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

You can use some macro magic:

template
struct UniqueDeleter : public std::unique_ptr {
    UniqueDeleter(T* t) : std::unique_ptr(t, d) { };
};

template
T decl_unique_deleter_ptr(R (*d)(T*));

#define DECL_UNIQUE_DELETER1(f) UniqueDeleter
#define DECL_UNIQUE_DELETER2(T, f) UniqueDeleter
#define DECL_UNIQUE_DELETER_GET_MACRO(_1, _2, NAME, ...) NAME
#define DECL_UNIQUE_DELETER_EXPAND(x) x
#define decl_unique_deleter(...) DECL_UNIQUE_DELETER_EXPAND(DECL_UNIQUE_DELETER_GET_MACRO( \
    __VA_ARGS__, DECL_UNIQUE_DELETER2, DECL_UNIQUE_DELETER1, _)(__VA_ARGS__))

Now you can use it like such:

decl_unique_deleter(int, free_int) ig(new int(2));

Or with using and some more magic:

using int_gptr = decl_unique_deleter(free_int); // If accepted pointer type matches.
int_gptr ig(new int(2));

---

I might also recommend an alternative solution:

auto i = new int(2);
BOOST_SCOPE_EXIT_ALL(&) { delete i; };