std::function still requires parameters when no place holders are coded in

I wanted to know what I might be doing wrong here. This is my code sample. Its just a rough code sample depicting the use of function pointers.

#include <iostream>
#include <functional>

using namespace std;

std::function<std::string(std::string)> fptr;

std::string foo()
{
    return fptr();  //--->statement A
}

std::string bar(std::string p)
{
    return p;
}

int main()
{
  fptr = std::bind(&bar,"Hello");
  std::cout << foo();
}

Notice in std::bind I did not use any placeholders and "Hello" is the parameter to the function bar. My question is why does

return fptr();

not work. If i do the following

return fptr("Bye");

It works and returns "Hello" . (No point of passing parameter during fptr call) Why does it not work without a parameter ?