Re: Derived class reference supplied to class reference template argument
Gabor Greif ha scritto:
I have trouble compiling following code
with various G++ versions (and comeau online too)
-----------------------------------------------------
struct A
{
static int foo(void)
{
return 42;
}
};
struct B : A
{
} b;
template <A& a>
struct C : A
{
static int foo(void)
{
return a.foo();
}
};
int main (void)
{
A& a(b); // OK
return C<b>::foo(); // Fails
}
-----------------------------------------------------
As far as I can see the standard does not
forbid passing b as a template parameter,
since
- b has external linkage
- b isa B -> b isa B& -> b isa A& (because of inheritance).
Can anybody explaing why this is illegal?
Because 14.3.2/5 says that "For a non-type template-parameter of type
reference to object, no conversions apply." So, despite intuitiveness,
the conversion from B& to A& is not applied. Notice that there's
actually no way to bind the template parameter to the base sub-object,
because explicit casts are forbidden according to 14.3.2/1 (except those
in integral constant expressions).
HTH,
Ganesh
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