Qt 5 class parameter as std::function<>

[ad5xj]

I am having difficulty with some borrowed code that is supposedly working. It contains a class defined like:

#include <functional>
using namespace std;
 
class ClassName
{
public:
    ClassName(parm1 parm, const std::function<double(quint16,quint16)>& func, parm3 param);
};

#include <functional>
using namespace std;

class ClassName
{
public:
    ClassName(parm1 parm, const std::function<double(quint16,quint16)>& func, parm3 param);
};

To copy to clipboard, switch view to plain text mode 

The Qt 5.5 compiler complains:

function does not name a type

and of course, a few errors that cascade after due to this one.

I am not sure what the compiler error is all about. The library <functional> is where the function<> is contained and is included.

What am I missing here?