Re: Isn't this a conversion bug in gcc?
On 30 Jul., 02:22, Nikolay Ivchenkov <ts...@mail.ru> wrote:
On 29 Jul, 17:48, Daniel Kr?gler <daniel.krueg...@googlemail.com>
wrote:
ad c) I agree that the Standard in general includes the instantiation
of definitions *when* they are required to exist and *when* they
are available.
#include <iostream>
int n = 0;
template <class T>
struct B
{
B()
{
if (n)
f();
// pure function call
// (see the suggested resolution for core issue 230)
}
virtual void f() = 0;
};
template <class T>
void B<T>::f()
{ std::cout << "B<T>::f()\n"; }
struct D : B<int>
{
D()
{ static_cast<B<int> &>(*this).f(); }
void f()
{ std::cout << "D::f()\n"; }
};
int main()
{
std::cin >> n;
D();
}
A definition of B<int>::f is not required to be exist. Thus, we can't
apply 14.7.1/2
"Unless a function template specialization has been explicitly
instantiated or explicitly specialized, the function template
specialization is implicitly instantiated when the specialization is
referenced in a context that requires a function definition to exist."
Must the definition of B<int>::f be instantiated?
Yes.
Even though B<int>::f is not required to exist in any general program
it is required to exist in this particular program, because it is
still used
in the sense of the *general* requirements specified in 3.2/2:
"An expression is potentially evaluated unless it is an unevaluated
operand (Clause 5) or a subexpression thereof. A variable or non-
overloaded function whose name appears as a potentially-evaluated
expression is used unless it is an object that satisfies the
requirements for appearing in a constant expression (5.19)
and the lvalue-to-rvalue conversion (4.1) is immediately applied.[..]"
The example invokes the constructor of B as part of a potentially
evaluated expression. Note that the refining sentence:
"A virtual member function is used if it is not pure."
does not declare a pure virtual function as *unused*, it only declares
*any* non-pure virtual function as *used*. This is a big difference,
because it does not make the former requirements invalid.
HTH & Greetings from Bremen,
Daniel Kr?gler
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