How to fix error refactoring decltype inside template

Question

edit Possibly can\'t be done, see Clean implementation of function template taking function pointer although answer 1 there has a C macro work-around https://stacko

Answer 1

I think for c++11 it is impossible to achieve what you want.

template
class unique_gptr : public std::unique_ptr {
    public: unique_gptr(T* t) : std::unique_ptr(t, D) { };
};

using int_gptr = unique_gptr;
int_gptr ig(new int(2));

Notice that decltype is applied on D, which is declared as a typename. So D is a type. But decltype can't be used on a type.

Firstly the code tries to get the address of a type (&). Secondly, the argument of decltype is expected to be an expression, but not a type or "the address of a type". To make it easier to understand, we can say that decltype expects its argument to be a “variable”, like the following example.

int main()
{
    int i = 10;
    decltype(i) j;
    decltype(int) k; /* Compiler error. */
    decltype(&int) l; /* Compiler error. */
    return 0;
}

You also want the compiler to replace D with ::free_int. And ::free_int is passed in as a non-type template argument. However D is a type template parameter. A non-type template parameter starts with an actual type (e.g. int, struct a* or a type template name). While a type template parameter starts with typename or class. An easier example for non-type template parameter,

template
void func2(void)
{
    decltype(INIT) j = INIT;
}

int main()
{
    func2<10>();
    return 0;
}

When you pass in a function pointer like ::free_int, you need a non-type template parameter, which must be preceded by a type. And you want the type of the function pointer to be "replaceable". So the type of the function pointer has to be a type template parameter. These make them two template parameters.

That's the reason you need three template parameters in template, which is the best result you have.