Why does clang take a string literal as a pointer rather than an array?

Question

#include <iostream>
using namespace std;

void f(const char* arg)
{
    cout << "arg is a pointer" << endl;
}

template<size_t N>
void f(const char (&arg)[N])
{
    cout << "arg is an array." << endl;
}

int main()
{
    f("");
}

My compiler is clang 3.8.

The output is:

arg is a pointer

However, according to cppreference.com,

The type of an unprefixed string literal is const char[].

Why does the overload resolution not behave as expected?

Answer 1

It does behave as expected, you just need to adjust your expectations ;-)

const char[1] and const char (&)[1] are different types.

The conversions to const char* (array-to-pointer conversion) and const (&char)[1] (identity conversion) are both considered exact matches, but a non-template is a better match than a template.

If you write a non-template size-specific overload,

void f(const char (&arg)[1])

you will get an error that the function call is ambiguous.

Answer 2

@molbdnilo's answer is correct. To add one detail: Your intuition would be correct and the compiler would prefer to avoid the array-to-pointer conversion by calling the template. But lvalue transformations (lvalue-to-rvalue, array-to-pointer, and function-to-pointer) are specifically ignored in overload ranking, according to [over.ics.rank] §13.3.3.2/3.2.1.

There is a workaround: add a fake volatile to restore the balance of overload preference. Just be sure to remove it by const_cast before using the parameter.