Question

I was looking for the best way to convert string to int and I came across a function I don’t understand:

template <class T>
bool from_string(T& t, 
                 const std::string& s, 
                 std::ios_base& (*f)(std::ios_base&))
{
  std::istringstream iss(s);
  return !(iss >> f >> t).fail();
}

I know what template is, I don’t know what it means:

std::ios_base& (*f)(std::ios_base&)

Is a new pointer being created here, why are there 2 expressions enclosed in parenthesis next to each other?

Answer 1

It’s a pointer to a function which takes a std::ios_base& as the
argument, and returns an std::ios_base&.

In fact, it’s one form of a manipulator which doesn’t take any
arguments. The << overload for this type simply calls the function;
the function then does whatever it likes on the stream, returning it.
Your function can thus be called with something like:

from_string( anInt, "0A", &std::hex );