Constructor that takes nulltpr_t: function definition does not declare parameters - c++

Question

I have the following code:
class C {
private:
void *data;
public:
constexpr C(nullptr_t) : data(nullptr) { }
C(int i) : data(new int(i)) { }
};
I have created a constructor which takes nullptr_t, so that I can have code similar to the following:
C foo(2);
// ...
foo = nullptr;
Code similar to this has worked previously on MSVC, however this code fails to compile on GCC 5.3.1 (using -std=c++14) with on the closing bracket of C(nullptr_t) with error: function definition does not declare parameters. Even if i give the parameter a name (in this case _), I get error: expected ')' before '_'. This also fails if the constexpr keyword is removed.
Why am I unable to declare such a constructor, and what are any possible workarounds?

Answer 1

You must be a fan of "using namespace std", and you just got tripped up by it:
constexpr C(std::nullptr_t) : data(nullptr) { }
gcc 5.3.1 compiles this, at --std=c++14 conformance level:
[mrsam#octopus tmp]$ cat t.C
#include <iostream>
class C {
private:
void *data;
public:
constexpr C(std::nullptr_t) : data(nullptr) { }
C(int i) : data(new int(i)) { }
};
[mrsam#octopus tmp]$ g++ -g -c --std=c++14 -o t.o t.C
[mrsam#octopus tmp]$ g++ --version
g++ (GCC) 5.3.1 20160406 (Red Hat 5.3.1-6)
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