array decay to pointer and overload resolution - c++

Question

I want to be able to differentiate array from pointers in overload resolution :
class string {
public:
string(const char* c_str);
template<int N>
string(const char (&str) [N]);
};
int main() {
const char* c_str = "foo";
string foo(c_str); // ok will call string(const char*)
string bar("bar"); // call string(const char*) instead of the array version
}
The best I have found so far is to use a reference to the pointer instead of a pointer :
class string {
public:
string(const char*& c_str);
template<int N>
string(const char (&str) [N]);
};
int main() {
const char* c_str = "foo";
string foo(c_str); // ok will call string(const char*)
string bar("bar"); // ok, will call the array version
}
it's not exactly the same thing and I want to know if a better way exist

Answer 1

You need to make the first overload a poorer choice when both are viable. Currently they are a tie on conversion ranking (both are "Exact Match"), and then the tie is broken because non-templates are preferred.
This ought to make the conversion ranking worse:
struct stg
{
struct cvt { const char* p; cvt(const char* p_p) : p(p_p) {} };
// matches const char*, but disfavored in overload ranking
stg(cvt c_str); // use c_str.p inside :( Or add an implicit conversion
template<int N>
stg(const char (&str) [N]);
};

Answer 2

You can use SFINAE. This might not be the best way, but it should work ok:
//thanks to dyp for further reduction
template<typename T, typename = typename std::enable_if<std::is_same<T, char>::value>::type>
string(const T * const &) {std::cout << "const char *\n";}
template<std::size_t N> //credit to jrok for noticing the unnecessary SFINAE
string(const char(&)[N]) {std::cout << "const char(&)[" << N << "]\n";}
Here's a live example.

Answer 3

A more generic version of this problem could be detected as follows.
template <class T>
void func(T,
typename std::enable_if<std::is_pointer<T>::value, void>::type * = 0)
{
// catch ptr
}
template <class T, int N>
void func(T (&)[N])
{
//catch array
}
int main(void)
{
int arr[5];
char *b = 0;
func(arr); // catch array
func(b); // catch ptr
}