potential base classes
(A,B) C inherits from. What I want to know is whether there is an
accepted name for this idiom.

To illustrate the point, here is some code which demonstrates it in
action...

// Selector classes - these are typically just empty structs
struct A_Tag { };
struct B_Tag { };

// Potential base classes
class A { /* ... */ };
class B { /* ... */ };

// Definition of the class which selects the appropriate base for C
template<class Tag> struct Selector { };

// Specialisations of the selector map tags onto base classes
template<> struct Selector<A_Tag> { typedef A base_t; };
template<> struct Selector<B_Tag> { typedef B base_t; };

// OK, here is the class which we actually instantiate
template<class Tag>
class C : public Selector<Tag>::base_t
{ /* ... */ };

// And now the instantiations...
C<A_Tag> c_inheriting_from_A;
C<B_Tag> c_inheriting_from_B;


Gareth

Why not simply:

template<class base>
class C : public base
{
....
}

?

/Peter

 

[Michael Kurz]

Why not simply:

template<class base>
class C : public base
{
...
}

I thought exactly this one indirection level is the idiom.

And if this is really the case, and the *tag classes do never carry
implementation,
we could discuss if the "switching" template variable should be really a
class or if a numeric type (together with some enum) would be a cleaner, but
perhapes not so flexible solution.


Regards
Michael

 

[Rob Williscroft]

Gianni Mariani wrote in in
comp.lang.c++:

I don't think so, but then I would probably not know if there was one.

...

This is a syntax error. Should be:

class C : public typename Selector<Tag>::base_t

I think this is the one place where typename is specificaly *not*
alowed, I've no idea why it is required after typedef though .

mscv 7.1
error C2899: typename cannot be used outside a template declaration

g++ 3.2 and 3.3
test.cpp:12: no bases given following `:'
test.cpp:12: parse error before `typename'

bccx (EDG):
test.cpp:12: error expected an identifier

g++ 3.4 & 4.0 (prerelease)
test.cpp:12: error: keyword `typename' not allowed outside of templates

Rob.

 

[Gareth Stockwell]

This is a syntax error. Should be:

class C : public typename Selector<Tag>::base_t

I think the typename keyword is only required inside the body of the
class, not in its definition. I don't have the standard handy, so
I'll be pragmatic ... the above compiles fine with GCC (all versions
3.3.2 up to 3.4.2), and Intel C++ 8.0

Gareth

 

[Gianni Mariani]

I think the typename keyword is only required inside the body of the
class, not in its definition. I don't have the standard handy, so
I'll be pragmatic ... the above compiles fine with GCC (all versions
3.3.2 up to 3.4.2), and Intel C++ 8.0

That makes sense. My bad.

G