Ensure template parameter derived from a particular type?

[kk_oop]

Hi. I have a template class Base_t<typename T1, typename T2>. I
intend to instantiate it like this:

class Derived : public Base_t<SomeClass1, SomeClass2>
{
....
};

I'd like to have the compiler ensure that any actual parameter used for
T1 is derived from Class A. So in the example above, I'd like a
compiler error to be generated if SomeClass1 is not derived from Class
A.

Is there a way to do this?

Thanks!

Ken

 

[Alan Johnson]

Hi. I have a template class Base_t<typename T1, typename T2>. I
intend to instantiate it like this:

class Derived : public Base_t<SomeClass1, SomeClass2>
{
...
};

I'd like to have the compiler ensure that any actual parameter used for
T1 is derived from Class A. So in the example above, I'd like a
compiler error to be generated if SomeClass1 is not derived from Class
A.

Is there a way to do this?

Thanks!

Ken

You could use Loki's SuperSubclassStrict template metafunction.

 

[Frederick Gotham]

posted:

I'd like to have the compiler ensure that any actual parameter used for
T1 is derived from Class A. So in the example above, I'd like a
compiler error to be generated if SomeClass1 is not derived from Class
A.

Is there a way to do this?

Here's something that just came to mind, but I haven't sat down and
pondered over the posibilities. It isn't perfect, but it's a step in the
right direction:

#include <cstddef>

class Base {};

class Derived : public Base {};

class NotDerived {};

template<class T>
class TemplateClass {
private:

std::size_t const static assert_base =
sizeof static_cast<const Base&>( T() );

};

int main()
{
TemplateClass<Derived> obj1;

TemplateClass<NotDerived> obj2; /* Compile ERROR */
}

 

[jmoy]

Hi. I have a template class Base_t<typename T1, typename T2>. I
intend to instantiate it like this:
...
I'd like to have the compiler ensure that any actual parameter used for
T1 is derived from Class A.

Look at the discussion of constraints in Stroustrup's C++ FAQ
(http://www.research.att.com/~bs/bs_faq2.html#constraints),
particularly the Derived_from template.