Templated derived class in CRTP (Curiously Recurring Template Pattern)
- by Butterwaffle
Hi, I have a use of the CRTP that doesn't compile with g++ 4.2.1, perhaps because the derived class is itself a template? Does anyone know why this doesn't work or, better yet, how to make it work? Sample code and the compiler error are below.
Source: foo.C
#include <iostream>
using namespace std;
template<typename X, typename D> struct foo;
template<typename X> struct bar : foo<X,bar<X> >
{
X evaluate() { return static_cast<X>( 5.3 ); }
};
template<typename X> struct baz : foo<X,baz<X> >
{
X evaluate() { return static_cast<X>( "elk" ); }
};
template<typename X, typename D> struct foo : D
{
X operator() () { return static_cast<D*>(this)->evaluate(); }
};
template<typename X, typename D>
void print_foo( foo<X,D> xyzzx )
{
cout << "Foo is " << xyzzx() << "\n";
}
int main()
{
bar<double> br;
baz<const char*> bz;
print_foo( br );
print_foo( bz );
return 0;
}
Compiler errors
foo.C: In instantiation of ‘foo<double, bar<double> >’:
foo.C:8: instantiated from ‘bar<double>’
foo.C:30: instantiated from here
foo.C:18: error: invalid use of incomplete type ‘struct bar<double>’
foo.C:8: error: declaration of ‘struct bar<double>’
foo.C: In instantiation of ‘foo<const char*, baz<const char*> >’:
foo.C:13: instantiated from ‘baz<const char*>’
foo.C:31: instantiated from here
foo.C:18: error: invalid use of incomplete type ‘struct baz<const char*>’
foo.C:13: error: declaration of ‘struct baz<const char*>’
