Is it possible to take the address of an ADL function?
argument-dependent-lookup, c++, function-pointers
Solution
Honestly, I don't know but I tend towards saying that this is not possible.
Depending on what you want to achieve I can suggest a workaround. More precisely, if you just need the address of a function that has the same semantics as `swap` called through ADL then you can use this:
template <typename T>
void (*get_swap())(T&, T&) {
return [](T& x, T& y) { return swap(x, y); };
}
For instance, the following code:
namespace a {
struct b {
int i;
};
void swap(b& x, b& y) {
std::swap(x.i, y.i);
}
}
int main() {
auto f0 = (void (*)(a::b&, a::b&)) a::swap;
auto f1 = get_swap<a::b>();
std::cout << std::hex;
std::cout << (unsigned long long) f0 << '\n';
std::cout << (unsigned long long) f1 << '\n';
}
compiled with gcc 4.8.1 (`-std=c++11 -O3`) on my machine gave:
4008a0
4008b0
The relevant assembly code (`objdump -dSC a.out`) is
00000000004008a0 <a::swap(a::b&, a::b&)>:
4008a0: 8b 07 mov (%rdi),%eax
4008a2: 8b 16 mov (%rsi),%edx
4008a4: 89 17 mov %edx,(%rdi)
4008a6: 89 06 mov %eax,(%rsi)
4008a8: c3 retq
4008a9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
00000000004008b0 <void (*get_swap<a::b>())(a::b&, a::b&)::{lambda(a::b&, a::b&)#1}::_FUN(a::b&, a::b&)>:
4008b0: 8b 07 mov (%rdi),%eax
4008b2: 8b 16 mov (%rsi),%edx
4008b4: 89 17 mov %edx,(%rdi)
4008b6: 89 06 mov %eax,(%rsi)
4008b8: c3 retq
4008b9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
As one can see the functions pointed by `f0` and `f1` (located at `0x4008a0` and `0x4008b0`, respectively) are binary identical. The same holds when compiled with clang 3.3.
If the linker can do identical COMDAT folding (ICF), I guess, we can even get `f0 == f1`. (For more on ICF see this post.)
Problem
Is it possible to take the address of a function that would be found through ADL? For example: ``` template<class T> void (*get_swap())(T &, T &) { return & _________; // how do I take the address of T's swap() function? } int main() { typedef some_type T; get_swap<T>(); } ```