This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 114a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-04-18
[Picked up by evolution group at October 2002 meeting.]
How can we write a function template, or member function of a class template that takes a C linkage function as a parameter when the function type depends on one of the template parameter types?
extern "C" void f(int);
void g(char);
template <class T> struct A {
A(void (*fp)(T));
};
A<char> a1(g); // okay
A<int> a2(f); // error
Another variant of the same problem is:
extern "C" void f(int);
void g(char);
template <class T> void h( void (*fp)(T) );
int main() {
h(g); // okay
h(f); // error
}
Somehow permit a language linkage to be specified as part of a function parameter declaration. i.e.
template <class T> struct A {
A( extern "C" void (*fp)(T) );
};
template <class T> void h( extern "C" void (*fp)(T) );
Suggested resolution: (Bill Gibbons)
The whole area of linkage needs revisiting. Declaring calling convention as a storage class was incorrect to begin with; it should be a function qualifier, as in:
void f( void (*pf)(int) c_linkage );
instead of the suggested:
void f( extern "C" void (*pf)(int) );
I would like to keep calling convention on the "next round" issues list,
including the alternative of using function qualifiers.
And to that end, I suggest that the use of linkage specifiers to specify calling convention be deprecated - which would make any use of linkage specifiers in a parameter declaration deprecated.
Martin Sebor: 9.11 [dcl.link], paragraph 4 says that "A linkage-specification shall occur only in namespace scope..." I'm wondering why this restriction is necessary since it prevents, among other things, the use of the functions defined <cmath> in generic code that involves function objects. For example, the program below is ill-formed since std::pointer_to_binary_function<> takes a pointer to a function with extern "C++" linkage which is incompatible with the type of the double overload of std::pow.
Relaxing the restriction to allow linkage specification in declarations of typedefs in class scope would allow std::pointer_to_binary_function<> ctor to be overloaded on both types (i.e., extern "C" and extern "C++"). An alternative would be to allow for the linkage specification to be deduced along with the type.
#include <cmath>
#include <functional>
#include <numeric>
int main () {
double a[] = { 1, 2, 3 };
return std::accumulate (a, a + 3, 2.0,
std::pointer_to_binary_function<double, double, double>(std::pow));
}
Rationale (February, 2014):
EWG determined that no action should be taken on this issue.